The Elements of Geology
by
William Harmon Norton
Part 2 out of 7
The profile of the bed of the Niagara along the gorge (Fig. 39)
shows alternating deeps and shallows which cannot be accounted
for, except in a single instance, by the relative hardness of the
rocks of the river bed. The deeps do not exceed that at the foot
of the Horseshoe Falls at the present time. When the gorge was
being cut along the shallows, how did the Falls compare in
excavating power, in force, and volume with the Niagara of to-day?
How did the rate of recession at those times compare with the
present rate? Is the assumption made above that the rate of
recession has been uniform correct?
The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-
five feet. From what data could such an estimate be made?
Suggest a reason why the Horseshoe Falls are convex upstream.
At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will
be the fate of the Falls left behind when the other has passed
beyond the head of the island?
The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-
jointed rocks? on horizontal strata or on strata steeply inclined?
DEPOSITION
While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to
many delays en route. Now and again it is laid aside, to be picked
up later and carried some distance farther on its way. One of the
most striking features of the river therefore is the waste
accumulated along its course, in bars and islands in the channel,
beneath its bed, and in flood plains along its banks. All this
alluvium, to use a general term for river deposits, with which the
valley is cumbered is really en route to the sea; it is only
temporarily laid aside to resume its journey later on. Constantly
the river is destroying and rebuilding its alluvial deposits, here
cutting and there depositing along its banks, here eroding and
there building a bar, here excavating its bed and there filling it
up, and at all times carrying the material picked up at one point
some distance on downstream before depositing it at another.
These deposits are laid down by slackening currents where the
velocity of the stream is checked, as on the inner side of curves,
and where the slope of the bed is diminished, and in the lee of
islands, bridge piers and projecting points of land. How slight is
the check required to cause a current to drop a large part of its
load may be inferred from the law of the relation of the
transporting power to the velocity. If the velocity is decreased
one half, the current can move fragments but one sixty-fourth the
size of those which it could move before, and must drop all those
of larger size.
Will a river deposit more at low water or at flood? when rising or
when falling?
STRATIFICATION. River deposits are stratified, as may be seen in
any fresh cut in banks or bars. The waste of which they are built
has been sorted and deposited in layers, one above another; some
of finer and some of coarser material. The sorting action of
running water depends on the fact that its transporting power
varies with the velocity. A current whose diminishing velocity
compels it to drop coarse gravel, for example, is still able to
move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower
currents may deposit on the gravel bed layers of sand, and, still
later, slack water may leave on these a layer of mud. In case of
materials lighter than water the transporting power does not
depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.
CROSS BEDDING. A section of a bar exposed at low water may show
that it is formed of layers of sand, or coarser stuff, inclined
downstream as steeply often as the angle of repose of the
material. From a boat anchored over the lower end of a submerged
sand bar we may observe the way in which this structure, called
cross bedding, is produced. Sand is continually pushed over the
edge of the bar at b (Fig. 42) and comes to rest in successive
layers on the sloping surface. At the same time the bar may be
worn away at the upper end, a, and thus slowly advance down
stream. While the deposit is thus cross bedded, it constitutes as
a whole a stratum whose upper and lower surfaces are about
horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.
WATER WEAR. The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are WATER
WORN, or rounded, except when near their source. Rolling along the
bottom they have been worn round by impact and friction as they
rubbed against one another and the rocky bed of the stream.
Experiments have shown that angular fragments of granite lose
nearly half their weight and become well rounded after traveling
fifteen miles in rotating cylinders partly filled with water.
Marbles are cheaply made in Germany out of small limestone cubes
set revolving in a current of water between a rotating bed of
stone and a block of oak, the process requiring but about fifteen
minutes. It has been found that in the upper reaches of mountain
streams a descent of less than a mile is sufficient to round
pebbles of granite.
LAND FORMS DUE TO RIVER EROSION
RIVER VALLEYS. In their courses to the sea, rivers follow valleys
of various forms, some shallow and some deep, some narrow and some
wide. Since rivers are known to erode their beds and banks, it is
a fair presumption that, aided by the weather, they have excavated
the valleys in which they flow.
Moreover, a bird's-eye view or a map of a region shows the
significant fact that the valleys of a system unite with one
another in a branch work, as twigs meet their stems and the
branches of a tree its trunk. Each valley, from that of the
smallest rivulet to that of the master stream, is proportionate to
the size of the stream which occupies it. With a few explainable
exceptions the valleys of tributaries join that of the trunk
stream at a level; there is no sudden descent or break in the bed
at the point of juncture. These are the natural consequences which
must follow if the land has long been worked upon by streams, and
no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been
formed by the river systems which drain them, aided by the work of
the weather; they are not gaping fissures in the earth's crust, as
early observers imagined, but are the furrows which running water
has drawn upon the land.
As valleys are made by the slow wear of streams and the action of
the weather, they pass in their development through successive
stages, each of which has its own characteristic features. We may
therefore classify rivers and valleys according to the stage which
they have reached in their life history from infancy to old age.
YOUNG RIVER VALLEYS
INFANCY. The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba
was so recently covered by the waters of an extinct lake, known as
Lake Agassiz, that the surface remains much as it was left when
the lake was drained away. The flat floor, spread smooth with
lake-laid silts, is still a plain, to the eye as level as the sea.
Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not
exceeding sixty feet in depth, their gradients differing little
from the general slopes of the region. The trunk streams have but
few tributaries; the river system, like a sapling with few limbs,
is still undeveloped. Along the banks of the trunk streams short
gullies are slowly lengthening headwards, like growing twigs which
are sometime to become large branches.
The flat interstream areas are as yet but little scored by
drainage lines, and in wet weather water lingers in ponds in any
initial depressions on the plain.
CONTOURS. In order to read the topographic maps of the text-book
and the laboratory the student should know that contours are lines
drawn on maps to represent relief, all points on any given contour
being of equal height above sea level. The CONTOUR INTERVAL is the
uniform vertical distance between two adjacent contours and varies
on different maps.
To express regions of faint relief a contour interval of ten or
twenty feet is commonly selected; while in mountainous regions a
contour interval of two hundred and fifty, five hundred, or even
one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.
Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or
on a spread of glacial drift left by the retreat of continental
ice sheets, such as covers much of Canada and the northeastern
parts of the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground
water stands high, and, exuding in the undrained initial
depressions, forms marshes and lakes.
LAKES. Lakes are perhaps the most obvious of these fleeting
features of infancy. They are short-lived, for their destruction
is soon accomplished by several means. As a river system advances
toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the
undrained depressions of the region. Lakes having outlets are
drained away as their basin rims are cut down by the outflowing
streams,--a slow process where the rim is of hard rock, but a
rapid one where it is of soft material such as glacial drift.
Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the
shore and strew the debris worn from it over the lake bed. Shallow
lakes are often filled with organic matter from decaying
vegetation.
Does the outflowing stream, from a lake carry sediment? How does
this fact affect its erosive power on hard rock? on loose
material?
Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters
of the lake for a length of twenty miles with the waste brought
down from the high Alps. For this distance there extends up the
Rhone Valley an alluvial plain, which has grown lakeward at the
rate of a mile and a half since Roman times, as proved by the
distance inland at which a Roman port now stands.
How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the
great dam across the Colorado River at Austin, Texas, sediments
thirty-nine feet deep gathered in seven years.
Lake Mendota, one of the many beautiful lakes of southern
Wisconsin, is rapidly cutting back the soft glacial drift of its
shores by means of the abrasion of its waves. While the shallow
basin is thus broadened, it is also being filled with the waste;
and the time is brought nearer when it will be so shoaled that
vegetation can complete the work of its effacement.
Along the margin of a shallow lake mosses, water lilies, grasses,
and other water-loving plants grow luxuriantly. As their decaying
remains accumulate on the bottom, the ring of marsh broadens
inwards, the lake narrows gradually to a small pond set in the
midst of a wide bog, and finally disappears. All stages in this
process of extinction may be seen among the countless lakelets
which occupy sags in the recent sheets of glacial drift in the
northern states; and more numerous than the lakes which still
remain are those already thus filled with carbonaceous matter
derived from the carbon dioxide of the atmosphere. Such fossil
lakes are marked by swamps or level meadows underlain with muck.
THE ADVANCE TO MATURITY. The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins
of its area, are gradually effaced. By the furrowing action of the
rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire
area, and not an acre is left on which the fallen raindrop does
not find already cut for it an uninterrupted downward path which
leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is
now wholly destroyed; the region is all reduced to valley slopes.
THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds
with the initial surface of the region on which the stream begins
to flow, although its way may lead through basins and down steep
descents. The successive profiles to which it reduces its bed are
illustrated in Figure 51. As the gradient, or rate of descent of
its bed, is lowered, the velocity of the river is decreased until
its lessening energy is wholly consumed in carrying its load and
it can no longer erode its bed. The river is now AT GRADE, and its
capacity is just equal to its load. If now its load is increased
the stream deposits, and thus builds up, or AGGRADES, its bed. On
the other hand, if its load is diminished it has energy to spare,
and resuming its work of erosion, DEGRADES its bed. In either case
the stream continues aggrading or degrading until a new gradient
is found where the velocity is just sufficient to move the load,
and here again it reaches grade.
V-VALLEYS. Vigorous rivers well armed with waste make short work
of cutting their beds to grade, and thus erode narrow, steep-sided
gorges only wide enough at the base to accommodate the stream. The
steepness of the valley slopes depends on the relative rates at
which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream
may saw out a gorge whose sides are nearly or even quite vertical,
but as a rule young valleys whose streams have not yet reached
grade are V-shaped; their sides flare at the top because here the
rocks have longest been opened up to the action of the weather.
Some of the deepest canyons may be found where a rising land mass,
either mountain range or plateau, has long maintained by its
continued uplift the rivers of the region above grade.
In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you
suggest a reason?
THE GRAND CANYON OF THE COLORADO RIVER IN ARIZONA. The Colorado
River trenches the high plateau of northern Arizona with a
colossal canyon two hundred and eighteen miles long and more than
a mile in greatest depth. The rocks in which the canyon is cut are
for the most part flat-lying, massive beds of limestones and
sandstones, with some shales, beneath which in places harder
crystalline rocks are disclosed. Where the canyon is deepest its
walls have been profoundly dissected. Lateral ravines have widened
into immense amphitheaters, leaving between them long ridges of
mountain height, buttressed and rebuttressed with flanking spurs
and carved into majestic architectural forms. From the extremity
of one of these promontories it is two miles or more across the
gulf to the point of the one opposite, and the heads of the
amphitheaters are thirteen miles apart.
The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the
river, a swift, powerful stream a few hundred feet wide, turbid
with reddish silt, by means of which it continually rasps its
rocky bed as it hurries on. The Colorado is still deepening its
gorge. In the Grand Canyon its gradient is seven and one half feet
to the mile, but, as in all ungraded rivers, the descent is far
from uniform. Graded reaches in soft rock alternate with steeper
declivities in hard rock, forming rapids such as, for example, a
stretch of ten miles where the fall averages twenty-one feet to
the mile. Because of these dangerous rapids the few exploring
parties who have traversed the Colorado canyon have done so at the
hazard of their lives.
The canyon has been shaped by several agencies. Its depth is due
to the river which has sawed its way far toward the base of a
lofty rising plateau. Acting alone this would have produced a
slitlike gorge little wider than the breadth of the stream. The
impressive width of the canyon and the magnificent architectural
masses which fill it are owing to two causes.: Running water has
gulched the walls and weathering has everywhere attacked and
driven them back. The horizontal harder beds stand out in long
lines of vertical cliffs, often hundreds of feet in height, at
whose feet talus slopes conceal the outcrop of the weaker strata.
As the upper cliffs have been sapped and driven back by the
weather, broad platforms are left at their bases and the sides of
the canyon descend to the river by gigantic steps. Far up and down
the canyon the eye traces these horizontal layers, like the
flutings of an elaborate molding, distinguishing each by its
contour as well as by its color and thickness.
The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no
more than well begun. It has not yet wholly reached grade, and the
great task of the river and its tributaries--the task of leveling
the lofty plateau to a low plain and of transporting it grain by
grain to the sea--still lies almost entirely in the future.
WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to
the initial surface over which it began its course; still more
commonly are they developed in the rock mass through which it is
cutting its valley. Thus, wherever a stream leaves harder rocks to
flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or
rapid, results.
At time of flood young tributaries with steeper courses than that
of the trunk stream may bring down stones and finer waste, which
the gentler current cannot move along, and throw them as a dam
across its way. The rapids thus formed are also ephemeral, for as
the gradient of the tributaries is lowered the main stream becomes
able to handle the smaller and finer load which they discharge.
A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in
the erosion of their beds because of their smaller volume, and
thus join it by plunging over the side of its gorge. But as the
river approaches grade and slackens its down cutting, the
tributaries sooner or later overtake it, and effacing their falls,
unite with it on a level.
Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have
been obliterated already.
LOCAL BASELEVELS. That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks,
ungraded outcrops of hard rocks remaining as barriers which give
rise to rapids or falls. Until these barriers are worn away they
constitute local baselevels, below which level the stream, up
valley from them, cannot cut. They are eroded to grade one after
another, beginning with the least strong, or the one nearest the
mouth of the stream. In a similar way the surface of a lake in a
river's course constitutes for all inflowing streams a local
baselevel, which disappears when the basin is filled or drained.
MATURE AND OLD RIVERS
Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great
task of wearing down the land mass which it drains and carrying it
particle by particle to the sea. The relief of the land is now at
its greatest; for the main channels have been sunk to grade, while
the divides remain but little worn below their initial altitudes.
Ground water now stands low. The run-off washes directly to the
streams, with the least delay and loss by evaporation in ponds and
marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley
slopes is now largest and sheds to the streams a heavier load of
waste than ever before. At maturity the river system is doing its
greatest amount of work both in erosion and in the carriage of
water and of waste to the sea.
LATERAL EROSION. On reaching grade a river ceases to scour its
bed, and it does not again begin to do so until some change in
load or volume enables it to find grade at a lower level. On the
other hand, a stream erodes its banks at all stages in its
history, and with graded rivers this process, called lateral
erosion, or PLANATION, is specially important. The current of a
stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and
continually wears and saps its banks. On the inner side deposition
takes place in the more shallow and slower-moving water. The inner
bank of bends is thus built out while the outer bank is worn away.
By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is
thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle
slopes. While widening their valleys streams maintain a constant
width of channel, so that a wide-floored valley does not signify
that it ever was occupied by a river of equal width.
THE GRADIENT. The gradients of graded rivers differ widely. A
large river with a light load reaches grade on a faint slope,
while a smaller stream heavily burdened with waste requires a
steep slope to give it velocity sufficient to move the load.
The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the
Great Plains and surcharged with the waste brought down both by
its branches in the mountains and by those whose tracks lie over
the soft rocks of the plains. It is compelled to maintain a
gradient of eight feet to the mile in western Nebraska. The Ohio
reaches grade with a slope of less than four inches to the mile
from Cincinnati to its mouth, and the powerful Mississippi washes
along its load with a fall of but three inches per mile from Cairo
to the Gulf.
Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream
supplied with gravel or one with silt?
Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the
load should be increased in amount or in coarseness?
THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed
over the gentler slopes of the lowering hills. With smaller loads
to carry, the rivers now deepen their valleys and find grade with
fainter declivities nearer the level of the sea. This limit of the
level of the sea beneath which they cannot erode is known as
baselevel. [Footnote: The term "baselevel" is also used to
designate the close approximation to sea level to which streams
are able to subdue the land.] As streams grow old they approach
more and more closely to baselevel, although they are never able
to attain it. Some slight slope is needed that water may flow and
waste be transported over the land. Meanwhile the relief of the
land has ever lessened. The master streams and their main
tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion
of their innumerable branches and the wear of the weather the
divides everywhere are lowered and subdued to more and more gentle
slopes. Mountains and high plateaus are thus reduced to rolling
hills, and at last to plains, surmounted only by such hills as may
still be unreduced to the common level, because of the harder
rocks of which they are composed or because of their distance from
the main erosion channels. Such regions of faint relief, worn down
to near base level by subaerial agencies, are known as PENEPLAINS
(almost plains). Any residual masses which rise above them are
called MONADNOCKS, from the name of a conical peak of New
Hampshire which overlooks the now uplifted peneplain of southern
New England.
In its old age a region becomes mantled with thick sheets of fine
and weathered waste, slowly moving over the faint slopes toward
the water ways and unbroken by ledges of bare rock. In other
words, the waste mantle also is now graded, and as waterfalls have
been effaced in the river beds, so now any ledges in the wide
streams of waste are worn away and covered beneath smooth slopes
of fine soil. Ground water stands high and may exude in areas of
swamp. In youth the land mass was roughhewn and cut deep by stream
erosion. In old age the faint reliefs of the land dissolve away,
chiefly under the action of the weather, beneath their cloak of
waste.
THE CYCLE OF EROSION. The successive stages through which a land
mass passes while it is being leveled to the sea constitute
together a cycle of erosion. Each stage of the cycle from infancy
to old age leaves, as we have seen, its characteristic records in
the forms sculptured on the land, such as the shapes of valleys
and the contours of hills and plains. The geologist is thus able
to determine by the land forms of any region the stage in the
erosion cycle to which it now belongs, and knowing what are the
earlier stages of the cycle, to read something of the geological
history of the region.
INTERRUPTED CYCLES. So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual
movements of the earth's crust, by which a region is either
depressed or elevated relative to sea level.
The DEPRESSION of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their
beds and carry their loads to a degree corresponding to the amount
of the depression, and by lessening the amount of work they have
to do. The slackened river currents deposit their waste in Hood
plains which increase in height as the subsidence continues. The
lower courses of the rivers are invaded by the sea and become
estuaries, while the lower tributaries are cut off from the trunk
stream.
ELEVATION, on the other hand, increases the activity of all
agencies of weathering, erosion, and transportation, restores the
region to its youth, and inaugurates a new cycle of erosion.
Streams are given a steeper gradient, greater velocity, and
increased energy to carry their loads and wear their beds. They
cut through the alluvium of their flood plains, leaving it on
either bank as successive terraces, and intrench themselves in the
underlying rock. In their older and wider valleys they cut narrow,
steep-walled inner gorges, in which they flow swiftly over rocky
floors, broken here and there by falls and rapids where a harder
layer of rock has been discovered. Winding streams on plains may
thus incise their meanders in solid rock as the plains are
gradually uplifted. Streams which are thus restored to their youth
are said to be REVIVED.
As streams cut deeper and the valley slopes are steepened, the
mantle of waste of the region undergoing elevation is set in more
rapid movement. It is now removed particle by particle faster than
it forms. As the waste mantle thins, weathering attacks the rocks
of the region more energetically until an equilibrium is reached
again; the rocks waste rapidly and their waste is as rapidly
removed.
DISSECTED PENEPLAINS. When a rise of the land brings one cycle to
an end and begins another, the characteristic land forms of each
cycle are found together and the topography of the region is
composite until the second cycle is so far advanced that the land
forms of the first cycle are entirely destroyed. The contrast
between the land surfaces of the later and the earlier cycles is
most striking when the earlier had advanced to age and the later
is still in youth. Thus many peneplains which have been elevated
and dissected have been recognized by the remnants of their
ancient erosion surfaces, and the length of time which has elapsed
since their uplift has been measured by the stage to which the new
cycle has advanced.
THE PIEDMONT BELT. As an example of an ancient peneplain uplifted
and dissected we may cite the Piedmont Belt, a broad upland lying
between the Appalachian Mountains and the Atlantic coastal plain.
The surface of the Piedmont is gently rolling. The divides, which
are often smooth areas of considerable width, rise to a common
plane, and from them one sees in every direction an even sky line
except where in places some lone hill or ridge may lift itself
above the general level (Fig. 62). The surface is an ancient one,
for the mantle of residual waste lies deep upon it, soils are
reddened by long oxidation, and the rocks are rotted to a depth of
scores of feet.
At present, however, the waste mantle is not forming so rapidly as
it is being removed. The streams of the upland are actively
engaged in its destruction. They flow swiftly in narrow, rock-
walled valleys over rocky beds. This contrast between the young
streams and the aged surface which they are now so vigorously
dissecting can only be explained by the theory that the region
once stood lower than at present and has recently been upraised.
If now we imagine the valleys refilled with the waste which the
streams have swept away, and the upland lowered, we restore the
Piedmont region to the condition in which it stood before its
uplift and dissection,--a gently rolling plain, surmounted here
and there by isolated hills and ridges.
The surface of the ancient Piedmont plain, as it may be restored
from the remnants of it found on the divides, is not in accordance
with the structures of the country rocks. Where these are exposed
to view they are seen to be far from horizontal. On the walls of
river gorges they dip steeply and in various directions and the
streams flow over their upturned edges. As shown in Figure 67, the
rocks of the Piedmont have been folded and broken and tilted.
It is not reasonable to believe that when the rocks of the
Piedmont were thus folded and otherwise deformed the surface of
the region was a plain. The upturned layers have not always
stopped abruptly at the even surface of the Piedmont plain which
now cuts across them. They are the bases of great folds and tilted
blocks which must once have risen high in air. The complex and
disorderly structures of the Piedmont rocks are those seen in
great mountain ranges, and there is every reason to believe that
these rocks after their deformation rose to mountain height.
The ancient Piedmont plain cuts across these upturned rocks as
independently of their structure as the even surface of the sawed
stump of some great tree is independent of the direction of its
fibers. Hence the Piedmont plain as it was before its uplift was
not a coastal plain formed of strata spread in horizontal sheets
beneath the sea and then uplifted; nor was it a structural plain,
due to the resistance to erosion of some hard, flat-lying layer of
rock. Even surfaces developed on rocks of discordant structure,
such as the Piedmont shows, are produced by long denudation, and
we may consider the Piedmont as a peneplain formed by the wearing
down of mountain ranges, and recently uplifted.
THE LAURENTIAN PENEPLAIN. This is the name given to a denuded
surface on very ancient rocks which extends from the Arctic Ocean
to the St. Lawrence River and Lake Superior, with small areas also
in northern Wisconsin and New York. Throughout this U-shaped area,
which incloses Hudson Bay within its arms, the country rocks have
the complicated and contorted structures which characterize
mountain ranges. But the surface of the area is by no means
mountainous. The sky line when viewed from the divides is unbroken
by mountain peaks or rugged hills. The surface of the arm west of
Hudson Bay is gently undulating and that of the eastern arm has
been roughened to low-rolling hills and dissected in places by
such deep river gorges as those of the Ottawa and Saguenay. This
immense area may be regarded as an ancient peneplain truncating
the bases of long-vanished mountains and dissected after
elevation.
In the examples cited the uplift has been a broad one and to
comparatively little height. Where peneplains have been uplifted
to great height and have since been well dissected, and where they
have been upfolded and broken and uptilted, their recognition
becomes more difficult. Yet recent observers have found evidences
of ancient lowland surfaces of erosion on the summits of the
Allegheny ridges, the Cascade Mountains (Fig. 69), and the western
slope of the Sierra Nevadas.
THE SOUTHERN APPALACHIAN REGION. We have here an example of an
area the latter part of whose geological history may be deciphered
by means of its land forms. The generalized section of Figure 70,
which passes from west to east across a portion of the region in
eastern Tennessee, shows on the west a part of the broad
Cumberland plateau. On the east is a roughened upland platform,
from which rise in the distance the peaks of the Great Smoky
Mountains. The plateau, consisting of strata but little changed
from their original flat-lying attitude, and the platform,
developed on rocks of disordered structure made crystalline by
heat and pressure, both stand at the common level of the line AB.
They are separated by the Appalachian valley, forty miles wide,
cut in strata which have been folded and broken into long narrow
blocks. The valley is traversed lengthwise by long, low ridges,
the outcropping edges of the harder strata, which rise to about
the same level,--that of the line cd. Between these ridges stretch
valley lowlands at the level ef excavated in the weaker rocks,
while somewhat below them lie the channels of the present streams
now busily engaged in deepening their beds.
THE VALLEY LOWLANDS. Were they planed by graded or ungraded
streams? Have the present streams reached grade? Why did the
streams cease widening the floors of the valley lowlands? How long
since? When will they begin anew the work of lateral planation?
What effect will this have on the ridges if the present cycle of
erosion continues long uninterrupted?
THE RIDGES OF THE APPALACHIAN VALLEY. Why do they stand above the
valley lowlands? Why do their summits lie in about the same plane?
Refilling the valleys intervening between these ridges with the
material removed by the streams, what is the nature of the surface
thus restored? Does this surface cd accord with the rock
structures on which' it has been developed? How may it have been
made? At what height did the land stand then, compared with its
present height? What elevations stood above the surface cd? Why?
What name may you use to designate them? How does the length of
time needed to develop the surface cd compare with that needed to
develop the valley lowlands?
THE PLATFORM AND PLATEAU. Why do they stand at a common level ab?
Of what surface may they be remnants? Is it accordant with the
rock structure? How was it produced? What unconsumed masses
overlooked it? Did the rocks of the Appalachian valley stand above
this surface when it was produced? Did they then stand below it?
Compare the time needed to develop this surface with that needed
to develop cd. Which surface is the older?
How many cycles of erosion are represented here? Give the erosion
history of the region by cycles, beginning with the oldest, the
work done in each and the work left undone, what brought each
cycle to a close, and how long relatively it continued.
CHAPTER IV
RIVER DEPOSITS
The characteristic features of river deposits and the forms which
they assume may be treated under three heads: (1) valley deposits,
(2) basin deposits, and (3) deltas.
VALLEY DEPOSITS
FLOOD PLAINS are the surfaces of the alluvial deposits which
streams build along their courses at times of flood. A swift
current then sweeps along the channel, while a shallow sheet of
water moves slowly over the flood plain, spreading upon it a thin
layer of sediment. It has been estimated that each inundation of
the Nile leaves a layer of fertilizing silt three hundredths of an
inch thick over the flood plain of Egypt.
Flood plains may consist of a thin spread of alluvium over the
flat rock floor of a valley which is being widened by the lateral
erosion of a graded stream (Fig. 60). Flood-plain deposits of
great thickness may be built by aggrading rivers even in valleys
whose rock floors have never been thus widened.
A cross section of a flood plain shows that it is highest next the
river, sloping gradually thence to the valley sides. These wide
natural embankments are due to the fact that the river deposit is
heavier near the bank, where the velocity of the silt-laden
channel current is first checked by contact with the slower-moving
overflow.
Thus banked off from the stream, the outer portions of a flood
plain are often ill-drained and swampy, and here vegetal deposits,
such as peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the Mississippi or
the Missouri (Fig. 77), shows that the courses of the tributaries
on entering it are deflected downstream. Why?
The aggrading streams by which flood plains are constructed
gradually build their immediate banks and beds to higher and
higher levels, and therefore find it easy at times of great floods
to break their natural embankments and take new courses over the
plain. In this way they aggrade each portion of it in turn by
means of their shifting channels,
BRAIDED CHANNELS. A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep
gradient and often flows down it in a fairly direct course and
through a network of braided channels. From time to time a channel
becomes choked with waste, and the water no longer finding room in
it breaks out and cuts and builds itself a new way which reunites
down valley with the other channels. Thus there becomes
established a network of ever-changing channels inclosing low
islands of sand and gravel.
TERRACES. While aggrading streams thus tend to shift their
channels, degrading streams, on the contrary, become more and more
deeply intrenched in their valleys. It often occurs that a stream,
after having built a flood plain, ceases to aggrade its bed
because of a lessened load or for other reasons, such as an uplift
of the region, and begins instead to degrade it. It leaves the
original flood plain out of reach of even the highest floods. When
again it reaches grade at a lower level it produces a new flood
plain by lateral erosion in the older deposits, remnants of which
stand as terraces on one or both sides of the valley. In this way
a valley may be lined with a succession of terraces at different
levels, each level representing an abandoned flood plain.
MEANDERS. Valleys aggraded with fine waste form well-nigh level
plains over which streams wind from side to side of a direct
course in symmetric bends known as meanders, from the name of a
winding river of Asia Minor. The giant Mississippi has developed
meanders with a radius of one and one half miles, but a little
creek may display on its meadow as perfect curves only a rod or so
in radius. On the flood plain of either river or creek we may find
examples of the successive stages in the development of the
meander, from its beginning in the slight initial bend sufficient
to deflect the current against the outer side. Eroding here and
depositing on the inner side of the bend, it gradually reaches
first the open bend whose width and length are not far from equal,
and later that of the horseshoe meander whose diameter transverse
to the course of the stream is much greater than that parallel
with it. Little by little the neck of land projecting into the
bend is narrowed, until at last it is cut through and a "cut-off"
is established. The old channel is now silted up at both ends and
becomes a crescentic lagoon, or oxbow lake, which fills gradually
to an arc-shaped shallow depression.
FLOOD PLAINS CHARACTERISTIC OF MATURE RIVERS. On reaching grade a
stream planes a flat floor for its continually widening valley.
Ever cutting on the outer bank of its curves, it deposits on the
inner bank scroll-like flood-plain patches. For a while the valley
bluffs do not give its growing meanders room to develop to their
normal size, but as planation goes on, the bluffs are driven back
to the full width of the meander belt and still later to a width
which gives room for broad stretches of flood plain on either
side.
Usually a river first attains grade near its mouth, and here first
sinks its bed to near baselevel. Extending its graded course
upstream by cutting away barrier after barrier, it comes to have a
widened and mature valley over its lower course, while its young
headwaters are still busily eroding their beds. Its ungraded
branches may thus bring down to its lower course more waste than
it is competent to carry on to the sea, and here it aggrades its
bed and builds a flood plain in order to gain a steeper gradient
and velocity enough to transport its load.
As maturity is past and the relief of the land is lessened, a
smaller and smaller load of waste is delivered to the river. It
now has energy to spare and again degrades its valley, excavating
its former flood plains and leaving them in terraces on either
side, and at last in its old age sweeping them away.
ALLUVIAL CONES AND FANS. In hilly and mountainous countries one
often sees on a valley side a conical or fan-shaped deposit of
waste at the mouth of a lateral stream. The cause is obvious: the
young branch has not been able as yet to wear its bed to accordant
level with the already deepened valley of the master stream. It
therefore builds its bed to grade at the point of juncture by
depositing here its load of waste,--a load too heavy to be carried
along the more gentle profile of the trunk valley.
Where rivers descend from a mountainous region upon the plain they
may build alluvial fans of exceedingly gentle slope. Thus the
rivers of the western side of the Sierra Nevada Mountains have
spread fans with a radius of as much as forty miles and a slope
too slight to be detected without instruments, where they leave
the rock-cut canyons in the mountains and descend upon the broad
central valley of California.
As a river flows over its fan it commonly divides into a
branchwork of shifting channels called DISTRIBUTARIES, since they
lead off the water from the main stream. In this way each part of
the fan is aggraded and its symmetric form is preserved.
PIEDMONT PLAINS. Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains.
These are especially characteristic of arid lands, where the
streams wither as they flow out upon the thirsty lowlands and are
therefore compelled to lay down a large portion of their load. In
humid climates mountain-born streams are usually competent to
carry their loads of waste on to the sea, and have energy to spare
to cut the lower mountain slopes into foothills. In arid regions
foothills are commonly absent and the ranges rise, as from
pedestals, above broad, sloping plains of stream-laid waste.
THE HIGH PLAINS. The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste
which stretches forward from the base of the mountains for
hundreds of miles and in places is five hundred feet thick (Fig.
80). That the deposit was made in ancient times on land and not in
the sea is proved by the remains which it contains of land animals
and plants of species now extinct. That it was laid by rivers and
not by fresh-water lakes is shown by its structure. Wide stretches
of flat-lying, clays and sands are interrupted by long, narrow
belts of gravel which mark the channels of the ancient streams.
Gravels, and sands are often cross bedded, and their well worn
pebbles may be identified with the rocks of the mountains. After
building this sheet of waste the streams ceased to aggrade and
began the work of destruction. Large uneroded remnants, their
surfaces flat as a floor, remain as the High Plains of western
Kansas and Nebraska.
RIVER DEPOSITS IN SUBSIDING TROUGHS. To a geologist the most
important river deposits are those which gather in areas of
gradual subsidence; they are often of vast extent and immense
thickness, and such deposits of past geological ages have not
infrequently been preserved, with all their records of the times
in which they were built, by being carried below the level of the
sea, to be brought to light by a later uplift. On the other hand,
river deposits which remain above baselevels of erosion are swept
away comparatively soon.
THE GREAT VALLEY OF CALIFORNIA is a monotonously level plain of
great fertility, four hundred miles in length and fifty miles in
average width, built of waste swept down by streams from the
mountain ranges which inclose it,--the Sierra Nevada on the east
and the Coast Range on the west. On the waste slopes at the foot
of the bordering hills coarse gravels and even bowlders are left,
while over the interior the slow-flowing streams at times of
flood spread wide sheets of silt. Organic deposits are now forming
by the decay of vegetation in swampy tule (reed) lands and in
shallow lakes which occupy depressions left by the aggrading
streams.
Deep borings show that this great trough is filled to a depth of
at least two thousand feet below sea level with recent
unconsolidated sands and silts containing logs of wood and fresh-
water shells. These are land deposits, and the absence of any
marine deposits among them proves that the region has not been
invaded by the sea since the accumulation began. It has therefore
been slowly subsiding and its streams, although continually
carried below grade, have yet been able to aggrade the surface as
rapidly as the region sank, and have maintained it, as at present,
slightly above sea level.
THE INDO-GANGETIC PLAIN, spread by the Brahmaputra, the Ganges,
and the Indus river systems, stretches for sixteen hundred miles
along the southern base of the Himalaya Mountains and occupies an
area of three hundred thousand square miles (Fig.342). It consists
of the flood plains of the master streams and the confluent fans
of the tributaries which issue from the mountains on the north.
Large areas are subject to overflow each season of flood, and
still larger tracts mark abandoned flood plains below which the
rivers have now cut their beds. The plain is built of far-
stretching beds of clay, penetrated by streaks of sand, and also
of gravel near the mountains. Beds of impure peat occur in it, and
it contains fresh-water shells and the bones of land animals of
species now living in northern India. At Lucknow an artesian well
was sunk to one thousand feet below sea level without reaching the
bottom of these river-laid sands and silts, proving a slow
subsidence with which the aggrading rivers have kept pace.
WARPED VALLEYS. It is not necessary that an area should sink below
sea level in order to be filled with stream-swept waste. High
valleys among growing mountain ranges may suffer warping, or may
be blockaded by rising mountain folds athwart them. Where the
deformation is rapid enough, the river may be ponded and the
valley filled with lake-laid sediments. Even when the river is
able to maintain its right of way it may yet have its declivity so
lessened that it is compelled to aggrade its course continually,
filling the valley with river deposits which may grow to an
enormous thickness.
Behind the outer ranges of the Himalaya Mountains lie several
waste-filled valleys, the largest of which are Kashmir and Nepal,
the former being an alluvial plain about as large as the state of
Delaware. The rivers which drain these plains have already cut
down their outlet gorges sufficiently to begin the task of the
removal of the broad accumulations which they have brought in from
the surrounding mountains. Their present flood plains lie as much
as some hundreds of feet below wide alluvial terraces which mark
their former levels. Indeed, the horizontal beds of the Hundes
Valley have been trenched to the depth of nearly three thousand
feet by the Sutlej River. These deposits are recent or subrecent,
for there have been found at various levels the remains of land
plants and land and fresh-water shells, and in some the bones of
such animals as the hyena and the goat, of species or of genera
now living. Such soft deposits cannot be expected to endure
through any considerable length of future time the rapid erosion
to which their great height above the level of the sea will
subject them.
CHARACTERISTICS OF RIVER DEPOSITS. The examples just cited teach
clearly the characteristic features of extensive river deposits.
These deposits consist of broad, flat-lying sheets of clay and
fine sand left by the overflow at time of flood, and traversed
here and there by long, narrow strips of coarse, cross-bedded
sands and gravels thrown down by the swifter currents of the
shifting channels. Occasional beds of muck mark the sites of
shallow lakelets or fresh-water swamps. The various strata also
contain some remains of the countless myriads of animals and
plants which live upon the surface of the plain as it is in
process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land
animals as suffer drowning at times of flood or are mired in
swampy places, logs of wood, and the stems and leaves of plants
are examples of the variety of the remains of land and fresh-water
organisms which are entombed in river deposits and sealed away as
a record of the life of the time, and as proof that the deposits
were laid by streams and not beneath the sea.
BASIN DEPOSITS
DEPOSITS IN DRY BASINS. On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of
central Asia, stream-swept waste accumulates indefinitely. The
rivers of the surrounding mountains, fed by the rains and melting
snows of these comparatively moist elevations, dry and soak away
as they come down upon the arid plains. They are compelled to lay
aside their entire load of waste eroded from the mountain valleys,
in fans which grow to enormous size, reaching in some instances
thousands of feet in thickness.
The monotonous levels of Turkestan include vast alluvial tracts
now in process of building by the floods of the frequently
shifting channels of the Oxus and other rivers of the region. For
about seven hundred miles from its mouth in Aral Lake the Oxus
receives no tributaries, since even the larger branches of its
system are lost in a network of distributaries and choked with
desert sands before they reach their master stream. These
aggrading rivers, which have channels but no valleys, spread their
muddy floods--which in the case of the Oxus sometimes equal the
average volume of the Mississippi--far and wide over the plain,
washing the bases of the desert dunes.
PLAYAS. In arid interior basins the central depressions may be
occupied by playas,--plains of fine mud washed forward from the
margins. In the wet season the playa is covered with a thin sheet
of muddy water, a playa lake, supplied usually by some stream at
flood. In the dry season the lake evaporates, the river which fed
it retreats, and there is left to view a hard, smooth, level floor
of sun-baked and sun-cracked yellow clay utterly devoid of
vegetation.
In the Black Rock desert of Nevada a playa lake spreads over an
area fifty miles long and twenty miles wide. In summer it
disappears; the Quinn River, which feeds it, shrinks back one
hundred miles toward its source, leaving an absolutely barren
floor of clay, level as the sea.
LAKE DEPOSITS. Regarding lakes as parts of river systems, we may
now notice the characteristic features of the deposits in lake
basins. Soundings in lakes of considerable size and depth show
that their bottoms are being covered with tine clays. Sand and
gravel are found along; their margins, being brought in by streams
and worn by waves from the shore, but there are no tidal or other
strong currents to sweep coarse waste out from shore to any
considerable distance. Where fine clays are now found on the land
in even, horizontal layers containing the remains of fresh-water
animals and plants, uncut by channels tilled with cross-bedded
gravels and sands and bordered by beach deposits of coarse waste,
we may safely infer the existence of ancient lakes.
MARL. Marl is a soft, whitish deposit of carbonate of lime,
mingled often with more or less of clay, accumulated in small
lakes whose feeding springs are charged with carbonate of lime and
into which little waste is washed from the land. Such lakelets are
not infrequent on the surface of the younger drift sheets of
Michigan and northern Indiana, where their beds of marl--sometimes
as much as forty feet thick--are utilized in the manufacture of
Portland cement. The deposit results from the decay of certain
aquatic plants which secrete lime carbonate from the water, from
the decomposition of the calcareous shells of tiny mollusks which
live in countless numbers on the lake floor, and in some cases
apparently from chemical precipitation.
PEAT. We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants
of the surface of the bog lies a spongy mass composed of dead
vegetable tissue, which passes downward gradually into PEAT,--a
dense, dark brown carbonaceous deposit in which, to the unaided
eye, little or no trace of vegetable structure remains. When
dried, peat forms a fuel of some value and is used either cut into
slabs and dried or pressed into bricks by machinery.
When vegetation decays in open air the carbon of its tissues,
taken from the atmosphere by the leaves, is oxidized and returned
to it in its original form of carbon dioxide. But decomposing in
the presence of water, as in a bog, where the oxygen of the air is
excluded, the carbonaceous matter of plants accumulates in
deposits of peat.
Peat bogs are numerous in regions lately abandoned by glacier ice,
where river systems are so immature that the initial depressions
left in the sheet of drift spread over the country have not yet
been drained. One tenth of the surface of Ireland is said to be
covered with peat, and small bogs abound in the drift-covered area
of New England and the states lying as far west as the Missouri
River. In Massachusetts alone it has been reckoned that there are
fifteen billion cubic feet of peat, the largest bog occupying
several thousand acres.
Much larger swamps occur on the young coastal plain of the
Atlantic from New Jersey to Florida. The Dismal Swamp, for
example, in Virginia and North Carolina is forty miles across. It
is covered with a dense growth of water-loving trees such as the
cypress and black gum. The center of the swamp is occupied by Lake
Drummond, a shallow lake seven miles in diameter, with banks of
pure-peat, and still narrowing from the encroachment of vegetation
along its borders.
SALT LAKES. In arid climates a lake rarely receives sufficient
inflow to enable it to rise to the basin rim and find an outlet.
Before this height is reached its surface becomes large enough to
discharge by evaporation into the dry air the amount of water that
is supplied by streams. As such a lake has no outlet, the minerals
in solution brought into it by its streams cannot escape from the
basin. The lake water becomes more and more heavily charged with
such substances as common salt and the sulphates and carbonates of
lime, of soda, and of potash, and these are thrown down from
solution one after another as the point of saturation for each
mineral is reached. Carbonate of lime, the least soluble and often
the most abundant mineral brought in, is the first to be
precipitated. As concentration goes on, gypsum, which is insoluble
in a strong brine, is deposited, and afterwards common salt. As
the saltness of the lake varies with the seasons and with climatic
changes, gypsum and salt are laid in alternate beds and are
interleaved with sedimentary clays spread from the waste brought
in by streams at times of flood. Few forms of life can live in
bodies of salt water so concentrated that chemical deposits take
place, and hence the beds of salt, gypsum, and silt of such lakes
are quite barren of the remains of life. Similar deposits are
precipitated by the concentration of sea water in lagoons and arms
of the sea cut off from the ocean.
LAKES BONNEVILLE AND LAHONTAN. These names are given to extinct
lakes which once occupied large areas in the Great Basin, the
former in Utah, the latter in northwestern Nevada. Their records
remain in old horizontal beach lines which they drew along their
mountainous shores at the different levels at which they stood,
and in the deposits of their beds. At its highest stage Lake
Bonneville, then one thousand feet deep, overflowed to the north
and was a fresh-water lake. As it shrank below the outlet it
became more and more salty, and the Great Salt Lake, its withered
residue, is now depositing salt along its shores. In its strong
brine lime carbonate is insoluble, and that brought in by streams
is thrown down at once in the form of travertine.
Lake Lahontan never had an outlet. The first chemical deposits to
be made along its shores were deposits of travertine, in places
eighty feet thick. Its floor is spread with fine clays, which must
have been laid in deep, still water, and which are charged with
the salts absorbed by them as the briny water of the lake dried
away. These sedimentary clays are in two divisions, the upper and
lower, each being about one hundred feet thick. They are separated
by heavy deposits of well-rounded, cross-bedded gravels and sands,
similar to those spread at the present time by the intermittent
streams of arid regions. A similar record is shown in the old
floors of Lake Bonneville. What conclusions do you draw from these
facts as to the history of these ancient lakes?
DELTAS
In the river deposits which are left above sea level particles of
waste are allowed to linger only for a time. From alluvial fans
and flood plains they are constantly being taken up and swept
farther on downstream. Although these land forms may long persist,
the particles which compose them are ever changing. We may
therefore think of the alluvial deposits of a valley as a stream
of waste fed by the waste mantle as it creeps and washes down the
valley sides, and slowly moving onwards to the sea.
In basins waste finds a longer rest, but sooner or later lakes and
dry basins are drained or filled, and their deposits, if above sea
level, resume their journey to their final goal. It is only when
carried below the level of the sea that they are indefinitely
preserved.
On reaching this terminus, rivers deliver their load to the ocean.
In some cases the ocean is able to take it up by means of strong
tidal and other currents, and to dispose of it in ways which we
shall study later. But often the load is so large, or the tides
are so weak, that much of the waste which the river brings in
settles at its mouth, there building up a deposit called the
DELTA, from the Greek letter of that name, whose shape it
sometimes resembles.
Deltas and alluvial fans have many common characteristics. Both
owe their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline,
the apex pointing upstream; and both are traversed by
distributaries which build up all parts in turn.
In a delta we may distinguish deposits of two distinct kinds,--
the submarine and the subaerial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves
and tides over the sea bottom adjacent to the river's mouth. The
origin of these deposits is recorded in the remains of marine
animals and plants which they contain.
As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaerial deposits
altogether similar to those of the flood plain, of which indeed
the subaerial delta is the prolongation. Here extended deposits of
peat may accumulate in swamps, and the remains of land and fresh-
water animals and plants swept down by the stream are imbedded in
the silts laid at times of flood.
Borings made in the deltas of great rivers such as the
Mississippi, the Ganges, and the Nile, show that the subaerial
portion often reaches a surprising thickness. Layers of peat, old
soils, and forest grounds with the stumps of trees are discovered
hundreds of feet below sea level. In the Nile delta some eight
layers of coarse gravel were found interbedded with river silts,
and in the Ganges delta at Calcutta a boring nearly five hundred
feet in depth stopped in such a layer.
The Mississippi has built a delta of twelve thousand three hundred
square miles, and is pushing the natural embankments of its chief
distributaries into the Gulf at a maximum rate of a mile in
sixteen years. Muddy shoals surround its front, shallow lakes,
e.g. lakes Pontchartrain and Borgne, are formed between the
growing delta and the old shore line, and elongate lakes and
swamps are inclosed between the natural embankments of the
distributaries.
The delta of the Indus River, India, lies so low along shore that
a broad tract of country is overflowed by the highest tides. The
submarine portion of the delta has been built to near sea level
over so wide a belt offshore that in many places large vessels
cannot come even within sight of land because of the shallow
water.
A former arm of the sea, the Rann of Cutch, adjoining the delta on
the east has been silted up and is now an immense barren flat of
sandy mud two hundred miles in length and one hundred miles in
greatest breadth. Each summer it is flooded with salt water when
the sea is brought in by strong southwesterly monsoon winds, and
the climate during the remainder of the year is hot and dry. By
the evaporation of sea water the soil is thus left so salty that
no vegetation can grow upon it, and in places beds of salt several
feet in thickness have accumulated. Under like conditions salt
beds of great thickness have been formed in the past and are now
found buried among the deposits of ancient deltas.
SUBSIDENCE OF GREAT DELTAS. As a rule great deltas are slowly
sinking. In some instances upbuilding by river deposits has gone
on as rapidly as the region has subsided. The entire thickness of
the Ganges delta, for example, so far as it has been sounded,
consists of deposits laid in open air. In other cases interbedded
limestones and other sedimentary rocks containing marine fossils
prove that at times subsidence has gained on the upbuilding and
the delta has been covered with the sea.
It is by gradual depression that delta deposits attain enormous
thickness, and, being lowered beneath the level of the sea, are
safely preserved from erosion until a movement of the earth's
crust in the opposite direction lifts them to form part of the
land. We shall read later in the hard rocks of our continent the
records of such ancient deltas, and we shall not be surprised to
find them as thick as are those now building at the mouths of
great rivers.
LAKE DELTAS. Deltas are also formed where streams lose their
velocity on entering the still waters of lakes. The shore lines of
extinct lakes, such as Lake Agassiz and Lakes Bonneville and
Lahontan, may be traced by the heavy deposits at the mouths of
their tributary streams.
We have seen that the work of streams is to drain the lands of the
water poured upon them by the rainfall, to wear them down, and to
carry their waste away to the sea, there to be rebuilt by other
agents into sedimentary rocks. The ancient strata of which the
continents are largely made are composed chiefly of material thus
worn from still more ancient lands--lands with their hills and
valleys like those of to-day--and carried by their rivers to the
ocean. In all geological times, as at the present, the work of
streams has been to destroy the lands, and in so doing to furnish
to the ocean the materials from which the lands of future ages
were to be made. Before we consider how the waste of the land
brought in by streams is rebuilt upon the ocean floor, we must
proceed to study the work of two agents, glacier ice and the wind,
which cooperate with rivers in the denudation of the land.
CHAPTER V
THE WORK OF GLACIERS
THE DRIFT. The surface of northeastern North America, as far south
as the Ohio and Missouri rivers, is generally covered by the
drift,--a formation which is quite unlike any which we have so far
studied. A section of it, such as that illustrated in Figure 87,
shows that for the most part it is unstratified, consisting of
clay, sand, pebbles, and even large bowlders, all mingled pell-
mell together. The agent which laid the drift is one which can
carry a load of material of all sizes, from the largest bowlder to
the finest clay, and deposit it without sorting.
The stones of the drift are of many kinds. The region from which
it was gathered may well have been large in order to supply these
many different varieties of rocks. Pebbles and bowlders have been
left far from their original homes, as may be seen in southern
Iowa, where the drift contains nuggets of copper brought from the
region about Lake Superior. The agent which laid the drift is one
able to gather its load over a large area and carry it a long way.
The pebbles of the drift are unlike those rounded by running water
or by waves. They are marked with scratches. Some are angular,
many have had their edges blunted, while others have been ground
flat and smooth on one or more sides, like gems which have been
faceted by being held firmly against the lapidary's wheel. In many
places the upper surface of the country rock beneath the drift has
been swept clean of residual clays and other waste. All rock
rotten has been planed away, and the ledges of sound rock to which
the surface has been cut down have been rubbed smooth and
scratched with long, straight, parallel lines. The agent which
laid the drift can hold sand and pebbles firmly in its grasp and
can grind them against the rock beneath, thus planing it down and
scoring it, while faceting the pebbles also.
Neither water nor wind can do these things. Indeed, nothing like
the drift is being formed by any process now at work anywhere in
the eastern United States. To find the agent which has laid this
extensive formation we must go to a region of different climatic
conditions.
THE INLAND ICE OF GREENLAND. Greenland is about fifteen hundred
miles long and nearly seven hundred miles in greatest width. With
the exception of a narrow fringe of mountainous coast land, it is
completely buried beneath a sheet of ice, in shape like a vast
white shield, whose convex surface rises to a height of nine
thousand feet above the sea. The few explorers who have crossed
the ice cap found it a trackless desert destitute of all life save
such lowly forms as the microscopic plant which produces the so-
called "red snow." On the smooth plain of the interior no rock
waste relieves the snow's dazzling whiteness; no streams of
running water are seen; the silence is broken only by howling
storm winds and the rustle of the surface snow which they drive
before them. Sounding with long poles, explorers find that below
the powdery snow of the latest snowfall lie successive layers of
earlier snows, which grow more and more compact downward, and at
last have altered to impenetrable ice. The ice cap formed by the
accumulated snows of uncounted centuries may well be more than a
mile in depth. Ice thus formed by the compacting of snow is
distinguished when in motion as GLACIER ICE.
The inland ice of Greenland moves. It flows with imperceptible
slowness under its own weight, like, a mass of some viscous or
plastic substance, such as pitch or molasses candy, in all
directions outward toward the sea. Near the edge it has so thinned
that mountain peaks are laid bare, these islands in the sea of ice
being known as NUNATAKS. Down the valleys of the coastal belt it
drains in separate streams of ice, or GLACIERS. The largest of
these reach the sea at the head of inlets, and are therefore
called TIDE GLACIERS. Their fronts stand so deep in sea water that
there is visible seldom more than three hundred feet of the wall
of ice, which in many glaciers must be two thousand and more feet
high. From the sea walls of tide glaciers great fragments break
off and float away as icebergs. Thus snows which fell in the
interior of this northern land, perhaps many thousands of years
ago, are carried in the form of icebergs to melt at last in the
North Atlantic.
Greenland, then, is being modeled over the vast extent of its
interior not by streams of running water, as are regions in warm
and humid climates, nor by currents of air, as are deserts to a
large extent, but by a sheet of flowing ice. What the ice sheet is
doing in the interior we may infer from a study of the separate
glaciers into which it breaks at its edge.
THE SMALLER GREENLAND GLACIERS. Many of the smaller glaciers of
Greenland do not reach the sea, but deploy on plains of sand and
gravel. The edges of these ice tongues are often as abrupt as if
sliced away with a knife (Fig. 92), and their structure is thus
readily seen. They are stratified, their layers representing in
part the successive snowfalls of the interior of the country. The
upper layers are commonly white and free from stones; but the
lower layers, to the height of a hundred feet or more, are dark
with debris which is being slowly carried on. So thickly studded
with stones is the base of the ice that it is sometimes difficult
to distinguish it from the rock waste which has been slowly
dragged beneath the glacier or left about its edges. The waste
beneath and about the glacier is unsorted. The stones are of many
kinds, and numbers of them have been ground to flat faces. Where
the front of the ice has retreated the rock surface is seen to be
planed and scored in places by the stones frozen fast in the sole
of the glacier.
We have now found in glacier ice an agent able to produce the
drift of North America. The ice sheet of Greenland is now doing
what we have seen was done in the recent past in our own land. It
is carrying for long distances rocks of many kinds gathered, we
may infer, over a large extent of country. It is laying down its
load without assortment in unstratified deposits. It grinds down
and scores the rock over which it moves, and in the process many
of the pebbles of its load are themselves also ground smooth and
scratched. Since this work can be done by no other agent, we must
conclude that the northeastern part of our own continent was
covered in the recent past by glacier ice, as Greenland is to-day.
VALLEY GLACIERS
The work of glacier ice can be most conveniently studied in the
separate ice streams which creep down mountain valleys in many
regions such as Alaska, the western mountains of the United States
and Canada, the Himalayas, and the Alps. As the glaciers of the
Alps have been studied longer and more thoroughly than any others,
we shall describe them in some detail as examples of valley
glaciers in all parts of the world.
CONDITIONS OF GLACIER FORMATION. The condition of the great
accumulation of snow to which glaciers are due--that more or less
of each winter's snow should be left over unmelted and
unevaporated to the next--is fully met in the Alps. There is
abundant moisture brought by the winds from neighboring seas. The
currents of moist air driven up the mountain slopes are cooled by
their own expansion as they rise, and the moisture which they
contain is condensed at a temperature at or below 32 degrees F.,
and therefore is precipitated in the form of snow. The summers are
cool and their heat does not suffice to completely melt the heavy
snow of the preceding winter. On the Alps the SNOW LINE--the lower
limit of permanent snow--is drawn at about eight thousand five
hundred feet above sea level. Above the snow line on the slopes
and crests, where these are not too steep, the snow lies the year
round and gathers in valley heads to a depth of hundreds of feet.
This is but a small fraction of the thickness to which snow would
be piled on the Alps were it not constantly being drained away.
Below the snow fields which mantle the heights the mountain
valleys are occupied by glaciers which extend as much as a
vertical mile below the snow line. The presence in the midst of
forests and meadows and cultivated fields of these tongues of ice,
ever melting and yet from year to year losing none of their bulk,
proves that their loss is made good in the only possible way. They
are fed by snow fields above, whose surplus of snow they drain
away in the form of ice. The presence of glaciers below the snow
line is a clear proof that, rigid and motionless as they appear,
glaciers really are in constant motion down valley.
THE NEVE FIELD. The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow. Great
peaks tower above it, and snowy slopes rise on either side on the
flanks of mountain spurs. From these heights fierce winds drift
the snows into the amphitheater, and avalanches pour in their
torrents of snow and waste. The snow of the amphitheater is like
that of drifts in late winter after many successive thaws and
freezings. It is made of hard grains and pellets and is called
NEVE. Beneath the surface of the neve field and at its outlet the
granular neve has been compacted to a mass of porous crystalline
ice. Snow has been changed to neve, and neve to glacial ice, both
by pressure, which drives the air from the interspaces of the
snowflakes, and also by successive meltings and freezings, much as
a snowball is packed in the warm hand and becomes frozen to a ball
of ice.
THE BERGSCHRUND. The neve is in slow motion. It breaks itself
loose from the thinner snows about it, too shallow to share its
motion, and from the rock rim which surrounds it, forming a deep
fissure called the bergschrund, sometimes a score and more feet
wide.
SIZE OF GLACIERS. The ice streams of the Alps vary in size
according to the amount of precipitation and the area of the neve
fields which they drain. The largest of Alpine glaciers, the
Aletsch, is nearly ten miles long and has an average width of
about a mile. The thickness of some of the glaciers of the Alps is
as much as a thousand feet. Giant glaciers more than twice the
length of the longest in the Alps occur on the south slope of the
Himalaya Mountains, which receive frequent precipitations of snow
from moist winds from the Indian Ocean. The best known of the many
immense glaciers of Alaska, the Muir, has an area of about eight
hundred square miles (Fig. 95).
GLACIER MOTION. The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the
enormous pressure of their weight and because of less marginal
resistance, move faster than small ones. The Muir advances at the
rate of seven feet a day, and some of the larger tide glaciers of
Greenland are reported to move at the exceptional rate of fifty
feet and more in the same time. Glaciers move faster by day than
by night, and in summer than in winter. Other laws of glacier
motion may be discovered by a study of Figures 96 and 97. It is
important to remember that glaciers do not slide bodily over their
beds, but urged by gravity move slowly down valley in somewhat the
same way as would a stream of thick mud. Although small pieces of
ice are brittle, the large mass of granular ice which composes a
glacier acts as a viscous substance.
CREVASSES. Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions
under which the ice cracks and opens in great fissures called
crevasses. At an abrupt descent in the bed the ice is shattered
into great fragments, which unite again below the icefall.
Crevasses are opened on lines at right angles to the direction of
the tension. TRANSVERSE CREVASSES are due to a convexity in the
bed which stretches the ice lengthwise (Fig. 99). MARGINAL
CREVASSES are directed upstream and inwards; RADIAL CREVASSES are
found where the ice stream deploys from some narrow valley and
spreads upon some more open space. What is the direction of the
tension which causes each and to what is it due?
LATERAL AND MEDIAL MORAINES. The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center
are called the medial moraines. The one on either margin is a
lateral moraine, and is clearly formed of waste which has fallen
on the edge of the ice from the valley slopes. A medial moraine
cannot be formed in this way, since no rock fragments can fall so
far out from the sides. But following it up the glacial stream,
one finds that a medial moraine takes its beginning at the
junction of the glacier and some tributary and is formed by the
union of their two adjacent lateral moraines. Each branch thus
adds a medial moraine, and by counting the number of medial
moraines of a trunk stream one may learn of how many branches it
is composed.
Surface moraines appear in the lower course of the glacier as
ridges, which may reach the exceptional height of one hundred
feet. The bulk of such a ridge is ice. It has been protected from
the sun by the veneer of moraine stuff; while the glacier surface
on either side has melted down at least the distance of the height
of the ridge. In summer the lowering of the glacial surface by
melting goes on rapidly. In Swiss glaciers it has been estimated
that the average lowering of the surface by melting and
evaporation amounts to ten feet a year. As a moraine ridge grows
higher and more steep by the lowering of the surface of the
surrounding ice, the stones of its cover tend to slip down its
sides. Thus moraines broaden, until near the terminus of a glacier
they may coalesce in a wide field of stony waste.
ENGLACIAL DRIFT. This name is applied to whatever debris is
carried within the glacier. It consists of rock waste fallen on
the neve and there buried by accumulations of snow, and of that
engulfed in the glacier where crevasses have opened beneath a
surface moraine. As the surface of the glacier is lowered by
melting, more or less englacial drift is brought again to open
air, and near the terminus it may help to bury the ice from view
beneath a sheet of debris.
THE GROUND MORAINE. The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been
torn and worn from the glacier's bed and banks. While the stones
of the surface moraines remain as angular as when they lodged on
the ice, many of those of the ground moraine have been blunted on
the edges and faceted and scratched by being ground against one
another and the rocky bed.
In glaciers such as those of Greenland, whose basal layers are
well loaded with drift and whose surface layers are nearly clean,
different layers have different rates of motion, according to the
amount of drift with which they are clogged. One layer glides over
another, and the stones inset in each are ground and smoothed and
scratched. Usually the sides of glaciated pebbles are more worn
than the ends, and the scratches upon them run with the longer
axis of the stone. Why?
THE TERMINAL MORAINE. As a glacier is in constant motion, it
brings to its end all of its load except such parts of the ground
moraine as may find permanent lodgment beneath the ice. Where the
glacier front remains for some time at one place, there is formed
an accumulation of drift known as the terminal moraine. In valley
glaciers it is shaped by the ice front to a crescent whose convex
side is downstream. Some of the pebbles of the terminal moraine
are angular, and some are faceted and scored, the latter having
come by the hard road of the ground moraine. The material of the
dump is for the most part unsorted, though the water of the
melting ice may find opportunity to leave patches of stratified
sands and gravels in the midst of the unstratified mass of drift,
and the finer material is in places washed away.
GLACIER DRAINAGE. The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as VALLEY DRIFT.
The sand brought out by streams from beneath a glacier differs
from river sand in that it consists of freshly broken angular
grains. Why?
The stream derives its water chiefly from the surface melting of
the glacier. As the ice is touched by the rays of the morning sun
in summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells
cut by their whirling waters where some crevasse has begun to open
across their path. These wells lead into chambers and tunnels by
which sooner or later their waters find way to the rock floor of
the valley and there unite in a subglacial stream.
THE LOWER LIMIT OF GLACIERS. The glaciers of a region do not by
any means end at a uniform height above sea level. Each terminates
where its supply is balanced by melting. Those therefore which are
fed by the largest and deepest neves and those also which are best
protected from the sun by a northward exposure or by the depth of
their inclosing valleys flow to lower levels than those whose
supply is less and whose exposure to the sun is greater.
A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years
causes them to wither and melt back. The variation in glaciers is
now carefully observed in many parts of the world. The Muir
glacier has retreated two miles in twenty years. The glaciers of
the Swiss Alps are now for the most part melting back, although a
well-known glacier of the eastern Alps, the Vernagt, advanced five
hundred feet in the year 1900, and was then plowing up its
terminal moraine.
How soon would you expect a glacier to advance after its neve
fields have been swollen with unusually heavy snows, as compared
with the time needed for the flood of a large river to reach its
mouth after heavy rains upon its headwaters?
On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig.
108). These "glacier tables" commonly slope more or less strongly
to the south, and thus may be used to indicate roughly the points
of the compass. Can you explain their formation and the direction
of their slope? On the other hand, a small and thin stone, or a
patch of dust, lying on the ice, tends to sink a few inches into
it. Why?
In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?
Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove
this fact?
What effect would you expect the laws of glacier motion to have on
the slant of the sides of transverse crevasses?
A trunk glacier has four medial moraines. Of how many tributaries
is it composed? Illustrate by diagram.
State all the evidences which you have found that glaciers move.
If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?
PIEDMONT GLACIERS
THE MALASPINA GLACIER. Piedmont (foot of the mountain) glaciers
are, as the name implies, ice fields formed at the foot of
mountains by the confluence of valley glaciers. The Malaspina
glacier of Alaska, the typical glacier of this kind, is seventy
miles wide and stretches for thirty miles from the foot of the
Mount Saint Elias range to the shore of the Pacific Ocean. The
valley glaciers which unite and spread to form this lake of ice
lie above the snow line and their moraines are concealed beneath
neve. The central area of the Malaspina is also free from debris;
but on the outer edge large quantities of englacial drift are
exposed by surface melting and form a belt of morainic waste a few
feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand
feet in depth. The glacier here is practically stagnant, and lakes
a few hundred yards across, which could not exist were the ice in
motion and broken with crevasses, gather on their beds sorted
waste from the moraine. The streams which drain the glacier have
cut their courses in englacial and subglacial tunnels; none flow
for any distance on the surface. The largest, the Yahtse River,
issues from a high archway in the ice,--a muddy torrent one
hundred feet wide and twenty feet deep, loaded with sand and
stones which it deposits in a broad outwash plain (Fig. 110).
Where the ice has retreated from the sea there is left a hummocky
drift sheet with hollows filled with lakelets. These deposits help
to explain similar hummocky regions of drift and similar plains of
coarse, water-laid material often found in the drift-covered area
of the northeastern United States.
THE GEOLOGICAL WORK OF GLACIER ICE
The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its
channel occupies but a small portion of the valley. The glacier is
slow and big; its rate of motion may be less than a millionth of
that of running water over the same declivity, and its bulk is
proportionately large and fills the valley to great depth.
Moreover, glacier ice is a solid body plastic under slowly applied
stresses, while the water of rivers is a nimble fluid.
TRANSPORTATION. Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand;
while streams push and roll much of their load along their beds,
and their power of transporting waste depends solely upon their
velocity. The amount of the surface load of glaciers is limited
only by the amount of waste received from the mountain slopes
above them. The moving floor of ice stretched high across a valley
sweeps along as lateral moraines much of the waste which a
mountain stream would let accumulate in talus and alluvial cones.
While a valley glacier carries much of its load on top, an ice
sheet, such as that of Greenland, is free from surface debris,
except where moraines trail away from some nunatak. If at its edge
it breaks into separate glaciers which drain down mountain
valleys, these tongues of ice will carry the selvages of waste
common to valley glaciers. Both ice sheets and valley glaciers
drag on large quantities of rock waste in their ground moraines.
Stones transported by glaciers are sometimes called erratics. Such
are the bowlders of the drift of our northern states. Erratics may
be set down in an insecure position on the melting of the ice.
DEPOSIT. Little need be added here to what has already been said
of ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged
beneath the glacier as ground moraine is often an extremely dense,
stony clay, having been compacted under the pressure of the
overriding ice.
EROSION. A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.
The rock bed over which a glacier has moved is seen in places to
have been abraded, or ground away, to smooth surfaces which are
marked by long, straight, parallel scorings aligned with the line
of movement of the ice and varying in size from hair lines and
coarse scratches to exceptional furrows several feet deep. Clearly
this work has been accomplished by means of the sharp sand, the
pebbles, and the larger stones with which the base of the glacier
is inset, and which it holds in a firm grasp as running water
cannot. Hard and fine-grained rocks, such as granite and
quartzite, are often not only ground down to a smooth surface but
are also highly polished by means of fine rock flour worn from the
glacier bed.
In other places the bed of the glacier is rough and torn. The
rocks have been disrupted and their fragments have been carried
away,--a process known as PLUCKING. Moving under immense pressure
the ice shatters the rock, breaks off projections, presses into
crevices and wedges the rocks apart, dislodges the blocks into
which the rock is divided by joints and bedding planes, and
freezing fast to the fragments drags them on. In this work the
freezing and thawing of subglacial waters in any cracks and
crevices of the rock no doubt play an important part. Plucking
occurs especially where the bed rock is weak because of close
jointing. The product of plucking is bowlders, while the product
of abrasion is fine rock flour and sand.
Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?
ROCHES MOUTONNEES AND ROUNDED HILLS. The prominences left between
the hollows due to plucking are commonly ground down and rounded
on the stoss side,--the side from which the ice advances,--and
sometimes on the opposite, the lee side, as well. In this way the
bed rock often comes to have a billowy surface known as roches
moutonnees (sheep rocks). Hills overridden by an ice sheet often
have similarly rounded contours on the stoss side, while on the
lee side they may be craggy, either because of plucking or because
here they have been less worn from their initial profile.
THE DIRECTION OF GLACIER MOVEMENT. The direction of the flow of
vanished glaciers and ice sheets is recorded both in the
differences just mentioned in the profiles of overridden hills and
also in the minute details of the glacier trail.
Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities,
on the other hand, have their edges worn on the lee side and left
sharp upon the stoss.
Surfaces worn and torn in the ways which we have mentioned are
said to be glaciated. But it must not be supposed that a glacier
everywhere glaciates its bed. Although in places it acts as a rasp
or as a pick, in others, and especially where its pressure is
least, as near the terminus, it moves over its bed in the manner
of a sled. Instances are known where glaciers have advanced over
deposits of sand and gravel without disturbing them to any notable
degree. Like a river, a glacier does not everywhere erode. In
places it leaves its bed undisturbed and in places aggrades it by
deposits of the ground moraine.
CIRQUES. Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet
waters reflect the sheltering ramparts of rugged rock which tower
far above them. Such mountain hollows are termed CIRQUES. As a
powerful spring wears back a recess in the valley side where it
discharges, so the fountain head of a glacier gradually wears back
a cirque. In its slow movement the neve field broadly scours its
bed to a flat or basined floor. Meanwhile the sides of the valley
head are steepened and driven back to precipitous walls. For in
winter the crevasse of the bergschrund which surrounds the neve
field is filled with snow and the neve is frozen fast to the rocky
sides of the valley. In early summer the neve tears itself free,
dislodging and removing any loosened blocks, and the open fissure
of the bergschrund allows frost and other agencies of weathering
to attack the unprotected rock. As cirques are thus formed and
enlarged the peaks beneath which they lie are sharpened, and the
mountain crests are scalloped and cut back from either side to
knife-edged ridges.
In the western mountains of the United States many cirques, now
empty of neve and glacier ice, and known locally as "basins,"
testify to the fact that in recent times the snow line stood
beneath the levels of their floors, and thus far below its present
altitude.
GLACIER TROUGHS. The channel worn to accommodate the big and
clumsy glacier differs markedly from the river valley cut as with
a saw by the narrow and flexible stream and widened by the weather
and the wash of rains. The valley glacier may easily be from one
thousand to three thousand feet deep and from one to three miles
wide. Such a ponderous bulk of slowly moving ice does not readily
adapt itself to sharp turns and a narrow bed. By scouring and
plucking all resisting edges it develops a fitting channel with a
wide, flat floor, and steep, smooth sides, above which are seen
the weathered slopes of stream-worn mountain valleys. Since the
trunk glacier requires a deeper channel than do its branches, the
bed of a branch glacier enters the main trough at some distance
above the floor of the latter, although the surface of the two ice
streams may be accordant. Glacier troughs can be studied best
where large glaciers have recently melted completely away, as is
the case in many valleys of the mountains of the western United
States and of central and northern Europe (Fig. 114). The typical
glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little
broken by projecting spurs and lateral ravines. It is as if a V-
valley cut by a river had afterwards been gouged deeper with a
gigantic chisel, widening the floor to the width of the chisel
blade, cutting back the spurs, and smoothing and steepening the
sides. A river valley could only be as wide-floored as this after
it had long been worn down to grade.
The floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a
thousand feet in height, over which the stream that now drains the
valley tumbles in waterfalls. Reaches between the steps are often
basined. Lakelets may occupy hollows excavated in solid rock, and
other lakes may be held behind terminal moraines left as dams
across the valley at pauses in the retreat of the glacier.
FJORDS are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow
bay with steep rock walls and basined floor. Fjords are found only
in regions which have suffered glaciation, such as Norway and
Alaska.
HANGING VALLEYS. These are lateral valleys which open on their
main valley some distance above its floor. They are conspicuous
features of glacier troughs from which the ice has vanished; for
the trunk glacier in widening and deepening its channel cut its
bed below the bottoms of the lateral valleys.
Since the mouths of hanging valleys are suspended on the walls of
the glacier trough, their streams are compelled to plunge down its
steep, high sides in waterfalls. Some of the loftiest and most
beautiful waterfalls of the world leap from hanging valleys,--
among them the celebrated Staubbach of the Lauterbrunnen valley of
Switzerland, and those of the fjords of Norway and Alaska.
Hanging valleys are found also in river gorges where the smaller
tributaries have not been able to keep pace with a strong master
stream in cutting down their beds. In this case, however, they are
a mark of extreme youth; for, as the trunk stream approaches grade
and its velocity and power to erode its bed decrease, the side
streams soon cut back their falls and wear their beds at their
mouths to a common level with that of the main river. The Grand
Canyon of the Colorado must be reckoned a young valley. At its
base it narrows to scarcely more than the width of the river, and
yet its tributaries, except the very smallest, enter it at a
common level.
Why could not a wide-floored valley, such as a glacier trough,
with hanging valleys opening upon it, be produced in the normal
development of a river valley?
THE TROUGHS OF YOUNG AND OF MATURE GLACIERS. The features of a
glacier trough depend much on the length of time the preexisting
valley was occupied with ice. During the infancy of a glacier, we
may believe, the spurs of the valley which it fills are but little
blunted and its bed is but little broken by steps. In youth the
glacier develops icefalls, as a river in youth develops
waterfalls, and its bed becomes terraced with great stairs. The
mature glacier, like the mature river, has effaced its falls and
smoothed its bed to grade. It has also worn back the projecting
spurs of its valley, making itself a wide channel with smooth
sides. The bed of a mature glacier may form a long basin, since it
abrades most in its upper and middle course, where its weight and
motion are the greatest. Near the terminus, where weight and
motion are the least, it erodes least, and may instead deposit a
sheet of ground moraine, much as a river builds a flood plain in
the same part of its course as it approaches maturity. The bed of
a mature glacier thus tends to take the form of a long, relatively
narrow basin, across whose lower end may be stretched the dam of
the terminal moraine. On the disappearance of the ice the basin is
rilled with a long, narrow lake, such as Lake Chelan in Washington
and many of the lakes in the Highlands of Scotland.
Piedmont glaciers apparently erode but little. Beneath their lake-
like expanse of sluggish or stagnant ice a broad sheet of ground
moraine is probably being deposited.
Cirques and glaciated valleys rapidly lose their characteristic
forms after the ice has withdrawn. The weather destroys all
smoothed, polished, and scored surfaces which are not protected
beneath glacial deposits. The oversteepened sides of the trough
are graded by landslips, by talus slopes, and by alluvial cones.
Morainic heaps of drift are dissected and carried away. Hanging
valleys and the irregular bed of the trough are both worn down to
grade by the streams which now occupy them. The length of time
since the retreat of the ice from a mountain valley may thus be
estimated by the degree to which the destruction of the
characteristic features of the glacier trough has been carried.
In Figure 104 what characteristics of a glacier trough do you
notice? What inference do you draw as to the former thickness of
the glacier?
Name all the evidences you would expect to find to prove the fact
that in the recent geological past the valleys of the Alps
contained far larger glaciers than at present, and that on the
north of the Alps the ice streams united in a piedmont glacier
which extended across the plains of Switzerland to the sides of
the Jura Mountains.
THE RELATIVE IMPORTANCE OF GLACIERS AND OF RIVERS. Powerful as
glaciers are, and marked as are the land forms which they produce,
it is easy to exaggerate their geological importance as compared
with rivers. Under present climatic conditions they are confined
to lofty mountains or polar lands. Polar ice sheets are permanent
only so long as the lands remain on which they rest. Mountain
glaciers can stay only the brief time during which the ranges
continue young and high. As lofty mountains, such as the Selkirks
and the Alps, are lowered by frost and glacier ice, the snowfall
will decrease, the line of permanent snow will rise, and as the
mountain hollows in which snow may gather are worn beneath the
snow line, the glaciers must disappear. Under present climatic
conditions the work of glaciers is therefore both local and of
short duration.
Even the glacial epoch, during which vast ice sheets deposited
drift over northeastern North America, must have been brief as
well as recent, for many lofty mountains, such as the Rockies and
the Alps, still bear the marks of great glaciers which then filled
their valleys. Had the glacial epoch been long, as the earth
counts time, these mountains would have been worn low by ice; had
the epoch been remote, the marks of glaciation would already have
been largely destroyed by other agencies.
On the other hand, rivers are well-nigh universally at work over
the land surfaces of the globe, and ever since the dry land
appeared they have been constantly engaged in leveling the
continents and in delivering to the seas the waste which there is
built into the stratified rocks.
ICEBERGS. Tide glaciers, such as those of Greenland and Alaska,
are able to excavate their beds to a considerable distance below
sea level. From their fronts the buoyancy of sea water raises and
breaks away great masses of ice which float out to sea as
icebergs. Only about one seventh of a mass of glacier ice floats
above the surface, and a berg three hundred feet high may be
estimated to have been detached from a glacier not less than two
thousand feet thick where it met the sea.
Icebergs transport on their long journeys whatever drift they may
have carried when part of the glacier, and scatter it, as they
melt, over the ocean floor. In this way pebbles torn by the inland
ice from the rocks of the interior of Greenland and glaciated
during their carriage in the ground moraine are dropped at last
among the oozes of the bottom of the North Atlantic.
CHAPTER VI
THE WORK OF THE WIND
We are now to study the geological work of the currents of the
atmosphere, and to learn how they erode, and transport and deposit
waste as they sweep over the land. Illustrations of the wind's
work are at hand in dry weather on any windy day.
Clouds of dust are raised from the street and driven along by the
gale. Here the roadway is swept bare; and there, in sheltered
places, the dust settles in little windrows. The erosive power of
waste-laden currents of air is suggested as the sharp grains of
flying sand sting one's face or clatter against the window. In the
country one sometimes sees the dust whirled in clouds from dry,
plowed fields in spring and left in the lee of fences in small
drifts resembling in form those of snow in winter.
THE ESSENTIAL CONDITIONS for the wind's conspicuous work are
illustrated in these simple examples; they are aridity and the
absence of vegetation. In humid climates these conditions are only
rarely and locally met; for the most part a thick growth of
vegetation protects the moist soil from the wind with a cover of
leaves and stems and a mattress of interlacing roots. But in arid
regions either vegetation is wholly lacking, or scant growths are
found huddled in detached clumps, leaving interspaces of
unprotected ground (Fig. 119). Here, too, the mantle of waste,
which is formed chiefly under the action of temperature changes,
remains dry and loose for long periods. Little or no moisture is
present to cause its particles to cohere, and they are therefore
readily lifted and drifted by the wind.
TRANSPORTATION BY THE WIND
In the desert the finer waste is continually swept to and fro by
the ever-shifting wind. Even in quiet weather the air heated by
contact with the hot sands rises in whirls, and the dust is lifted
in stately columns, sometimes as much as one thousand feet in
height, which march slowly across the plain. In storms the sand is
driven along the ground in a continuous sheet, while the air is
tilled with dust. Explorers tell of sand storms in the deserts of
central Asia and Africa, in which the air grows murky and
suffocating. Even at midday it may become dark as night, and
nothing can be heard except the roar of the blast and the whir of
myriads of grains of sand as they fly past the ear.
Sand storms are by no means uncommon in the arid regions of the
western United States. In a recent year, six were reported from
Yuma, Arizona. Trains on transcontinental railways are
occasionally blockaded by drifting sand, and the dust sifts into
closed passenger coaches, covering the seats and floors. After
such a storm thirteen car loads of sand were removed from the
platform of a station on a western railway.
DUST FALLS. Dust launched by upward-whirling winds on the swift
currents of the upper air is often blown for hundreds of miles
beyond the arid region from which it was taken. Dust falls from
western storms are not unknown even as far east as the Great
Lakes. In 1896 a "black snow" fell in Chicago, and in another dust
storm in the same decade the amount of dust carried in the air
over Rock Island, Ill., was estimated at more than one thousand
tons to the cubic mile.
In March, 1901, a cyclonic storm carried vast quantities of dust
from the Sahara northward across the Mediterranean to fall over
southern and central Europe. On March 8th dust storms raged in
southern Algeria; two days later the dust fell in Italy; and on
the 11th it had reached central Germany and Denmark. It is
estimated that in these few days one million eight hundred
thousand tons of waste were carried from northern Africa and
deposited on European soil.
We may see from these examples the importance of the wind as an
agent of transportation, and how vast in the aggregate are the
loads which it carries. There are striking differences between air
and water as carriers of waste. Rivers flow in fixed and narrow
channels to definite goals. The channelless streams of the air
sweep across broad areas, and, shifting about continually, carry
their loads back and forth, now in one direction and now in
another.
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