The Elements of Geology
by
William Harmon Norton
Part 7 out of 7
THE GREAT LAKES. The basins of the Great Lakes are broad
preglacial river valleys, warped by movements of the crust still
in progress, enlarged by the erosive action of lobes of the
continental ice sheets, and blockaded by their drift. The
complicated glacial and postglacial history of the lakes is
recorded in old strand lines which have been traced at various
heights about them, showing their areas and the levels at which
their waters stood at different times.
With the retreat of the lobate Wisconsin ice sheet toward the
north and east, the southern and western ends of the basins of the
Great Lakes were uncovered first; and here, between the receding
ice front and the slopes of land which faced it, lakes gathered
which increased constantly in size.
The lake which thus came to occupy the western end of the Lake
Superior basin discharged over the divide at Duluth down the St.
Croix River, as an old outlet channel proves; that which held the
southern end of the basin of Lake Michigan sent its overflow
across the divide at Chicago via the Illinois River to the
Mississippi; the lake which covered the lowlands about the western
end of Lake Erie discharged its waters at Fort Wayne into the
Wabash River.
The ice still blocked the Mohawk and St. Lawrence valleys on the
east, while on the west it had retreated far to the north. The
lakes become confluent in wide expanses of water, whose depths and
margins, as shown by their old lake beaches, varied at different
times with the position of the confining ice and with warpings of
the land. These vast water bodies, which at one or more periods
were greater than all the Great Lakes combined, discharged at
various times across the divide at Chicago, near Syracuse, New
York, down the Mohawk valley, and by a channel from Georgian Bay
into the Ottawa River. Last of all the present outlet by the St.
Lawrence was established.
The beaches of the glacial lakes just mentioned are now far from
horizontal. That of the lake which occupied the Ontario basin has
an elevation of three hundred and sixty-two feet above tide at the
west and of six hundred and seventy-five feet at the northeast,
proving here a differential movement of the land since glacial
times amounting to more than three hundred feet. The beaches which
mark the successive heights of these glacial lakes are not
parallel; hence the warping began before the Glacial epoch closed.
We have already seen that the canting of the region is still in
progress.
THE CHAMPLAIN SUBSIDENCE. As the Glacial epoch approached its end,
and the Labrador ice field melted back for the last time to near
its source, the land on which the ice had lain in eastern North
America was so depressed that the sea now spread far and wide up
the St. Lawrence valley. It joined with Lake Ontario, and
extending down the Champlain and Hudson valleys, made an island of
New England and the maritime provinces of Canada.
The proofs of this subsidence are found in old sea beaches and
sea-laid clays resting on Wisconsin till. At Montreal such
terraces are found six hundred and twenty feet above sea level,
and along Lake Champlain--where the skeleton of a whale was once
found among them--at from five hundred to four hundred feet. The
heavy delta which the Mohawk River built at its mouth in this arm
of the sea now stands something more than three hundred feet above
sea level. The clays of the Champlain subsidence pass under water
near the mouth of the Hudson, and in northern New Jersey they
occur two hundred feet below tide. In these elevations we have
measures of the warping of the region since glacial times.
THE WESTERN UNITED STATES IN GLACIAL TIMES. The western United
States was not covered during the Pleistocene by any general ice
sheet, but all the high ranges were capped with permanent snow and
nourished valley glaciers, often many times the size of the
existing glaciers of the Alps. In almost every valley of the
Sierras and the Rockies the records of these vanished ice streams
may be found in cirques, glacial troughs, roches moutonnecs, and
morainic deposits.
It was during the Glacial epoch that Lakes Bonneville and Lahontan
were established in the Great Basin, whose climate must then have
been much more moist than now.
THE DRIFTLESS AREA. In the upper Mississippi valley there is an
area of about ten thousand square miles in southwestern Wisconsin
and the adjacent parts of Iowa and Minnesota, which escaped the
ice invasions. The rocks are covered with residual clays, the
product of long preglacial weathering. The region is an ancient
peneplain, uplifted and dissected in late Tertiary times, with
mature valleys whose gentle gradients are unbroken by waterfalls
and rapids. Thus the driftless area is in strong contrast with the
immature drift topography about it, where lakes and waterfalls are
common. It is a bit of preglacial landscape, showing the condition
of the entire region before the Glacial epoch.
The driftless area lay to one side of the main track of both the
Keewatin and the Labrador ice fields, and at the north it was
protected by the upland south of Lake Superior, which weakened and
retarded the movement of the ice.
South of the driftless area the Mississippi valley was invaded at
different times by ice sheets from the west,--the Kansan and the
Iowan,--and again by the Illinoian ice sheet from the east. Again
and again the Mississippi River was pushed to one side or the
other of its path. The ancient channel which it held along the
Illinoian ice front has been traced through southeastern Iowa for
many miles.
BENEFITS OF GLACIATION. Like the driftless area, the preglacial
surface over which the ice advanced seems to have been well
dissected after the late Tertiary uplifts, and to have been carved
in many places to steep valley slopes and rugged hills. The
retreating ice sheets, which left smooth plains and gently rolling
country over the wide belt where glacial deposition exceeded
glacial erosion, have made travel and transportation easier than
they otherwise would have been.
The preglacial subsoils were residual clays and sands, composed
of the insoluble elements of the country rock of the locality,
with some minglings of its soluble parts still undissolved. The
glacial subsoils are made of rocks of many kinds, still undecayed
and largely ground to powder. They thus contain an inexhaustible
store of the mineral foods of plants, and in a form
made easily ready for plant use.
On the preglacial hillsides the humus layer must have been
comparatively thin, while the broad glacial plains have gathered
deep black soils, rich in carbon and nitrogen taken from the
atmosphere. To these soils and subsoils a large part of the wealth
and prosperity of the glaciated regions of our country must be
attributed.
The ice invasions have also added very largely to the water power
of the country. The rivers which in preglacial times were flowing
over graded courses for the most part, were pushed from their old
valleys and set to flow on higher levels, where they have
developed waterfalls and rapids. This power will probably be fully
utilized long before the coal beds of the country are exhausted,
and will become one of the chief sources of the national wealth.
THE RECENT EPOCH. The deposits laid since glacial times graduate
into those now forming along the ocean shores, on lake beds, and
in river valleys. Slow and comparatively slight changes, such as
the warpings of the region of the Great Lakes, have brought about
the geographical conditions of the present. The physical history
of the Recent epoch needs here no special mention.
THE LIFE OF THE QUATERNARY
During the entire Quaternary, invertebrates and plants suffered
little change in species,--so slowly are these ancient and
comparatively simple organisms modified. The Mammalia, on the
other hand, have changed much since the beginning of Quaternary
time: the various species of the present have been evolved, and
some lines have become extinct. These highly organized vertebrates
are evidently less stable than are lower types of animals, and
respond more rapidly to changes in the environment.
PLEISTOCENE MAMMALS. In the Pleistocene the Mammalia reached their
culmination both in size and in variety of forms, and were
superior in both these respects to the mammals of to-day. In
Pleistocene times in North America there were several species of
bison,--one whose widespreading horns were ten feet from tip to
tip,--a gigantic moose elk, a giant rodent (Castoroides) five feet
long, several species of musk oxen, several species of horses,--
more akin, however, to zebras than to the modern horse,--a huge
lion, several saber-tooth tigers, immense edentates of several
genera, and largest of all the mastodon and mammoth.
The largest of the edentates was the Megatherium, a. clumsy ground
sloth bigger than a rhinoceros. The bones of the Megatherium are
extraordinarily massive,--the thigh bone being thrice as thick as
that of an elephant,--and the animal seems to have been well able
to get its living by overthrowing trees and stripping off their
leaves. The Glyptodon was a mailed edentate, eight feet long,
resembling the little armadillo. These edentates survived from
Tertiary times, and in the warmer stages of the Pleistocene ranged
north as far as Ohio and Oregon.
The great proboscidians of the Glacial epoch were about the size
of modern elephants, and somewhat smaller than their ancestral
species in the Pliocene. The MASTODON ranged over all North
America south of Hudson Bay, but had become extinct in the Old
World at the end of the Tertiary. The elephants were represented
by the MAMMOTH, which roamed in immense herds from our middle
states to Alaska, and from Arctic Asia to the Mediterranean and
Atlantic.
It is an oft-told story how about a century ago, near the Lena
River in Siberia, there was found the body of a mammoth which had
been safely preserved in ice for thousands of years, how the flesh
was eaten by dogs and bears, and how the eyes and hoofs and
portions of the hide were taken with the skeleton to St.
Petersburg. Since then several other carcasses of the mammoth,
similarly preserved in ice, have been found in the same region,--
one as recently as 1901. We know from these remains that the
animal was clothed in a coat of long, coarse hair, with thick
brown fur beneath.
THE DISTRIBUTION OF ANIMALS AND PLANTS. The distribution of
species in the Glacial epoch was far different from that of the
present. In the glacial stages arctic species ranged south into
what are now temperate latitudes. The walrus throve along the
shores of Virginia and the musk ox grazed in Iowa and Kentucky. In
Europe the reindeer and arctic fox reached the Pyrenees. During
the Champlain depression arctic shells lived along the shore of
the arm of the sea which covered the St. Lawrence valley. In
interglacial times of milder climate the arctic fauna-flora
retreated, and their places were taken by plants and animals from
the south. Peccaries, now found in Texas, ranged into Michigan and
New York, while great sloths from South America reached the middle
states. Interglacial beds at Toronto, Canada, contain remains of
forests of maple, elm, and papaw, with mollusks now living in the
Mississippi basin.
What changes in the forests of your region would be brought about,
and in what way, if the climate should very gradually grow colder?
What changes if it should grow warmer?
On the Alps and the highest summits of the White Mountains of New
England are found colonies of arctic species of plants and
insects. How did they come to be thus separated from their home
beyond the arctic circle by a thousand miles and more of temperate
climate impossible to cross?
MAN. Along with the remains of the characteristic animals of the
time which are now extinct there have been found in deposits of
the Glacial epoch in the Old World relics of Pleistocene MAN, his
bones, and articles of his manufacture. In Europe, where they have
best been studied, human relics occur chiefly in peat bogs, in
loess, in caverns where man made his home, and in high river
terraces sometimes eighty and a hundred feet above the present
flood plains of the streams.
In order to understand the development of early man, we should
know that prehistoric peoples are ranked according to the
materials of which their tools were made and the skill shown in
their manufacture. There are thus four well-marked stages of human
culture preceding the written annals of history:
4 The Iron stage.
3 The Bronze stage.
2 The Neolithic (recent stone) stage.
1 The Paleolithic (ancient stone) stage.
In the Neolithic stage the use of the metals had not yet been
learned, but tools of stone were carefully shaped and polished. To
this stage the North American Indian belonged at the time of the
discovery of the continent. In the Paleolithic stage, stone
implements were chipped to rude shapes and left unpolished. This,
the lowest state of human culture, has been outgrown by nearly
every savage tribe now on earth. A still earlier stage may once
have existed, when man had not learned so much as to shape his
weapons to his needs, but used chance pebbles and rock splinters
in their natural forms; of such a stage, however, we have no
evidence.
PALEOLITHIC MAN IN EUROPE. It was to the Paleolithic stage that
the earliest men belonged whose relics are found in Europe. They
had learned to knock off two-edged flakes from flint pebbles, and
to work them into simple weapons. The great discovery had been
made that fire could be kindled and made use of, as the charcoal
and the stones discolored by heat of their ancient hearths attest.
Caves and shelters beneath overhanging cliffs were their homes or
camping places. Paleolithic man was a savage of the lowest type,
who lived by hunting the wild beasts of the time.
Skeletons found in certain caves in Belgium and France represent
perhaps the earliest race yet found in Europe. These short, broad-
shouldered men, muscular, with bent knees and stooping gait, low-
browed and small of brain, were of little intelligence and yet
truly human.
The remains of Pleistocene man are naturally found either in
caverns, where they escaped destruction by the ice sheets, or in
deposits outside the glaciated area. In both cases it is extremely
difficult, or quite impossible, to assign the remains to definite
glacial or interglacial times. Their relative age is best told by
the fauna with which they are associated. Thus the oldest relics
of man are found with the animals of the late Tertiary or early
Quaternary, such as a species of hippopotamus and an elephant more
ancient than the mammoth. Later in age are the remains found along
with the mammoth, cave bear and cave hyena, and other animals of
glacial time which are now extinct; while more recent still are
those associated with the reindeer, which in the last ice invasion
roamed widely with the mammoth over central Europe.
THE CAVES OF SOUTHERN FRANCE. These contain the fullest records of
the race, much like the Eskimos in bodily frame, which lived in
western Europe at the time of the mammoth and the reindeer. The
floors of these caves are covered with a layer of bone fragments,
the remains of many meals, and here are found also various
articles of handicraft. In this way we know that the savages who
made these caves their homes fished with harpoons of bone, and
hunted with spears and darts tipped with flint and horn. The
larger bones are split for the extraction of the marrow. Among
such fragments no split human bones are found; this people,
therefore, were not cannibals. Bone needles imply the art of
sewing, and therefore the use of clothing, made no doubt of skins;
while various ornaments, such as necklaces of shells, show how
ancient is the love of personal adornment. Pottery was not yet
invented. There is no sign of agriculture. No animals had yet been
domesticated; not even man's earliest friend, the dog. Certain
implements, perhaps used as the insignia of office, suggest a rude
tribal organization and the beginnings of the state. The remains
of funeral feasts in front of caverns used as tombs point to a
religion and the belief in a life beyond the grave. In the caverns
of southern France are found also the beginnings of the arts of
painting and of sculpture. With surprising skill these Paleolithic
men sketched on bits of ivory the mammoth with his long hair and
huge curved tusks, frescoed their cavern walls with pictures of
the bison and other animals, and carved reindeer on their dagger
heads.
EARLY MAN ON OTHER CONTINENTS. Paleolithic flints curiously like
those of western Europe are found also in many regions of the Old
World,--in India, Egypt, and Asia Minor,--beneath the earliest
vestiges of the civilization of those ancient seats, and sometimes
associated with the fauna of the Glacial epoch.
In Java there were found in 1891, in strata early Quaternary or
late Pliocene in age, parts of a skeleton of lower grade, if not
of greater antiquity, than any human remains now known.
Pithecanthropus erectus, as the creature has been named, walked
erect, as its thigh bone shows, but the skull and teeth indicate a
close affinity with the ape.
In North America there have been reported many finds of human
relics in valley trains, loess, old river gravels buried beneath
lava flows, and other deposits of supposed glacial age; but in the
opinion of some geologists sufficient proof of the existence of
man in America in glacial times has not as yet been found.
These finds in North America have been discredited for various
reasons. Some were not made by scientific men accustomed to the
closest scrutiny of every detail. Some were reported after a
number of years, when the circumstances might not be accurately
remembered; while in a number of instances it seems possible that
the relics might have been worked into glacial deposits by natural
causes from the surface.
Man, we may believe, witnessed the great ice fields of Europe, if
not of America, and perhaps appeared on earth under the genial
climate of preglacial times. Nothing has yet been found of the
line of man's supposed descent from the primates of the early
Tertiary, with the possible exception of the Java remains just
mentioned. The structures of man's body show that he is not
descended from any of the existing genera of apes. And although he
may not have been exempt from the law of evolution,--that method
of creation which has made all life on earth akin,--yet his
appearance was an event which in importance ranks with the advent
of life upon the planet, and marks a new manifestation of creative
energy upon a higher plane. There now appeared intelligence,
reason, a moral nature, and a capacity for self-directed progress
such as had never been before on earth.
THE RECENT EPOCH. The Glacial epoch ends with the melting of the
ice sheets of North America and Europe, and the replacement of the
Pleistocene mammalian fauna by present species. How gradually the
one epoch shades into the other is seen in the fact that the
glaciers which still linger in Norway and Alaska are the lineal
descendants or the renewed appearances of the ice fields of
glacial times.
Our science cannot foretell whether all traces of the Great Ice
Age are to disappear, and the earth is to enjoy again the genial
climate of the Tertiary, or whether the present is an interglacial
epoch and the northern lands are comparatively soon again to be
wrapped in ice.
NEOLITHIC MAN. The wild Paleolithic men vanished from Europe with
the wild beasts which they hunted, and their place was taken by
tribes, perhaps from Asia, of a higher culture. The remains of
Neolithic man are found, much as are those of the North American
Indians, upon or near the surface, in burial mounds, in shell
heaps (the refuse heaps of their settlements), in peat bogs,
caves, recent flood-plain deposits, and in the beds of lakes near
shore where they sometimes built their dwellings upon piles.
The successive stages in European culture are well displayed in
the peat bogs of Denmark. The lowest layers contain the polished
STONE implements of Neolithic man, along with remains of the
SCOTCH FIR. Above are OAK trunks with implements of BRONZE, while
the higher layers hold iron weapons and the remains of a BEECH
forest.
Neolithic man in Europe had learned to make pottery, to spin and
weave linen, to hew timbers and build boats, and to grow wheat and
barley. The dog, horse, ox, sheep, goat, and hog had been
domesticated, and, as these species are not known to have existed
before in Europe, it is a fair inference that they were brought by
man from another continent of the Old World. Neolithic man knew
nothing of the art of extracting the metals from their ores, nor
had he a written language.
The Neolithic stage of culture passes by insensible gradations
into that of the age of bronze, and thus into the Recent epoch.
In the Recent epoch the progress of man in language, in social
organization, in the arts of life, in morals and religion, has
left ample records which are for other sciences than ours to read;
here, therefore, geology gives place to archaeology and history.
Our brief study of the outlines of geology has given us, it is
hoped, some great and lasting good. To conceive a past so
different from the present has stimulated the imagination, and to
follow the inferences by which the conclusions of our science have
been reached has exercised one of the noblest faculties of the
mind,--the reason. We have learned to look on nature in new ways:
every landscape, every pebble now has a meaning and tells
something of its origin and history, while plants and animals have
a closer interest since we have traced the long lines of their
descent. The narrow horizons of human life have been broken
through, and we have caught glimpses of that immeasurable reach of
time in which nebulae and suns and planets run their courses.
Moreover, we have learned something of that orderly and world-
embracing progress by which the once uninhabitable globe has come
to be man's well-appointed home, and life appearing in the
lowliest forms has steadily developed higher and still higher
types. Seeing this process enter human history and lift our race
continually to loftier levels, we find reason to believe that the
onward, upward movement of the geological past is the
manifestation of the same wise Power which makes for righteousness
and good and that this unceasing purpose will still lead on to
nobler ends.
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