The Elements of Geology
by
William Harmon Norton
Part 3 out of 7
WIND DEPOSITS
The mantle of waste of deserts is rapidly sorted by the wind. The
coarser rubbish, too heavy to be lifted into the air, is left to
strew wide tracts with residual gravels (Fig. 120). The sand
derived from the disintegration of desert rocks gathers in vast
fields. About one eighth of the surface of the Sahara is said to
be thus covered with drifting sand. In desert mountains, as those
of Sinai, it lies like fields of snow in the high valleys below
the sharp peaks. On more level tracts it accumulates in seas of
sand, sometimes, as in the deserts of Arabia, two hundred and more
feet deep.
DUNES. The sand thus accumulated by the wind is heaped in wavelike
hills called dunes. In the desert of northwestern India, where the
prevalent wind is of great strength, the sand is laid in
longitudinal dunes, i.e. in stripes running parallel with the
direction of the wind; but commonly dunes lie, like ripple marks,
transverse to the wind current. On the windward side they show a
long, gentle slope, up which grains of sand can readily be moved;
while to the lee their slope is frequently as great as the angle
of repose (Fig. 122). Dunes whose sands are not fixed by
vegetation travel slowly with the wind; for their material is ever
shifted forward as the grains are driven up the windward slope
and, falling over the crest, are deposited in slanting layers in
the quiet of the lee.
Like river deposits, wind-blown sands are stratified, since they
are laid by currents of air varying in intensity, and therefore
in transporting power, which carry now finer and now coarser
materials and lay them down where their velocity is checked (Fig.
123). Since the wind varies in direction, the strata dip in
various directions. They also dip at various angles, according to
the inclination of the surface on which they were laid.
Dunes occur not only in arid regions, but also wherever loose sand
lies unprotected by vegetation from the wind. From the beaches of
sea and lake shores the wind drives inland the surface sand left
dry between tides and after storms, piling it in dunes which may
invade forests and fields and bury villages beneath their slowly
advancing waves. On flood plains during summer droughts river
deposits are often worked over by the wind; the sand is heaped in
hummocks and much of the fine silt is caught and held by the
forests and grassy fields of the bordering hills.
The sand of shore dunes differs little in composition and the
shape of its grains from that of the beach from which it was
derived. But in deserts, by the long wear of grain on grain as
they are blown hither and thither by the wind, all soft minerals
are ground to powder and the sand comes to consist almost wholly
of smooth round grams of hard quartz.
Some marine sandstones, such as the St. Peter sandstone of the
upper Mississippi valley, are composed so entirely of polished
spherules of quartz that it has been believed by some that their
grains were long blown about in ancient deserts before they were
deposited in the sea.
DUST DEPOSITS. As desert sands are composed almost wholly of
quartz, we may ask what has become of the softer minerals of which
the rocks whose disintegration has supplied the sand were in part,
and often in large part, composed. The softer minerals have been
ground to powder, and little by little the quartz sand also is
worn by attrition to fine dust. Yet dust deposits are scant and
few in great deserts such as the Sahara. The finer waste is blown
beyond its limits and laid in adjacent oceans, where it adds to
the muds and oozes of their floors, and on bordering steppes and
forest lands, where it is bound fast by vegetation and slowly
accumulates in deposits of unstratified loose yellow earth. The
fine waste of the Sahara has been identified in dredgings from the
bottom of the Atlantic Ocean, taken hundreds of miles from the
coast of Africa.
LOESS. In northern China an area as large as France is deeply
covered with a yellow pulverulent earth called loess (German,
loose), which many consider a dust deposit blown from the great
Mongolian desert lying to the west. Loess mantles the recently
uplifted mountains to the height of eight thousand feet and
descends on the plains nearly to sea level. Its texture and lack
of stratification give it a vertical cleavage; hence it stands in
steep cliffs on the sides of the deep and narrow trenches which
have been cut in it by streams.
On loess hillsides in China are thousands of villages whose
eavelike dwellings have been excavated in this soft, yet firm, dry
loam. While dust falls are common at the present time in this
region, the loess is now being rapidly denuded by streams, and its
yellow silt gives name to the muddy Hwang-ho (Yellow River), and
to the Yellow Sea, whose waters it discolors for scores of miles
from shore.
Wind deposits both of dust and of sand may be expected to contain
the remains of land shells, bits of wood, and bones of land
animals, testifying to the fact that they were accumulated in open
air and not in the sea or in bodies of fresh water.
WIND EROSION
Sand-laden currents of air abrade and smooth and polish exposed
rock surfaces, acting in much the same way as does the jet of
steam fed with sharp sand, which is used in the manufacture of
ground glass. Indeed, in a single storm at Cape Cod a plate glass
of a lighthouse was so ground by flying sand that its transparency
was destroyed and its removal made necessary.
Telegraph poles and wires whetted by wind-blown sands are
destroyed within a few years. In rocks of unequal resistance the
harder parts are left in relief, while the softer are etched away.
Thus in the pass of San Bernardino, Cal., through which strong
winds stream from the west, crystals of garnet are left projecting
on delicate rock fingers from the softer rock in which they were
imbedded.
Wind-carved pebbles are characteristically planed, the facets
meeting along a summit ridge or at a point like that of a pyramid.
We may suppose that these facets were ground by prevalent winds
from certain directions, or that from time to time the stone was
undermined and rolled over as the sand beneath it was blown away
on the windward side, thus exposing fresh surfaces to the driving
sand. Such wind-carved pebbles are sometimes found in ancient
rocks and may be accepted as evidence that the sands of which the
rocks are composed were blown about by the wind.
DEFLATION. In the denudation of an arid region, wind erosion is
comparatively ineffective as compared with deflation (Latin, de,
from; flare, to blow),--a term by which is meant the constant
removal of waste by the wind, leaving the rocks bare to the
continuous attack of the weather. In moist climates denudation is
continually impeded by the mantle of waste and its cover of
vegetation, and the land surface can be lowered no faster than the
waste is removed by running water. Deep residual soils come to
protect all regions of moderate slope, concealing from view the
rock structure, and the various forms of the land are due more to
the agencies of erosion and transportation than to differences in
the resistance of the underlying rocks.
But in arid regions the mantle is rapidly removed, even from well-
nigh level plains and plateaus, by the sweep of the wind and the
wash of occasional rains. The geological structure of these
regions of naked rock can be read as far as the eye can see, and
it is to this structure that the forms of the land are there
largely due. In a land mass of horizontal strata, for example, any
softer surface rocks wear down to some underlying, resistant
stratum, and this for a while forms the surface of a level plateau
(Fig. 129). The edges of the capping layer, together with those of
any softer layers beneath it, wear back in steep cliffs, dissected
by the valleys of wet-weather streams and often swept bare to the
base by the wind. As they are little protected by talus, which
commonly is removed about as fast as formed, these escarpments and
the walls of the valleys retreat indefinitely, exposing some hard
stratum beneath which forms the floor of a widening terrace.
The high plateaus of northern Arizona and southern Utah, north of
the Grand Canyon of the Colorado River, are composed of stratified
rocks more than ten thousand feet thick and of very gentle
inclination northward. From the broad plat form in which the
canyon has been cut rises a series of gigantic stairs, which are
often more than one thousand feet high and a score or more of
miles in breadth. The retreating escarpments, the cliffs of the
mesas and buttes which they have left behind as outliers, and the
walls of the ravines are carved into noble architectural forms--
into cathedrals, pyramids, amphitheaters, towers, arches, and
colonnades--by the processes of weathering aided by deflation. It
is thus by the help of the action of the wind that great plateaus
in arid regions are dissected and at last are smoothed away to
waterless plains, either composed of naked rock, or strewed with
residual gravels, or covered with drifting residual sand.
The specific gravity of air is 1/823 that of water. How does this
fact affect the weight of the material which each can carry at the
same velocity?
If the rainfall should lessen in your own state to from five to
ten inches a year, what changes would take place in the vegetation
of the country? in the soil? in the streams? in the erosion of
valleys? in the agencies chiefly at work in denuding the land?
In what way can a wind-carved pebble be distinguished from a
river-worn pebble? from a glaciated pebble?
CHAPTER VII
THE SEA AND ITS SHORES
We have already seen that the ocean is the goal at which the waste
of the land arrives. The mantle of rock waste, creeping down
slopes, is washed to the sea by streams, together with the
material which the streams have worn from their beds and that
dissolved by underground waters. In arid regions the winds sweep
waste either into bordering oceans or into more humid regions
where rivers take it up and carry it on to the sea. Glaciers
deliver the load of their moraines either directly to the sea or
leave it for streams to transport to the same goal. All deposits
made on the land, such as the flood plains of rivers, the silts of
lake beds, dune sands, and sheets of glacial drift, mark but
pauses in the process which is to bring all the materials of the
land now above sea level to rest upon the ocean bed.
But the sea is also at work along all its shores as an agent of
destruction, and we must first take up its work in erosion before
we consider how it transports and deposits the waste of the land.
SEA EROSION
THE SEA CLIFF AND THE ROCK BENCH. On many coasts the land fronts
the ocean in a line of cliffs. To the edge of the cliffs there
lead down valleys and ridges, carved by running water, which, if
extended, would meet the water surface some way out from shore.
Evidently they are now abruptly cut short at the present shore
line because the land has been cut back.
Along the foot of the cliff lies a gently shelving bench of rock,
more or less thickly veneered with sand and shingle. At low tide
its inner margin is laid bare, but at high tide it is covered
wholly, and the sea washes the base of the cliffs. A notch, of
which the SEA CLIFF and the ROCK BENCH are the two sides, has been
cut along the shore.
WAVES. The position of the rock bench, with its inner margin
slightly above low tide, shows that it has been cut by some agent
which acts like a horizontal saw set at about sea level. This
agent is clearly the surface agitation of the water; it is the
wind-raised wave.
As a wave comes up the shelving bench the crest topples forward
and the wave "breaks," striking a blow whose force is measured by
the momentum of all its tons of falling water. On the coast of
Scotland the force of the blows struck by the waves of the
heaviest storms has sometimes exceeded three tons to the square
foot. But even a calm sea constantly chafes the shore. It heaves
in gentle undulations known as the ground swell, the result of
storms perhaps a thousand miles distant, and breaks on the shore
in surf.
The blows of the waves are not struck with clear water only, else
they would have little effect on cliffs of solid rock. Storm waves
arm themselves with the sand and gravel, the cobbles, and even the
large bowlders which lie at the base of the cliff, and beat
against it with these hammers of stone.
Where a precipice descends sheer into deep water, waves swash up
and down the face of the rocks but cannot break and strike
effective blows. They therefore erode but little until the talus
fallen from the cliff is gradually built up beneath the sea to the
level at which the waves drag bottom upon it and break.
Compare the ways in which different agents abrade. The wind
lightly brushes sand and dust over exposed surfaces of rock.
Running water sweeps fragments of various sizes along its
channels, holding them with a loose hand. Glacial ice grinds the
stones of its ground moraine against the underlying rock with the
pressure of its enormous weight. The wave hurls fragments of rock
against the sea cliff, bruising and battering it by the blow. It
also rasps the bench as it drags sand and gravel to and fro upon
it.
WEATHERING OF SEA CLIFFS. The sea cliff furnishes the weapons for
its own destruction. They are broken from it not only by the wave
but also by the weather. Indeed the sea cliff weathers more
rapidly, as a rule, than do rock ledges inland. It is abundantly
wet with spray. Along its base the ground water of the neighboring
land finds its natural outlet in springs which under mine it.
Moreover, it is unprotected by any shield of talus. Fragments of
rock as they fall from its face are battered to pieces by the
waves and swept out to sea. The cliff is thus left exposed to the
attack of the weather, and its retreat would be comparatively
rapid for this reason alone.
Sea cliffs seldom overhang, but commonly, as in Figure 134, slope
seaward, showing that the upper portion has retreated at a more
rapid rate than has the base. Which do you infer is on the whole
the more destructive agent, weathering or the wave?
Draw a section of a sea cliff cut in well jointed rocks whose
joints dip toward the land. Draw a diagram of a sea cliff where
the joints dip toward the sea.
SEA CAVES. The wave does not merely batter the face of the cliff.
Like a skillful quarryman it inserts wedges in all natural
fissures, such as joints, and uses explosive forces. As a wave
flaps against a crevice it compresses the air within with the
sudden stroke; as it falls back the air as suddenly expands. On
lighthouses heavily barred doors have been burst outward by the
explosive force of the air within, as it was released from
pressure when a partial vacuum was formed by the refluence of the
wave. Where a crevice is filled with water the entire force of the
blow of the wave is transmitted by hydraulic pressure to the sides
of the fissure. Thus storm waves little by little pry and suck the
rock loose, and in this way, and by the blows which they strike
with the stones of the beach, they quarry out about a joint, or
wherever the rock may be weak, a recess known as a SEA CAVE,
provided that the rock above is coherent enough to form a roof.
Otherwise an open chasm results.
BLOWHOLES AND SEA ARCHES. As a sea cave is drilled back into the
rock, it may encounter a joint or crevice opened to the surface by
percolating water. The shock of the waves soon enlarges this to a
blowhole, which one may find on the breezy upland, perhaps a
hundred yards and more back from the cliff's edge. In quiet
weather the blowhole is a deep well; in storm it plays a fountain
as the waves drive through the long tunnel below and spout their
spray high in air in successive jets. As the roof of the cave thus
breaks down in the rear, there may remain in front for a while a
sea arch, similar to the natural bridges of land caverns.
STACKS AND WAVE-CUT ISLANDS. As the sea drives its tunnels and
open drifts into the cliff, it breaks through behind the
intervening portions and leaves them isolated as stacks, much as
monuments are detached from inland escarpments by the weather; and
as the sea cliff retreats, these remnant masses may be left behind
as rocky islets. Thus the rock bench is often set with stacks,
islets in all stages of destruction, and sunken reefs, all wrecks
of the land testifying to its retreat before the incessant attack
of the waves.
COVES. Where zones of soft or closely jointed rock outcrop along a
shore, or where minor water courses conic down to the sea and aid
in erosion, the shore is worn back in curved reentrants called
coves; while the more resistant rocks on either hand are left
projecting as headlands (Fig. 139). After coves are cut back a
short distance by the waves, the headlands come to protect them,
as with breakwaters, and prevent their indefinite retreat. The
shore takes a curve of equilibrium, along which the hard rock of
the exposed headland and the weak rock of the protected cove wear
back at an equal rate.
RATE OF RECESSION. The rate at which a shore recedes depends on
several factors. In soft or incoherent rocks exposed to violent
storms the retreat is so rapid as to be easily measured. The coast
of Yorkshire, England, whose cliffs are cut in glacial drift,
loses seven feet a year on the average, and since the Norman
conquest a strip a mile wide, with farmsteads and villages and
historic seaports, has been devoured by the sea. The sandy south
shore of Martha's Vineyard wears back three feet a year. But hard
rocks retreat so slowly that their recession has seldom been
measured by the records of history.
SHORE DRIFT
BOWLDER AND PEBBLE BEACHES. About as fast as formed the waste of
the sea cliff is swept both along the shore and out to sea. The
road of waste along shore is the BEACH. We may also define the
beach as the exposed edge of the sheet of sediment formed by the
carriage of land waste out to sea. At the foot of sea cliffs,
where the waves are pounding hardest, one commonly finds the rock
bench strewn on its inner margin with large stones, dislodged by
the waves and by the weather and some-what worn on their corners
and edges. From this BOWLDER BEACH the smaller fragments of waste
from the cliff and the fragments into which the bowlders are at
last broken drift on to more sheltered places and there accumulate
in a PEBBLE BEACH, made of pebbles well rounded by the wear which
they have suffered. Such beaches form a mill whose raw material is
constantly supplied by the cliff. The breakers of storms set it in
motion to a depth of several feet, grinding the pebbles together
with a clatter to be heard above the roar of the surf. In such a
rock crusher the life of a pebble is short. Where ships have
stranded on our Atlantic coast with cargoes of hard-burned brick
or of coal, a year of time and a drift of five miles along the
shore have proved enough to wear brick and coal to powder. At no
great distance from their source, therefore, pebble beaches give
place to beaches of sand, which occupy the more sheltered reaches
of the shore.
SAND BEACHES. The angular sand grains of various minerals into
which pebbles are broken by the waves are ground together under
the beating surf and rounded, and those of the softer minerals are
crushed to powder. The process, however, is a slow one, and if we
study these sand grains under a lens we may be surprised to see
that, though their corners and edges have been blunted, they are
yet far from the spherical form of the pebbles from which they
were derived. The grains are small, and in water they have lost
about half their weiglit in air; the blows which they strike one
another are therefore weak. Besides, each grain of sand of the wet
beach is protected by a cushion of water from the blows of its
neighbors.
The shape and size of these grains and the relative proportion of
grains of the softer minerals which still remain give a rough
measure of the distance in space and time which they have traveled
from their source. The sand of many beaches, derived from the
rocks of adjacent cliffs or brought in by torrential streams from
neighboring highlands, is dark with grains of a number of minerals
softer than quartz. The white sand of other beaches, as those of
the east coast of Florida, is almost wholly composed of quartz
grains; for in its long travel down the Atlantic coast the weaker
minerals have been worn to powder and the hardest alone survive.
How does the absence of cleavage in quartz affect the durability
of quartz sand?
HOW SHORE DRIFT MIGRATES. It is under the action of waves and
currents that shore drift migrates slowly along a coast. Where
waves strike a coast obliquely they drive the waste before them
little by little along the shore. Thus on a north-south coast,
where the predominant storms are from the northeast, there will be
a migration of shore drift southwards.
All shores are swept also by currents produced by winds and tides.
These are usually far too gentle to transport of themselves the
coarse materials of which beaches are made. But while the wave
stirs the grains of sand and gravel, and for a moment lifts them
from the bottom, the current carries them a step forward on their
way. The current cannot lift and the wave cannot carry, but
together the two transport the waste along the shore. The road of
shore drift is therefore the zone of the breaking waves.
THE BAY-HEAD BEACH. As the waste derived from the wear of waves
and that brought in by streams is trailed along a coast it
assumes, under varying conditions, a number of distinct forms.
When swept into the head of a sheltered bay it constitutes the
bay-head beach. By the highest storm waves the beach is often
built higher than the ground immediately behind it, and forms a
dam inclosing a shallow pond or marsh.
THE BAY BAR. As the stream of shore drift reaches the mouth of a
bay of some size it often occurs that, instead of turning in, it
sets directly across toward the opposite headland. The waste is
carried out from shore into the deeper waters of the bay mouth;
where it is no longer supported by the breaking waves, and sinks
to the bottom. The dump is gradually built to the surface as a
stubby spur, pointing across the bay, and as it reaches the zone
of wave action current and wave can now combine to carry shore
drift along it, depositing their load continually at the point of
the spur. An embankment is thus constructed in much the same
manner as a railway fill, which, while it is building, serves as a
roadway along which the dirt from an adjacent cut is carted to be
dumped at the end. When the embankment is completed it bridges the
bay with a highway along which shore drift now moves without
interruption, and becomes a bay bar.
INCOMPLETE BAY BARS. Under certain conditions the sea cannot carry
out its intention to bridge a bay. Rivers discharging in bays
demand open way to the ocean. Strong tidal currents also are able
to keep open channels scoured by their ebb and flow. In such cases
the most that land waste can do is to build spits and shoals,
narrowing and shoaling the channel as much as possible. Incomplete
bay bars sometimes have their points recurved by currents setting
at right angles to the stream of shore drift and are then
classified as HOOKS (Fig. 142).
SAND REEFS. On low coasts where shallow water extends some
distance out, the highway of shore drift lies along a low, narrow
ridge, termed the sand reef, separated from the land by a narrow
stretch of shallow water called the LAGOON. At intervals the reef
is held open by INLETS,--gaps through which the tide flows and
ebbs, and by which the water of streams finds way to the sea.
No finer example of this kind of shore line is to be found in the
world than the coast of Texas. From near the mouth of the Rio
Grande a continuous sand reef draws its even curve for a hundred
miles to Corpus Christi Pass, and the reefs are but seldom
interrupted by inlets as far north as Galveston Harbor. On this
coast the tides are variable and exceptionally weak, being less
than one foot in height, while the amount of waste swept along the
shore is large. The lagoon is extremely shallow, and much of it is
a mud flat too shoal for even small boats. On the coast of New
Jersey strong tides are able to keep open inlets at intervals of
from two to twenty miles in spite of a heavy alongshore drift.
Sand reefs are formed where the water is so shallow near shore
that storm waves cannot run in it and therefore break some
distance out from land. Where storm waves first drag bottom they
erode and deepen the sea floor, and sweep in sediment as far as
the line where they break. Here, where they lose their force, they
drop their load and beat up the ridge which is known as the sand
reef when it reaches the surface.
SHORES OF ELEVATION AND DEPRESSION
Our studies have already brought to our notice two distinct forms
of strand lines,--one the high, rocky coast cut back to cliffs by
the attack of the waves, and the other the low, sandy coast where
the waves break usually upon the sand reef. To understand the
origin of these two types we must know that the meeting place of
sea and land is determined primarily by movements of the earth's
crust. Where a coast land emerges the--shore line moves seaward;
where it is being submerged the shore line advances on the land.
SHORES OF ELEVATION. The retreat of the sea, either because of a
local uplift of the land or for any other reason, such as the
lowering of any portion of ocean bottom, lays bare the inner
margin of the sea floor. Where the sea floor has long received the
waste of the land it has been built up to a smooth, subaqueous
plain, gently shelving from the land. Since the new shore line is
drawn across this even surface it is simple and regular, and is
bordered on the one side by shallow water gradually deepening
seaward, and on the other by low land composed of material which
has not yet thoroughly consolidated to firm rock. A sand reef is
soon beaten up by the waves, and for some time conditions will
favor its growth. The loss of sand driven into the lagoon beyond,
and of that ground to powder by the surf and carried out to sea,
is more than made up by the stream of alongshore drift, and
especially by the drag of sediments to the reef by the waves as
they deepen the sea floor on its seaward side.
Meanwhile the lagoon gradually fills with waste from the reef and
from the land. It is invaded by various grasses and reeds which
have learned to grow in salt and brackish water; the marsh, laid
bare only at low tide, is built above high tide by wind drift and
vegetable deposits, and becomes a meadow, soldering the sand reef
to the mainland.
While the lagoon has been filling, the waves have been so
deepening the sea floor off the sand reef that at last they are
able to attack it vigorously. They now wear it back, and, driving
the shore line across the lagoon or meadow, cut a line of low
cliffs on the mainland. Such a shore is that of Gascony in
southwestern France,--a low, straight, sandy shore, bordered by
dunes and unprotected by reefs from the attack of the waves of the
Bay of Biscay.
We may say, then, that on shores of elevation the presence of sand
reefs and lagoons indicates the stage of youth, while the absence
of these features and the vigorous and unimpeded attack by the sea
upon the mainland indicate the stage of maturity. Where much waste
is brought in by rivers the maturity of such a coast may be long
delayed. The waste from the land keeps the sea shallow offshore
and constantly renews the sand reef. The energy of the waves is
consumed in handling shore drift, and no energy is left for an
effective attack upon the land. Indeed, with an excessive amount
of waste brought down by streams the land may be built out and
encroach temporarily upon the sea; and not until long denudation
has lowered the land, and thus decreased the amount of waste from
it, may the waves be able to cut through the sand reef and thus
the coast reach maturity.
SHORES OF DEPRESSION
Where a coastal region is undergoing submergence the shore line
moves landward. The horizontal plane of the sea now intersects an
old land surface roughened by subaerial denudation. The shore line
is irregular and indented in proportion to the relief of the land
and the amount of the submergence which the land has suffered. It
follows up partially submerged valleys, forming bays, and bends
round the divides, leaving them to project as promontories and
peninsulas. The outlines of shores of depression are as varied as
are the forms of the land partially submerged. We give a few
typical illustrations.
The characteristics of the coast of Maine are due chiefly to the
fact that a mountainous region of hard rocks, once worn to a
peneplain, and after a subsequent elevation deeply dissected by
north-south valleys, has subsided, the depression amounting on its
southern margin to as much as six hundred feet below sea level.
Drowned valleys penetrate the land in long, narrow bays, and
rugged divides project in long, narrow land arms prolonged seaward
by islands representing the high portions of their extremities. Of
this exceedingly ragged shore there are said to be two thousand
miles from the New Brunswick boundary as far west as Portland,--a
straight-line distance of but two hundred miles. Since the time of
its greatest depression the land is known to have risen some three
hundred feet; for the bays have been shortened, and the waste with
which their floors were strewn is now in part laid bare as clay
plains about the bay heads and in narrow selvages about the
peninsulas and islands.
The coast of Dalmatia, on the Adriatic Sea, is characterized by
long land arms and chains of long and narrow islands, all parallel
to the trend of the coast. A region of parallel mountain ranges
has been depressed, and the longitudinal valleys which lie between
them are occupied by arms of the sea.
Chesapeake Bay is a branching bay due to the depression of an
ancient coastal plain which, after having emerged from the sea,
was channeled with broad, shallow valleys. The sea has invaded the
valley of the trunk stream and those of its tributaries, forming a
shallow bay whose many branches are all directed toward its axis
(Fig. 146).
Hudson Bay, and the North, the Baltic, and the Yellow seas are
examples where the sinking of the land has brought the sea in over
low plains of large extent, thus deeply indenting the continental
out-line. The rise of a few hundred feet would restore these
submerged plains to the land.
THE CYCLE OF SHORES OF DEPRESSION. In its infantile stage the
outline of a shore of depression depends almost wholly on the
previous relief of the land, and but little on erosion by the sea.
Sea cliffs and narrow benches appear where headlands and outlying
islands have been nipped by the waves. As yet, little shore waste
has been formed. The coast of Maine is an example of this stage.
In early youth all promontories have been strongly cliffed, and
under a vigorous attack of the sea the shore of open bays may be
cut back also. Sea stacks and rocky islets, caves and coves, make
the shore minutely ragged. The irregularity of the coast, due to
depression, is for a while increased by differential wave wear on
harder and softer rocks. The rock bench is still narrow. Shore
waste, though being produced in large amounts, is for the most
part swept into deeper water and buried out of sight. Examples of
this stage are the east coast of Scotland and the California coast
near San Francisco.
Later youth is characterized by a large accumulation of shore
waste. The rock bench has been cut back so that it now furnishes a
good roadway for shore drift. The stream of alongshore drift grows
larger and larger, filling the heads of the smaller bays with
beaches, building spits and hooks, and tying islands with sand
bars to the mainland. It bridges the larger bays with bay bars,
while their length is being reduced as their inclosing
promontories are cut back by the waves. Thus there comes to be a
straight, continuous, and easy road, no longer interrupted by
headlands and bays, for the transportation of waste alongshore.
The Baltic coast of Germany is in this stage.
All this while streams have been busy filling with delta deposits
the bays into which they empty. By these steps a coast gradually
advances to MATURITY, the stage when the irregularities due to
depression have been effaced, when outlying islands formed by
subsidence have been planed away, and when the shore line has been
driven back behind the former bay heads. The sea now attacks the
land most effectively along a continuous and fairly straight line
of cliffs. Although the first effect of wave wear was to increase
the irregularities of the shore, it sooner or later rectifies it,
making it simple and smooth. Northwestern France may be cited as
an upland plain, dissected and depressed, whose coast has reached
maturity.
In the OLD AGE of coasts the rock bench is cut back so far that
the waves can no longer exert their full effect upon the shore.
Their energy is dissipated in moving shore drift hither and
thither and in abrading the bench when they drag bottom upon it.
Little by little the bench is deepened by tidal currents and the
drag of waves; but this process is so slow that meanwhile the sea
cliffs melt down under the weather, and the bench becomes a broad
shoal where waves and tides gradually work over the waste from the
land to greater fineness and sweep it out to sea.
PLAINS OF MARINE ABRASION. While subaerial denudation reduces the
land to baselevel, the sea is sawing its edges to WAVE BASE, i.e.
the lowest limit of the wave's effective wear. The widened rock
bench forms when uplifted a plain of marine abrasion, which like
the peneplain bevels across strata regardless of their various
inclinations and various degrees of hardness.
How may a plain of marine abrasion be expected to differ from a
peneplain in its mantle of waste?
Compared with subaerial denudation, marine abrasion is a
comparatively feeble agent. At the rate of five feet per century--
a higher rate than obtains on the youthful rocky, coast of
Britain--it would require more than ten million years to pare a
strip one hundred miles wide from the margin of a continent, a
time sufficient, at the rate at which the Mississippi valley is
now being worn away, for subaerial denudation to lower the lands
of the globe to the level of the sea.
Slow submergence favors the cutting of a wide rock bench. The
water continually deepens upon the bench; storm waves can
therefore always ride in to the base of the cliffs and attack them
with full force; shore waste cannot impede the onset of the waves,
for it is continually washed out in deeper water below wave base.
BASAL CONGOLMERATES. As the sea marches across the land during a
slow submergence, the platform is covered with sheets of sea-laid
sediments. Lowest of these is a conglomerate,--the bowlder and
pebble beach, widened indefinitely by the retreat of the cliffs at
whose base it was formed, and preserved by the finer deposits laid
upon it in the constantly deepening water as the land subsides.
Such basal conglomerates are not uncommon among the ancient rocks
of the land, and we may know them by their rounded pebbles and
larger stones, composed of the same kind of rock as that of the
abraded and evened surface on which they lie.
CHAPTER VIII
OFFSHORE AND DEEP-SEA DEPOSITS
The alongshore deposits which we have now studied are the exposed
edge of a vast subaqueous sheet of waste which borders the
continents and extends often for as much as two or three hundred
miles from land. Soundings show that offshore deposits are laid in
belts parallel to the coast, the coarsest materials lying nearest
to the land and the finest farthest out. The pebbles and gravel
and the clean, coarse sand of beaches give place to broad
stretches of sand, which grows finer and finer until it is
succeeded by sheets of mud. Clearly there is an offshore movement
of waste by which it is sorted, the coarser being sooner dropped
and the finer being carried farther out.
OFFSHORE DEPOSITS
The debris torn by waves from rocky shores is far less in amount
than the waste of the land brought down to the sea by rivers,
being only one thirty-third as great, according to a conservative
estimate. Both mingle alongshore in all the forms of beach and bar
that have been described, and both are together slowly carried out
to sea. On the shelving ocean floor waste is agitated by various
movements of the unquiet water,--by the undertow (an outward-
running bottom current near the shore), by the ebb and flow of
tides, by ocean currents where they approach the land, and by
waves and ground swells, whose effects are sometimes felt to a
depth of six hundred feet. By all these means the waste is slowly
washed to and fro, and as it is thus ground finer and finer and
its soluble parts are more and more dissolved, it drifts farther
and farther out from land. It is by no steady and rapid movement
that waste is swept from the shore to its final resting place. Day
after day and century after century the grains of sand and
particles of mud are shifted to and fro, winnowed and spread in
layers, which are destroyed and rebuilt again and again before
they are buried safe from further disturbance.
These processes which are hidden from the eye are among the most
important of those with which our science has to do; for it is
they which have given shape to by far the largest part of the
stratified rocks of which the land is made.
THE CONTINENTAL DELTA. This fitting term has been recently
suggested for the sheet of waste slowly accumulating along the
borders of the continents. Within a narrow belt, which rarely
exceeds two or three hundred miles, except near the mouths of
muddy rivers such as the Amazon and Congo, nearly all the waste of
the continent, whether worn from its surface by the weather, by
streams, by glaciers, or by the wind, or from its edge by the
chafing of the waves, comes at last to its final resting place.
The agencies which spread the material of the continental delta
grow more and more feeble as they pass into deeper and more quiet
water away from shore. Coarse materials are therefore soon dropped
along narrow belts near land. Gravels and coarse sands lie in
thick, wedge-shaped masses which thin out seaward rapidly and give
place to sheets of finer sand.
SEA MUDS. Outermost of the sediments derived from the waste of the
continents is a wide belt of mud; for fine clays settle so slowly,
even in sea water,--whose saltness causes them to sink much faster
than they would in fresh water,--that they are wafted far before
they reach a bottom where they may remain undisturbed. Muds are
also found near shore, carpeting the floors of estuaries, and
among stretches of sandy deposits in hollows where the more quiet
water has permitted the finer silt to rest.
Sea muds are commonly bluish and consolidate to bluish shales; the
red coloring matter brought from land waste--iron oxide--is
altered to other iron compounds by decomposing organic matter in
the presence of sea water. Yellow and red muds occur where the
amount of iron oxide in the silt brought down to the sea by rivers
is too great to be reduced, or decomposed, by the organic matter
present.
Green muds and green sand owe their color to certain chemical
changes which take place where waste from the land accumulates on
the sea floor with extreme slowness. A greenish mineral called
GLAUCONITE--a silicate of iron and alumina--is then formed. Such
deposits, known as GREEN SAND, are now in process of making in
several patches off the Atlantic coast, and are found on the
coastal plain of New Jersey among the offshore deposits of earlier
geological ages.
ORGANIC DEPOSITS. Living creatures swarm along the shore and on
the shallows out from land as nowhere else in the ocean. Seaweed
often mantles the rock of the sea cliff between the levels of high
and low tide, protecting it to some degree from the blows of
waves. On the rock bench each little pool left by the ebbing tide
is an aquarium abounding in the lowly forms of marine life. Below
low-tide level occur beds of molluscous shells, such as the
oyster, with countless numbers of other humble organisms. Their
harder parts--the shells of mollusks, the white framework of
corals, the carapaces of crabs and other crustaceans, the shells
of sea urchins, the bones and teeth of fishes--are gradually
buried within the accumulating sheets of sediment, either whole
or, far more often, broken into fragments by the waves.
By means of these organic remains each layer of beach deposits and
those of the continental delta may contain a record of the life of
the time when it was laid. Such a record has been made ever since
living creatures with hard parts appeared upon the globe. We shall
find it sealed away in the stratified rocks of the continents,--
parts of ancient sea deposits now raised to form the dry land.
Thus we have in the traces of living creatures found in the rocks,
i.e. in fossils, a history of the progress of life upon the
planet.
MOLLUSCOUS SHELL DEPOSITS. The forms of marine life of importance
in rock making thrive best in clear water, where little sediment
is being laid, and where at the same time the depth is not so
great as to deprive them of needed light, heat, and of sufficient
oxygen absorbed by sea water from the air. In such clear and
comparatively shallow water there often grow countless myriads of
animals, such as mollusks and corals, whose shells and skeletons
of carbonate of lime gradually accumulate in beds of limestone.
A shell limestone made of broken fragments cemented together is
sometimes called COQUINA, a local term applied to such beds
recently uplifted from the sea along the coast of Florida (Fig.
149).
OOLITIC limestone (oon, an egg; lithos, a stone) is so named from
the likeness of the tiny spherules which compose it to the roe of
fish. Corals and shells have been pounded by the waves to
calcareous sand, and each grain has been covered with successive
concentric coatings of lime carbonate deposited about it from
solution.
The impalpable powder to which calcareous sand is ground by the
waves settles at some distance from shore in deeper and quieter
water as a limy silt, and hardens into a dense, fine-grained
limestone in which perhaps no trace of fossil is found to suggest
the fact that it is of organic origin.
From Florida Keys there extends south to the trough of Florida
Straits a limestone bank covered by from five hundred and forty to
eighteen hundred feet of water. The rocky bottom consists of
limestone now slowly building from the accumulation of the remains
of mollusks, small corals, sea urchins, worms with calcareous
tubes, and lime-secreting seaweed, which live upon its surface.
Where sponges and other silica-secreting organisms abound on
limestone banks, silica forms part of the accumulated deposit,
either in its original condition, as, for example, the spicules of
sponges, or gathered into concretions and layers of flint.
Where considerable mud is being deposited along with carbonate of
lime there is in process of making a clayey limestone or a limy
shale; where considerable sand, a sandy limestone or a limy
sandstone.
CONSOLIDATION OF OFFSHORE DEPOSITS. We cannot doubt that all these
loose sediments of the sea floor are being slowly consolidated to
solid rock. They are soaked with water which carries in solution
lime carbonate and other cementing substances. These cements are
deposited between the fragments of shells and corals, the grains
of sand and the particles of mud, binding them together into firm
rock. Where sediments have accumulated to great thickness the
lower portions tend also to consolidate under the weight of the
overlying beds. Except in the case of limestones, recent sea
deposits uplifted to form land are seldom so well cemented as are
the older strata, which have long been acted upon by underground
waters deep below the surface within the zone of cementation, and
have been exposed to view by great erosion.
RIPPLE MARKS, SUN CRACKS, ETC. The pulse of waves and tidal
currents agitates the loose material of offshore deposits,
throwing it into fine parallel ridges called ripple marks. One may
see this beautiful ribbing imprinted on beach sands uncovered by
the outgoing tide, and it is also produced where the water is of
considerable depth. While the tide is out the surface of shore
deposits may be marked by the footprints of birds and other
animals, or by the raindrops of a passing shower.
The mud of flats, thus exposed to the sun and dried, cracks in a
characteristic way. Such markings may be covered over with a thin
layer of sediment at the next flood tide and sealed away as a
lasting record of the manner and place in which the strata were
laid. In Figure 150 we have an illustration of a very ancient
ripple-marked sand consolidated to hard stone, uplifted and set on
edge by movements of the earth's crust, and exposed to open air
after long erosion.
STRATIFICATION. For the most part the sheet of sea-laid waste is
hidden from our sight. Where its edge is exposed along the shore
we may see the surface markings which have just been noticed.
Soundings also, and the observations made in shallow waters by
divers, tell something of its surface; but to learn more of its
structures we must study those ancient sediments which have been
lifted from the sea and dissected by subaerial agencies. From them
we ascertain that sea deposits are stratified. They lie in
distinct layers which often differ from one another in thickness,
in size of particles, and perhaps in color. They are parted by
bedding planes, each of which represents either a change in
material or a pause during which deposition ceased and the
material of one layer had time to settle and become somewhat
consolidated before the material of the next was laid upon
it. Stratification is thus due to intermittently acting forces,
such as the agitation of the water during storms, the flow and ebb
of the tide, and the shifting channels of tidal currents. Off the
mouths of rivers, stratification is also caused by the coarser and
more abundant material brought down at time of floods being laid
on the finer silt which is discharged during ordinary stages.
How stratified deposits are built up is well illustrated in the
flats which border estuaries, such as the Bay of Fundy. Each
advance of the tide spreads a film of mud, which dries and hardens
in the air during low water before another film is laid upon it by
the next incoming tidal flood. In this way the flats have been
covered by a clay which splits into leaves as thin as sheets of
paper.
It is in fine material, such as clays and shales and limestones,
that the thinnest and most uniform layers, as well as those of
widest extent, occur. On the other hand, coarse materials are
commonly laid in thick beds, which soon thin out seaward and give
place to deposits of finer stuff. In a general way strata are laid
in well-nigh horizontal sheets, for the surface on which they are
laid is generally of very gentle inclination. Each stratum,
however, is lenticular, or lenslike, in form, having an area where
it is thickest, and thinning out thence to its edges, where it is
overlapped by strata similar in shape.
CROSS BEDDING. There is an apparent exception to this rule where
strata whose upper and lower surfaces may be about horizontal are
made up of layers inclined at angles which may be as high as the
angle of repose. In this case each stratum grew by the addition
along its edge of successive layers of sediment, precisely as does
a sand bar in a river, the sand being pushed continuously over the
edge and coming to rest on a sloping surface. Shoals built by
strong and shifting tidal currents often show successive strata in
which the cross bedding is inclined in different directions.
THICKNESS OF SEA DEPOSITS. Remembering the vast amount of
material denuded from the land and deposited offshore, we should
expect that with the lapse of time sea deposits would have grown
to an enormous thickness. It is a suggestive fact that, as a rule,
the profile of the ocean bed is that of a soup plate,--a basin
surrounded by a flaring rim. On the CONTINENTAL SHELF, as the rim
is called, the water is seldom more than six hundred feet in depth
at the outer edge, and shallows gradually towards shore. Along the
eastern coast of the United States the continental shelf is from
fifty to one hundred and more miles in width; on the Pacific coast
it is much narrower. So far as it is due to upbuilding, a wide
continental shelf, such as that of the Atlantic coast, implies a
massive continental delta thousands of feet in thickness. The
coastal plain of the Atlantic states may be regarded as the
emerged inner margin of this shelf, and borings made along the
coast probe it to the depth of as much as three thousand feet
without finding the bottom of ancient offshore deposits.
Continental shelves may also be due in part to a submergence of
the outer margin of a continental plateau and to marine abrasion.
DEPOSITION OF SEDIMENTS AND SUBSIDENCE. The stratified rocks of
the land show in many places ancient sediments which reach a
thickness which is measured in miles, and which are yet the
product of well-nigh continuous deposition. Such strata may prove
by their fossils and by their composition and structure that they
were all laid offshore in shallow water. We must infer that,
during the vast length of time recorded by the enormous pile, the
floor of the sea along the coast was slowly sinking, and that the
trough was constantly being filled, foot by foot, as fast as it
was depressed. Such gradual, quiet movements of the earth's crust
not only modify the outline of coasts, as we have seen, but are of
far greater geological importance in that they permit the making
of immense deposits of stratified rock.
A slow subsidence continued during long time is recorded also in
the succession of the various kinds of rock that come to be
deposited in the same area. As the sea transgresses the land, i.e.
encroaches upon it, any given part of the sea bottom is brought
farther and farther from the shore. The basal conglomerate formed
by bowlder and pebble beaches comes to be covered with sheets of
sand, and these with layers of mud as the sea becomes deeper and
the shore more remote; while deposits of limestone are made when
at last no waste is brought to the place from the now distant
land, and the water is left clear for the growth of mollusks and
other lime-secreting organisms.
RATE OF DEPOSITION. As deposition in the sea corresponds to
denudation on the land, we are able to make a general estimate of
the rate at which the former process is going on. Leaving out of
account the soluble matter removed, the Mississippi is lowering
its basin at the rate of one foot in five thousand years, and we
may assume this as the average rate at which the earth's land
surface of fifty-seven million square miles is now being denuded
by the removal of its mechanical waste. But sediments from the
land are spread within a zone but two or three hundred miles in
width along the margin of the continents, a line one hundred
thousand miles long. As the area of deposition--about twenty-five
million square miles--is about one half the area of denudation,
the average rate of deposition must be twice the average rate of
denudation, i.e. about one foot in twenty-five hundred years. If
some deposits are made much more rapidly than this, others are
made much more slowly. If they were laid no faster than the
present average rate, the strata of ancient sea deposits exposed
in a quarry fifty feet deep represent a lapse of at least one
hundred and twenty-five thousand years, and those of a formation
five hundred feet thick required for their accumulation one
million two hundred and fifty thousand years.
THE SEDIMENTARY RECORD AND THE DENUDATION CYCLE. We have seen that
the successive stages in a cycle of denudation, such as that by
which a land mass of lofty mountains is worn to low plains, are
marked each by its own peculiar land forms, and that the forms of
the earlier stages are more or less completely effaced as the
cycle draws toward an end. Far more lasting records of each stage
are left in the sedimentary deposits of the continental delta.
Thus, in the youth of such a land mass as we have mentioned,
torrential streams flowing down the steep mountain sides deliver
to the adjacent sea their heavy loads of coarse waste, and thick
offshore deposits of sand and gravel (Fig. 156) record the high
elevation of the bordering land. As the land is worn to lower
levels, the amount and coarseness of the waste brought to the sea
diminishes, until the sluggish streams carry only a fine silt
which settles on the ocean floor near to land in wide sheets of
mud which harden into shale. At last, in the old age of the region
(Fig. 157), its low plains contribute little to the sea except the
soluble elements of the rocks, and in the clear waters near the
land lime-secreting organisms flourish and their remains
accumulate in beds of limestone. When long-weathered lands
mantled with deep, well-oxidized waste are uplifted by a gradual
movement of the earth's crust, and the mantle is rapidly stripped
off by the revived streams, the uprise is recorded in wide
deposits of red and yellow clays and sands upon the adjacent ocean
floor.
Where the waste brought in is more than the waves can easily
distribute, as off the mouths of turbid rivers which drain
highlands near the sea, deposits are little winnowed, and are laid
in rapidly alternating, shaly sandstones and sandy shales.
Where the highlands are of igneous rock, such as granite, and
mechanical disintegration is going on more rapidly than chemical
decay, these conditions are recorded in the nature of the deposits
laid offshore. The waste swept in by streams contains much
feldspar and other minerals softer and more soluble than quartz,
and where the waves have little opportunity to wear and winnow it,
it comes to rest in beds of sandstone in which grains of feldspar
and other soft minerals are abundant. Such feldspathic sandstones
are known as ARKOSE.
On the other hand, where the waste supplied to the sea comes
chiefly from wide, sandy, coastal plains, there are deposited off-
shore clean sandstones of well-worn grains of quartz alone. In
such coastal plains the waste of the land is stored for ages.
Again and again they are abandoned and invaded by the sea as from
time to time the land slowly emerges and is again submerged. Their
deposits are long exposed to the weather, and sorted over by the
streams, and winnowed and worked over again and again by the
waves. In the course of long ages such deposits thus become
thoroughly sorted, and the grains of all minerals softer than
quartz are ground to mud.
DEEP-SEA OOZES AND CLAYS
GLOBIGERINA OOZE. Beyond the reach of waste from the land the
bottom of the deep sea is carpeted for the most part with either
chalky ooze or a fine red clay. The surface waters of the warm
seas swarm with minute and lowly animals belonging to the order of
the Foraminifera, which secrete shells of carbonate of lime. At
death these tiny white shells fall through the sea water like
snowflakes in the air, and, slowly dissolving, seem to melt quite
away before they can reach depths greater than about three miles.
Near shore they reach bottom, but are masked by the rapid deposit
of waste derived from the land. At intermediate depths they mantle
the ocean floor with a white, soft lime deposit known as
Globigerina ooze, from a genus of the Foraminifera which
contributes largely to its formation.
RED CLAY. Below depths of from fifteen to eighteen thousand feet
the ocean bottom is sheeted with red or chocolate colored clay. It
is the insoluble residue of seashells, of the debris of submarine
volcanic eruptions, of volcanic dust wafted by the winds, and of
pieces of pumice drifted by ocean currents far from the volcanoes
from which they were hurled. The red clay builds up with such
inconceivable slowness that the teeth of sharks and the hard ear
bones of whales may be dredged in large numbers from the deep
ocean bed, where they have lain unburied for thousands of years;
and an appreciable part of the clay is also formed by the dust of
meteorites consumed in the atmosphere,--a dust which falls
everywhere on sea and land, but which elsewhere is wholly masked
by other deposits.
The dark, cold abysses of the ocean are far less affected by
change than any other portion of the surface of the lithosphere.
These vast, silent plains of ooze lie far below the reach of
storms. They know no succession of summer and winter, or of night
and day. A mantle of deep and quiet water protects them from the
agents of erosion which continually attack, furrow, and destroy
the surface of the land. While the land is the area of erosion,
the sea is the area of deposition. The sheets of sediment which
are slowly spread there tend to efface any inequalities, and to
form a smooth and featureless subaqueous plain.
With few exceptions, the stratified rocks of the land are proved
by their fossils and composition to have been laid in the sea; but
in the same way they are proved to be offshore, shallow-water
deposits, akin to those now making on continental shelves. Deep-
sea deposits are absent from the rocks of the land, and we may
therefore infer that the deep sea has never held sway where the
continents now are,--that the continents have ever been, as now,
the elevated portions of the lithosphere, and that the deep seas
of the present have ever been its most depressed portions.
THE REEF-BUILDING CORALS
In warm seas the most conspicuous of rock-making organisms are the
corals known as the reef builders. Floating in a boat over a coral
reef, as, for example, off the south coast of Florida or among the
Bahamas, one looks down through clear water on thickets of
branching coral shrubs perhaps as much as eight feet high, and
hemispherical masses three or four feet thick, all abloom with
countless minute flowerlike coral polyps, gorgeous in their colors
of yellow, orange, green, and red. In structure each tiny polyp is
little more than a fleshy sac whose mouth is surrounded with
petal-like tentacles, or feelers. From the sea water the polyps
secrete calcium carbonate and build it up into the stony framework
which supports their colonies. Boring mollusks, worms, and sponges
perforate and honeycomb this framework even while its surface is
covered with myriads of living polyps. It is thus easily broken by
the waves, and white fragments of coral trees strew the ground
beneath. Brilliantly colored fishes live in these coral groves,
and countless mollusks, sea urchins, and other forms of marine
life make here their home. With the debris from all these sources
the reef is constantly built up until it rises to low-tide level.
Higher than this the corals cannot grow, since they are killed by
a few hours' exposure to the air.
When the reef has risen to wave base, the waves abrade it on the
windward side and pile to leeward coral blocks torn from their
foundation, filling the interstices with finer fragments. Thus
they heap up along the reef low, narrow islands (Fig. 160).
Reef building is a comparatively rapid progress. It has been
estimated that off Florida a reef could be built up to the surface
from a depth of fifty feet in about fifteen hundred years.
CORAL LIMESTONES. Limestones of various kinds are due to the reef
builders. The reef rock is made of corals in place and broken
fragments of all sizes, cemented together with calcium carbonate
from solution by infiltrating waters. On the island beaches coral
sand is forming oolitic limestone, and the white coral mud with
which the sea is milky for miles about the reef in times of storm
settles and concretes into a compact limestone of finest grain.
Corals have been among the most important limestone builders of
the sea ever since they made their appearance in the early
geological ages.
The areas on which coral limestone is now forming are large. The
Great Barrier Reef of Australia, which lies off the north-eastern
coast, is twelve hundred and fifty miles long, and has a width of
from ten to ninety miles. Most of the islands of the tropics are
either skirted with coral reefs or are themselves of coral
formation.
CONDITIONS OF CORAL GROWTH. Reef-building corals cannot live
except in clear salt water less, as a rule, than one hundred and
fifty feet in depth, with a winter temperature not lower than 68
degrees F. An important condition also is an abundant food supply,
and this is best secured in the path of the warm oceanic currents.
Coral reefs may be grouped in three classes,--fringing reefs,
barrier reefs, and atolls.
FRINGING REEFS. These take their name from the fact that they are
attached as narrow fringes to the shore. An example is the reef
which forms a selvage about a mile wide along the northeastern
coast of Cuba. The outer margin, indicated by the line of white
surf, where the corals are in vigorous growth, rises from about
forty feet of water. Between this and the shore lies a stretch of
shoal across which one can wade at low water, composed of coral
sand with here and there a clump of growing coral.
BARRIER REEFS. Reefs separated from the shore by a ship channel of
quiet water, often several miles in width and sometimes as much as
three hundred feet in depth, are known as barrier reefs. The
seaward face rises abruptly from water too deep for coral growth.
Low islands are cast up by the waves upon the reef, and inlets
give place for the ebb and flow of the tides. Along the west coast
of the island of New Caledonia a barrier reef extends for four
hundred miles, and for a length of many leagues seldom approaches
within eight miles of the shore.
ATOLLS. These are ring-shaped or irregular coral islands, or
island-studded reefs, inclosing a central lagoon. The narrow zone
of land, like the rim of a great bowl sunken to the water's edge,
rises hardly more than twenty feet at most above the sea, and is
covered with a forest of trees such as the cocoanut, whose seeds
can be drifted to it uninjured from long distances. The white
beach of coral sand leads down to the growing reef, on whose outer
margin the surf is constantly breaking. The sea face of the reef
falls off abruptly, often to depths of thousands of feet, while
the lagoon varies in depth from a few feet to one hundred and
fifty or two hundred, and exceptionally measures as much as three
hundred and fifty feet.
THEORIES OF CORAL REEFS. Fringing reefs require no explanation,
since the depth of water about them is not greater than that at
which coral can grow; but barrier reefs and atolls, which may rise
from depths too great for coral growth demand a theory of their
origin.
Darwin's theory holds that barrier reefs and atolls are formed
from fringing reefs by SUBSIDENCE. The rate of sinking cannot be
greater than that of the upbuilding of the reef, since otherwise
the corals would be carried below their depth and drowned. The
process is illustrated in Figure 161, where v represents a
volcanic island in mid ocean undergoing slow depression, and ss
the sea level before the sinking began, when the island was
surrounded by a fringing reef. As the island slowly sinks, the
reef builds up with equal pace. It rears its seaward face more
steep than the island slope, and thus the intervening space
between the sinking, narrowing land and the outer margin of the
reef constantly widens. In this intervening space the corals are
more or less smothered with silt from the outer reef and from the
land, and are also deprived in large measure of the needful supply
of food and oxygen by the vigorous growth of the corals on the
outer rim. The outer rim thus becomes a barrier reef and the inner
belt of retarded growth is deepened by subsidence to a ship
channel, s's' representing sea level at this time. The final
stage, where the island has been carried completely beneath the
sea and overgrown by the contracting reef, whose outer ring now
forms an atoll, is represented by s"s".
In very many instances, however, atolls and barrier reefs may be
explained without subsidence. Thus a barrier reef may be formed by
the seaward growth of a fringing reef upon the talus of its sea
face. In Figure 162 f is a fringing reef whose outer wall rises
from about one hundred and fifty feet, the lower limit of the
reef-building species. At the foot of this submarine cliff a talus
of fallen blocks t accumulates, and as it reaches the zone of
coral growth becomes the foundation on which the reef is steadily
extended seaward. As the reef widens, the polyps of the
circumference flourish, while those of the inner belt are retarded
in their growth and at last perish. The coral rock of the inner
belt is now dissolved by sea water and scoured out by tidal
currents until it gives place to a gradually deepening ship
channel, while the outer margin is left as a barrier reef.
In much the same way atolls may be built on any shoal which lies
within the zone of coral growth. Such shoals may be produced when
volcanic islands are leveled by waves and ocean currents, and when
submarine plateaus, ridges, and peaks are built up by various
organic agencies, such as molluscous and foraminiferal shell
deposits. The reef-building corals, whose eggs are drifted widely
over the tropic seas by ocean currents, colonize such submarine
foundations wherever the conditions are favorable for their
growth. As the reef approaches the surface the corals of the inner
area are smothered by silt and starved, and their Submarine
Volcanic Peak hard parts are dissolved and scoured away; while
those of the circumference, with abundant food supply, nourish and
build the ring of the atoll. Atolls may be produced also by the
backward drift of sand from either end of a crescentic coral reef
or island, the spits uniting in the quiet water of the lee to
inclose a lagoon. In the Maldive Archipelago all gradations
between crescent-shaped islets and complete atoll rings have been
observed.
In a number of instances where coral reefs have been raised by
movements of the earth's crust, the reef formation is found to be
a thin veneer built upon a foundation of other deposits. Thus
Christmas Island, in the Indian Ocean, is a volcanic pile rising
eleven hundred feet above sea level and fifteen thousand five
hundred feet above the bottom of the sea. The summit is a plateau
surrounded by a rim of hills of reef formation, which represent
the ring of islets of an ancient atoll. Beneath the reef are thick
beds of limestone, composed largely of the remains of
foraminifers, which cover the lavas and fragraental materials of
the old submarine volcano.
Among the ancient sediments which now form the stratified rocks of
the land there occur many thin reef deposits, but none are known
of the immense thickness which modern reefs are supposed to reach
according to the theory of subsidence.
Barrier and fringing reefs are commonly interrupted off the mouths
of rivers. Why?
SUMMARY. We have seen that the ocean bed is the goal to which the
waste of the rocks of the land at last arrives. Their soluble
parts, dissolved by underground waters and carried to the sea by
rivers, are largely built up by living creatures into vast sheets
of limestone. The less soluble portions--the waste brought in by
streams and the waste of the shore--form the muds and sands of
continental deltas. All of these sea deposits consolidate and
harden, and the coherent rocks of the land are thus reconstructed
on the ocean floor. But the destination is not a final one. The
stratified rocks of the land are for the most part ancient
deposits of the sea, which have been lifted above sea level; and
we may believe that the sediments now being laid offshore are the
"dust of continents to be," and will some time emerge to form
additions to the land. We are now to study the movements of the
earth's crust which restore the sediments of the sea to the light
of day, and to whose beneficence we owe the habitable lands of the
present.
PART II
INTERNAL GEOLOGICAL AGENCIES
CHAPTER IX
MOVEMENTS OF THE EARTH'S CRUST
The geological agencies which we have so far studied--weathering,
streams, underground waters, glaciers, winds, and the ocean--all
work upon the earth from without, and all are set in motion by an
energy external to the earth, namely, the radiant energy of the
sun. All, too, have a common tendency to reduce the inequalities
of the earth's surface by leveling the lands and strewing their
waste beneath the sea.
But despite the unceasing efforts of these external agencies, they
have not destroyed the continents, which still rear their broad
plains and great plateaus and mountain ranges above the sea.
Either, then, the earth is very young and the agents of denudation
have not yet had time to do their work, or they have been opposed
successfully by other forces.
We enter now upon a department of our science which treats of
forces which work upon the earth from within, and increase the
inequalities of its surface. It is they which uplift and recreate
the lands which the agents of denudation are continually
destroying; it is they which deepen the ocean bed and thus
withdraw its waters from the shores. At times also these forces
have aided in the destruction of the lands by gradually lowering
them and bringing in the sea. Under the action of forces resident
within the earth the crust slowly rises or sinks; from time to
time it has been folded and broken; while vast quantities of
molten rock have been pressed up into it from beneath and
outpoured upon its surface. We shall take up these phenomena in
the following chapters, which treat of upheavals and depressions
of the crust, foldings and fractures of the crust, earthquakes,
volcanoes, the interior conditions of the earth, mineral veins,
and metamorphism.
OSCILLATIONS OF THE CRUST
Of the various movements of the crust due to internal agencies we
will consider first those called oscillations, which lift or
depress large areas so slowly that a long time is needed to
produce perceptible changes of level, and which leave the strata
in nearly their original horizontal attitude. These movements are
most conspicuous along coasts, where they can be referred to the
datum plane of sea level; we will therefore take our first
illustrations from rising and sinking shores.
NEW JERSEY. Along the coasts of New Jersey one may find awash at
high tide ancient shell heaps, the remains of tribal feasts of
aborigines. Meadows and old forest grounds, with the stumps still
standing, are now overflowed by the sea, and fragments of their
turf and wood are brought to shore by waves. Assuming that the sea
level remains constant, it is clear that the New Jersey coast is
now gradually sinking. The rate of submergence has been estimated
at about two feet per century.
On the other hand, the wide coastal plain of New Jersey is made of
stratified sands and clays, which, as their marine fossils show,
were outspread beneath the sea. Their present position above sea
level proves that the land now subsiding emerged in the recent
past.
The coast of New Jersey is an example of the slow and tranquil
oscillations of the earth's unstable crust now in progress along
many shores. Some are emerging from the sea, some are sinking
beneath it; and no part of the land seems to have been exempt from
these changes in the past.
EVIDENCES OF CHANGES OF LEVEL. Taking the surface of the sea as a
level of reference, we may accept as proofs of relative upheaval
whatever is now found in place above sea level and could have been
formed only at or beneath it, and as proofs of relative subsidence
whatever is now found beneath the sea and could only have been
formed above it.
Thus old strand lines with sea cliffs, wave-cut rock benches, and
beaches of wave-worn pebbles or sand, are striking proofs of
recent emergence to the amount of their present height above tide.
No less conclusive is the presence of sea-laid rocks which we may
find in the neighboring quarry or outcrop, although it may have
been long ages since they were lifted from the sea to form part of
the dry land.
Among common proofs of subsidence are roads and buildings and
other works of man, and vegetal growths and deposits, such as
forest grounds and peat beds, now submerged beneath the sea. In
the deltas of many large rivers, such as the Po, the Nile, the
Ganges, and the Mississippi, buried soils prove subsidences of
hundreds of feet; and in several cases, as in the Mississippi
delta, the depression seems to be now in progress.
Other proofs of the same movement are drowned land forms which are
modeled only in open air. Since rivers cannot cut their valleys
farther below the baselevel of the sea than the depths of their
channels, DROWNED VALLEYS are among the plainest proofs of
depression. To this class belong Narragansett, Delaware,
Chesapeake, Mobile, and San Francisco bays, and many other similar
drowned valleys along the coasts of the United States. Less
conspicuous are the SUBMARINE CHANNELS which, as soundings show,
extend from the mouths of a number of rivers some distance out to
sea. Such is the submerged channel which reaches from New York Bay
southeast to the edge of the continental shelf, and which is
supposed to have been cut by the Hudson River when this part of
the shelf was a coastal plain.
WARPING. In a region undergoing changes of level the rate of
movement commonly varies in different parts. Portions of an area
may be rising or sinking, while adjacent portions are stationary
or moving in the opposite direction. In this way a land surface
becomes WARPED. Thus, while Nova Scotia and New Brunswick are now
rising from the level of the sea, Prince Edward Island and Cape
Breton Island are sinking, and the sea now flows over the site of
the famous old town of Louisburg destroyed in 1758.
Since the close of the glacial epoch the coasts of Newfoundland
and Labrador have risen hundreds of feet, but the rate of
emergence has not been uniform. The old strand line, which stands
at five hundred and seventy-five feet above tide at St. John's,
Newfoundland, declines to two hundred and fifty feet near the
northern point of Labrador.
THE GREAT LAKES is now under-going perceptible warping. Rivers
enter the lakes from the south and west with sluggish currents and
deep channels resembling the estuaries of drowned rivers; while
those that enter from opposite directions are swift and shallow.
At the western end of Lake Erie are found submerged caves
containing stalactites, and old meadows and forest grounds are now
under water. It is thus seen that the water of the lakes is rising
along their southwestern shores, while from their north-eastern
shores it is being withdrawn. The region of the Great Lakes is
therefore warping; it is rising in the northeast as compared with
the southwest.
From old bench marks and records of lake levels it has been
estimated that the rate of warping amounts to five inches a
century for every one hundred miles. It is calculated that the
water of Lake Michigan is rising at Chicago at the rate of nine or
ten inches per century. The divide at this point between the
tributaries of the Mississippi and Lake Michigan is but eight feet
above the mean stage of the lake. If the canting of the region
continues at its present rate, in a thousand years the waters of
the lake will here overflow the divide. In three thousand five
hundred years all the lakes except Ontario will discharge by this
outlet, via the Illinois and Mississippi rivers, into the Gulf of
Mexico. The present outlet by the Niagara River will be left dry,
and the divide between the St. Lawrence and the Mississippi
systems will have shifted from Chicago to the vicinity of Buffalo.
PHYSIOGRAPHIC EFFECTS OF OSCILLATIONS. We have already mentioned
several of the most important effects of movements of elevation
and depression, such as their effects on rivers, the mantle of
waste, and the forms of coasts. Movements of elevation--including
uplifts by folding and fracture of the crust to be noticed later--
are the necessary conditions for erosion by whatever agent. They
determine the various agencies which are to be chiefly concerned m
the wear of any land,--whether streams or glaciers, weathering or
the wind,--and the degree of their efficiency. The lands must be
uplifted before they can be eroded, and since they must be eroded
before their waste can be deposited, movements of elevation are a
prerequisite condition for sedimentation also. Subsidence is a
necessary condition for deposits of great thickness, such as those
of the Great Valley of California and the Indo-Gangetic plain (p.
101), the Mississippi delta (p. 109), and the still more important
formations of the continental delta in gradually sinking troughs
(p. 183). It is not too much to say that the character and
thickness of each formation of the stratified rocks depend
primarily on these crustal movements.
Along the Baltic coast of Sweden, bench marks show that the sea is
withdrawing from the land at a rate which at the north amounts to
between three and four feet per century; Towards the south the
rate decreases. South of Stockholm, until recent years, the sea
has gained upon the land, and here in several seaboard towns
streets by the shore are still submerged. The rate of oscillation
increases also from the coast inland. On the other hand, along the
German coast of the Baltic the only historic fluctuations of sea
level are those which may be accounted for by variations due to
changes in rainfall. In 1730 Celsius explained the changes of
level of the Swedish coast as due to a lowering of the Baltic
instead of to an elevation of the land. Are the facts just stated
consistent with his theory?
At the little town of Tadousac--where the Saguenay River empties
into the St. Lawrence--there are terraces of old sea beaches, some
almost as fresh as recent railway fills, the highest standing two
hundred and thirty feet above the river. Here the Saguenay is
eight hundred and forty feet in depth, and the tide ebbs and flows
far up its stream. Was its channel cut to this depth by the river
when the land was at its present height? What oscillations are
here recorded, and to what amount?
A few miles north of Naples, Italy, the ruins of an ancient Roman
temple lie by the edge of the sea, on a narrow plain which is
overlooked in the rear by an old sea cliff (Fig. 166). Three
marble pillars are still standing. For eleven feet above their
bases these columns are uninjured, for to this height they were
protected by an accumulation of volcanic ashes; but from eleven to
nineteen feet they are closely pitted with the holes of boring
marine mollusks. From these facts trace the history of the
oscillations of the region.
FOLDINGS OF THE CRUST
The oscillations which we have just described leave the strata not
far from their original horizontal attitude. Figure 167 represents
a region in which movements of a very different nature have taken
place. Here, on either side of the valley V, we find outcrops of
layers tilted at high angles. Sections along the ridge r show that
it is composed of layers which slant inward from either side. In
places the outcropping strata stand nearly on edge, and on the
right of the valley they are quite overturned; a shale SH has come
to overlie a limestone LM although the shale is the older rock,
whose original position was beneath the limestone.
It is not reasonable to suppose that these rocks were deposited in
the attitude in which we find them now; we must believe that, like
other stratified rocks, they were outspread in nearly level sheets
upon the ocean floor. Since that time they must have been
deformed. Layers of solid rock several miles in thickness have
been crumpled and folded like soft wax in the hand, and a vast
denudation has worn away the upper portions of the folds, in part
represented in our section by dotted lines.
DIP AND STRIKE. In districts where the strata have been disturbed
it is desirable to record their attitude. This is most easily done
by taking the angle at which the strata are inclined and the
compass direction in which they slant. It is also convenient to
record the direction in which the outcrop of the strata trends
across the country.
The inclination of a bed of rocks to the horizon is its DIP. The
amount of the dip is the angle made with a horizontal plane. The
dip of a horizontal layer is zero, and that of a vertical layer is
90 degrees. The direction of the dip is taken with the compass.
Thus a geologist's notebook in describing the attitude of
outcropping strata contains many such entries as these: dip 32
degrees north, or dip 8 degrees south 20 degrees west,--meaning in
the latter case that the amount of the dip is 8 degrees and the
direction of the dip bears 20 degrees west of south.
The line of intersection of a layer with the horizontal plane is
the STRIKE. The strike always runs at right angles to the dip.
Dip and strike may be illustrated by a book set aslant on a shelf.
The dip is the acute angle made with the shelf by the side of the
book, while the strike is represented by a line running along the
book's upper edge. If the dip is north or south, the strike runs
east and west.
FOLDED STRUCTURES. An upfold, in which the strata dip away from a
line drawn along the crest and called the axis of the fold, is
known as an ANTICLINE. A downfold, where the strata dip from
either side toward the axis of the trough, is called a SYNCLINE.
There is sometimes seen a downward bend in horizontal or gently
inclined strata, by which they descend to a lower level. Such a
single flexure is a MONOCLINE.
DEGREES OF FOLDING. Folds vary in degree from broad, low swells,
which can hardly be detected, to the most highly contorted and
complicated structures. In SYMMETRIC folds the dips of the rocks
on each side the axis of the fold are equal. In UNSYMMETRICAL
folds one limb is steeper than the other, as in the anticline in
Figure 167. In OVERTURNED folds one limb is inclined beyond the
perpendicular. FAN FOLDS have been so pinched that the original
anticlines are left broader at the top than at the bottom.
In folds where the compression has been great the layers are often
found thickened at the crest and thinned along the limbs. Where
strong rocks such as heavy limestones are folded together with
weak rocks such as shales, the strong rocks are often bent into
great simple folds, while the weak rocks are minutely crumpled.
SYSTEMS OF FOLDS. As a rule, folds occur in systems. Over the
Appalachian mountain belt, for example, extending from
northeastern Pennsylvania to northern Alabama and Georgia, the
earth's crust has been thrown into a series of parallel folds
whose axes run from northeast to southwest (Fig. 175). In
Pennsylvania one may count a score or more of these earth waves,--
some but from ten to twenty miles in length, and some extending as
much as two hundred miles before they die away. On the eastern
part of this belt the folds are steeper and more numerous than on
the western side.
CAUSE AND CONDITIONS OF FOLDING. The sections which we have
studied suggest that rocks are folded by lateral pressure. While a
single, simple fold might be produced by a heave, a series of
folds, including overturns, fan folds, and folds thickened on
their crests at the expense of their limbs, could only be made in
one way,--by pressure from the side. Experiment has reproduced all
forms of folds by subjecting to lateral thrust layers of plastic
material such as wax.
Vast as the force must have been which could fold the solid rocks
of the crust as one may crumple the leaves of a magazine in the
fingers, it is only under certain conditions that it could have
produced the results which we see. Rocks are brittle, and it is
only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED,
that they can thus be folded and bent instead of being crushed to
pieces. Under these conditions, experiments prove that not only
metals such as steel, but also brittle rocks such as marble, can
be deformed and molded and made to flow like plastic clay.
ZONE OF FLOW, ZONE OF FLOW AND FRACTURE, AND ZONE OF FRACTURE. We
may believe that at depths which must be reckoned in tens of
thousands of feet the load of overlying rocks is so great that
rocks of all kinds yield by folding to lateral pressure, and flow
instead of breaking. Indeed, at such profound depths and under
such inconceivable weight no cavity can form, and any fractures
would be healed at once by the welding of grain to grain. At less
depths there exists a zone where soft rocks fold and flow under
stress, and hard rocks are fractured; while at and near the
surface hard and soft rocks alike yield by fracture to strong
pressure.
STRUCTURES DEVELOPED IN COMPRESSED ROCKS
Deformed rocks show the effects of the stresses to which they have
yielded, not only in the immense folds into which they have been
thrown but in their smallest parts as well. A hand specimen of
slate, or even a particle under the microscope, may show
plications similar in form and origin to the foldings which have
produced ranges of mountains. A tiny flake of mica in the rocks of
the Alps may be puckered by the same resistless forces which have
folded miles of solid rock to form that lofty range.
SLATY CLEAVAGE. Rocks which have yielded to pressure often split
easily in a certain direction across the bedding planes. This
cleavage is known as slaty cleavage, since it is most perfectly
developed in fine-grained, homogeneous rocks, such as slates,
which cleave to the thin, smooth-surfaced plates with which we are
familiar in the slates used in roofing and for ciphering and
blackboards. In coarse-grained rocks, pressure develops more
distant partings which separate the rocks into blocks.
Slaty cleavage cannot be due to lamination, since it commonly
crosses bedding planes at an angle, while these planes have been
often well-nigh or quite obliterated. Examining slate with a
microscope, we find that its cleavage is due to the grain of the
rock. Its particles are flattened and lie with their broad faces
in parallel planes, along which the rock naturally splits more
easily than in any other direction. The irregular grains of the
mud which has been altered to slate have been squeezed flat by a
pressure exerted at right angles to the plane of cleavage.
Cleavage is found only in folded rocks, and, as we may see in
Figure 176, the strike of the cleavage runs parallel to the strike
of the strata and the axis of the folds. The dip of the cleavage
is generally steep, hence the pressure was nearly horizontal. The
pressure which has acted at right angles to the cleavage, and to
which it is due, is the same lateral pressure which has thrown the
strata into folds.
We find additional proof that slates have undergone compression at
right angles to their cleavage in the fact that any inclusions in
them, such as nodules and fossils, have been squeezed out of shape
and have their long diameters lying in the planes of cleavage.
That pressure is competent to cause cleavage is shown by
experiment. Homogeneous material of fine grain, such as beeswax,
when subjected to heavy pressure cleaves at right angles to the
direction of the compressing force.
RATE OF FOLDING. All the facts known with regard to rock
deformation agree that it is a secular process, taking place so
slowly that, like the deepening of valleys by erosion, it escapes
the notice of the inhabitants of the region. It is only under
stresses slowly applied that rocks bend without breaking. The
folds of some of the highest mountains have risen so gradually
that strong, well-intrenched rivers which had the right of way
across the region were able to hold to their courses, and as a
circular saw cuts its way through the log which is steadily driven
against it, so these rivers sawed their gorges through the fold as
fast as it rose beneath them. Streams which thus maintain the
course which they had antecedent to a deformation of the region
are known as ANTECEDENT streams. Examples of such are the Sutlej
and other rivers of India, whose valleys trench the outer ranges
of the Himalayas and whose earlier river deposits have been
upturned by the rising ridges. On the other hand, mountain crests
are usually divides, parting the head waters of different drainage
systems. In these cases the original streams of the region have
been broken or destroyed by the uplift of the mountain mass across
their paths.
On the whole, which have worked more rapidly, processes of
deformation or of denudation?
LAND FORMS DUE TO FOLDING
As folding goes on so slowly, it is never left to form surface
features unmodified by the action of other agencies. An anticlinal
fold is attacked by erosion as soon as it begins to rise above the
original level, and the higher it is uplifted, and the stronger
are its slopes, the faster is it worn away. Even while rising, a
young upfold is often thus unroofed, and instead of appearing as a
long, Smooth, boat-shaped ridge, it commonly has had opened along
the rocks of the axis, when these are weak, a valley which is
overlooked by the infacing escarpments of the hard layers of the
sides of the fold. Under long-continued erosion, anticlines may be
degraded to valleys, while the synclines of the same system may be
left in relief as ridges.
FOLDED MOUNTAINS. The vastness of the forces which wrinkle the
crust is best realized in the presence of some lofty mountain
range. All mountains, indeed, are not the result of folding. Some,
as we shall see, are due to upwarps or to fractures of the crust;
some are piles of volcanic material; some are swellings caused by
the intrusion of molten matter beneath the surface; some are the
relicts left after the long denudation of high plateaus.
But most of the mountain ranges of the earth, and some of the
greatest, such as the Alps and the Himalayas, were originally
mountains of folding. The earth's crust has wrinkled into a fold;
or into a series of folds, forming a series of parallel ridges and
intervening valleys; or a number of folds have been mashed
together into a vast upswelling of the crust, in which the layers
have been so crumpled and twisted, overturned and crushed, that it
is exceedingly difficult to make out the original structure.
The close and intricate folds seen in great mountain ranges were
formed, as we have seen, deep below the surface, within the zone
of folding. Hence they may never have found expression in any
individual surface features. As the result of these deformations
deep under ground the surface was broadly lifted to mountain
height, and the crumpled and twisted mountain structures are now
to be seen only because erosion has swept away the heavy cover of
surface rocks under whose load they were developed.
When the structure of mountains has been deciphered it is possible
to estimate roughly the amount of horizontal compression which the
region has suffered. If the strata of the folds of the Alps were
smoothed out, they would occupy a belt seventy-four miles wider
than that to which they have been compressed, or twice their
present width. A section across the Appalachian folds in
Pennyslvania shows a compression to about two thirds the original
width; the belt has been shortened thirty-five miles in every
hundred.
Considering the thickness of their strata, the compression which
mountains have undergone accounts fully for their height, with
enough to spare for all that has been lost by denudation.
The Appalachian folds involve strata thirty thousand feet in
thickness. Assuming that the folded strata rested on an unyielding
foundation, and that what was lost in width was gained in height,
what elevation would the range have reached had not denudation
worn it as it rose?
THE LIFE HISTORY OF MOUNTAINS. While the disturbance and uplift of
mountain masses are due to deformation, their sculpture into
ridges and peaks, valleys and deep ravines, and all the forms
which meet the eye in mountain scenery, excepting in the very
youngest ranges, is due solely to erosion. We may therefore
classify mountains according to the degree to which they have been
dissected. The Juras are an example of the stage of early youth,
in which the anticlines still persist as ridges and the synclines
coincide with the valleys; this they owe as much to the slight
height of their uplift as to the recency of its date.
The Alps were upheaved at various times, the last uplift being
later than the uplift of the Juras, but to so much greater height
that erosion has already advanced them well on towards maturity.
The mountain mass has been cut to the core, revealing strange
contortions of strata which could never have found expression at
the surface. Sharp peaks, knife-edged crests, deep valleys with
ungraded slopes subject to frequent landslides, are all features
of Alpine scenery typical of a mountain range at this stage in its
life history. They represent the survival of the hardest rocks and
the strongest structures, and the destruction of the weaker in
their long struggle for existence against the agents of erosion.
Although miles of rock have been removed from such ranges as the
Alps, we need not suppose that they ever stood much, if any,
higher than at present. All this vast denudation may easily have
been accomplished while their slow upheaval was going on; in
several mountain ranges we have evidence that elevation has not
yet ceased.
Under long denudation mountains are subdued to the forms
characteristic of old age. The lofty peaks and jagged crests of
their earlier life are smoothed down to low domes and rounded
crests. The southern Appalachians and portions of the Hartz
Mountains in Germany are examples of mountains which have reached
this stage.
There are numerous regions of upland and plains in which the rocks
are found to have the same structure that we have seen in folded
mountains; they are tilted, crumpled, and overturned, and have
clearly suffered intense compression. We may infer that their
folds were once lifted to the height of mountains and have since
been wasted to low-lying lands. Such a section as that of Figure
67 illustrates how ancient mountains may be leveled to their
roots, and represents the final stage to which even the Alps and
the Himalayas must sometime arrive. Mountains, perhaps of Alpine
height, once stood about Lake Superior; a lofty range once
extended from New England and New Jersey southwestward to Georgia
along the Piedmont belt. In our study of historic geology we shall
see more clearly how short is the life of mountains as the earth
counts time, and how great ranges have been lifted, worn away, and
again upheaved into a new cycle of erosion.
THE SEDIMENTARY HISTORY OF FOLDED MOUNTAINS. We may mention here
some of the conditions which have commonly been antecedent to
great foldings of the crust.
1. Mountain ranges are made of belts of enormously and
exceptionally thick sediments. The strata of the Appalachians are
thirty thousand feet thick, while the same formations thin out to
five thousand feet in the Mississippi valley. The folds of the
Wasatch Mountains involve strata thirty thousand feet thick, which
thin to two thousand feet in the region of the Plains.
2. The sedimentary strata of which mountains are made are for the
most part the shallow-water deposits of continental deltas.
Mountain ranges have been upfolded along the margins of
continents.
3. Shallow-water deposits of the immense thickness found in
mountain ranges can be laid only in a gradually sinking area. A
profound subsidence, often to be reckoned in tens of thousands of
feet, precedes the upfolding of a mountain range.
Thus the history of mountains of folding is as follows: For long
ages the sea bottom off the coast of a continent slowly subsides,
and the great trough, as fast as it forms, is filled with
sediments, which at last come to be many thousands of feet thick.
The downward movement finally ceases. A slow but resistless
pressure sets in, and gradually, and with a long series of many
intermittent movements, the vast mass of accumulated sediments is
crumpled and uplifted into a mountain range.
FRACTURES AND DISLOCATIONS OF THE CRUST
Considering the immense stresses to which the rocks of the crust
are subjected, it is not surprising to find that they often yield
by fracture, like brittle bodies, instead of by folding and
flowing, like plastic solids. Whether rocks bend or break depends
on the character and condition of the rocks, the load of overlying
rocks which they bear, and the amount of the force and the
slowness with which it is applied.
JOINTS. At the surface, where their load is least, we find rocks
universally broken into blocks of greater or less size by partings
known as joints. Under this name are included many division planes
caused by cooling and drying; but it is now generally believed
that the larger and more regular joints, especially those which
run parallel to the dip and strike of the strata, are fractures
due to up-and-down movements and foldings and twistings of the
rocks.
Joints are used to great advantage in quarrying, and we have seen
how they are utilized by the weather in breaking up rock masses,
by rivers in widening their valleys, by the sea in driving back
its cliffs, by glaciers in plucking their beds, and how they are
enlarged in soluble rocks to form natural passageways for
underground waters. The ends of the parted strata match along both
sides of joint planes; in. joints there has been little or no
displacement of the broken rocks.
FAULTS. In Figure 184 the rocks have been both broken and
dislocated along the plane ff'. One side must have been moved up
or down past the other. Such a dislocation is called a fault. The
amount of the displacement, as measured by the vertical distance
between the ends of a parted layer, is the throw. The angle which
the fault plane makes with the vertical is the HADE. In Figure 184
the right side has gone down relatively to the left; the right is
the side of the downthrow, while the left is the side of the
upthrow. Where the fault plane is not vertical the surfaces on the
two sides may be distinguished as the HANGING WALL and the FOOT
WALL. Faults differ in throw from a fraction of an inch to many
thousands of feet.
SLICKENSIDES. If we examine the walls of a fault, we may find
further evidence of movement in the fact that the surfaces are
polished and grooved by the enormous friction which they have
suffered as they have ground one upon the other. These
appearances, called sliekensides, have sometimes been mistaken for
the results of glacial action.
NORMAL FAULTS. Faults are of two kinds,--normal faults and thrust
faults. Normal faults, of which Figure 184 is an example, hade to
the downthrow; the hanging wall has gone down. The total length of
the strata has been increased by the displacement. It seems that
the strata have been stretched and broken, and that the blocks
have readjusted themselves under the action of gravity as they
settled.
THRUST FAULTS. Thrust faults hade to the upthrow; the hanging wall
has gone up. Clearly such faults, where the strata occupy less
space than before, are due to lateral thrust. Folds and thrust
faults are closely associated. Under lateral pressure strata may
fold to a certain point and then tear apart and fault along the
surface of least resistance. Under immense pressure strata also
break by shear without folding. Thus, in Figure 185, the rigid
earth block under lateral thrust has found it easier to break
along the fault plane than to fold. Where such faults are nearly
horizontal they are distinguished as THRUST PLANES.
In all thrust faults one mass has been pushed over another, so as
to bring the underlying and older strata upon younger beds; and
when the fault planes are nearly horizontal, and especially when
the rocks have been broken into many slices which have slidden far
one upon another, the true succession of strata is extremely hard
to decipher.
In the Selkirk Mountains of Canada the basement rocks of the
region have been driven east for seven miles on a thrust plane,
over rocks which originally lay thousands of feet above them.
Along the western Appalachians, from Virginia to Georgia, the
mountain folds are broken by more than fifteen parallel thrust
planes, running from northeast to southwest, along which the older
strata have been pushed westward over the younger. The longest
continuous fault has been traced three hundred and seventy-five
miles, and the greatest horizontal displacement has been estimated
at not less than eleven miles.
CRUSH BRECCIA. Rocks often do not fault with a clean and simple
fracture, but along a zone, sometimes several yards in width, in
which they are broken to fragments. It may occur also that strata
which as a whole yield to lateral thrust by folding include beds
of brittle rocks, such as thin-layered limestones, which are
crushed to pieces by the strain. In either case the fragments when
recemented by percolating waters form a rock known as a CRUSH
BRECCIA (pronounced BRETCHA).
Breccia is a term applied to any rock formed of cemented ANGULAR
fragments. This rock may be made by the consolidation of volcanic
cinders, of angular waste at the foot of cliffs, or of fragments
of coral torn by the waves from coral reefs, as well as of strata
crushed by crustal movements.
SURFACE FEATURES DUE TO DISLOCATIONS
FAULT SCARPS. A fault of recent date may be marked at surface by a
scarp, because the face of the upthrown block has not yet been
worn to the level of the downthrow side.
After the upthrown block has been worn down to this level,
differential erosion produces fault scarps wherever weak rocks and
resistant rocks are brought in contact along the fault plane; and
the harder rocks, whether on the upthrow or the downthrow side,
emerge in a line of cliffs. Where a fault is so old that no abrupt
scarps appear, its general course is sometimes marked by the line
of division between highland and lowland or hill and plain. Great
faults have sometimes brought ancient crystalline rocks in contact
with weaker and younger sedimentary rocks, and long after erosion
has destroyed all fault scarps the harder crystallines rise in an
upland of rugged or mountainous country which meets the lowland
along the line of faulting.
The vast majority of faults give rise to no surface features. The
faulted region may be old enough to have been baseleveled, or the
rocks on both sides of the line of dislocation may be alike in
their resistance to erosion and therefore have been worn down to a
common slope. The fault may be entirely concealed by the mantle of
waste, and in such cases it can be inferred from abrupt changes in
the character or the strike and dip of the strata where they may
outcrop near it.
The plateau trenched by the Grand Canyon of the Colorado River
exhibits a series of magnificent fault scarps whose general course
is from north to south, marking the edges of the great crust
blocks into which the country has been broken. The highest part of
the plateau is a crust block ninety miles long and thirty-five
miles in maximum width, which has been hoisted to nine thousand
three hundred feet above, sea level. On the east it descends four
thousand feet by a monoclinal fold, which passes into a fault
towards the north. On the west it breaks down by a succession of
terraces faced by fault scarps. The throw of these faults varies
from seven hundred feet to more than a mile. The escarpments,
however, are due in a large degree to the erosion of weaker rock
on the downthrow side.
The Highlands of Scotland meet the Lowlands on the south with a
bold front of rugged hills along a line of dislocation which runs
across the country from sea to sea. On the one side are hills of
ancient crystalline rocks whose crumpled structures prove that
they are but the roots of once lofty mountains; on the other lies
a lowland of sandstone and other stratified rocks formed from the
waste of those long-vanished mountain ranges. Remnants of
sandstone occur in places on the north of the great fault, and are
here seen to rest on the worn and fairly even surface of the
crystallines. We may infer that these ancient mountains were
reduced along their margins to low plains, which were slowly
lowered beneath the sea to receive a cover of sedimentary rocks.
Still later came an uplift and dislocation. On the one side
erosion has since stripped off the sandstones for the most part,
but the hard crystalline rocks yet stand in bold relief. On the
other side the weak sedimentary rocks have been worn down to
lowlands.
RIFT VALLEYS. In a broken region undergoing uplift or the unequal
settling which may follow, a slice inclosed between two fissures
may sink below the level of the crust blocks on either side, thus
forming a linear depression known as a rift valley, or valley of
fracture.
One of the most striking examples of this rare type of valley is
the long trough which runs straight from the Lebanon Mountains of
Syria on the north to the Red Sea on the south, and whose central
portion is occupied by the Jordan valley and the Dead Sea. The
plateau which it gashes has been lifted more than three thousand
feet above sea level, and the bottom of the trough reaches a depth
of two thousand six hundred feet below that level in parts of the
Dead Sea. South of the Dead Sea the floor of the trough rises
somewhat above sea level, and in the Gulf of Akabah again sinks
below it. This uneven floor could be accounted for either by the
profound warping of a valley of erosion or by the unequal
depression of the floor of a rift valley. But that the trough is a
true valley of fracture is proved by the fact that on either side
it is bounded by fault scarps and monoclinal folds. The keystone
of the arch has subsided. Many geologists believe that the Jordan-
Akabah trough, the long narrow basin of the Red Sea, and the chain
of down-faulted valleys which in Africa extends from the strait of
Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain
more than thirty lakes--belong to a single system of dislocation.
Should you expect the lateral valleys of a rift valley at the time
of its formation to enter it as hanging valleys or at a common
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