Books: The Elements of Geology

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BOWLDERS OF THE DRIFT. The pebbles and bowlders of the drift are
in part stream gravels, bowlders of weathering, and other coarse
rock waste picked up from the surface of the country by the
advancing ice, and in part are fragments plucked from ledges of
sound rock after the mantle of waste had been removed. Many of the
stones of the till are dressed as only glacier ice can do; their
sharp edges have been blunted and their sides faceted and scored.

We may easily find all stages of this process represented among
the pebbles of the till. Some are little worn, even on their
edges; some are planed and scored on one side only; while some in
their long journey have been ground down to many facets and have
lost much of their original bulk. Evidently the ice played fast
and loose with a stone carried in its basal layers, now holding it
fast and rubbing it against the rock beneath, now loosening its
grasp and allowing the stone to turn.

Bowlders of the drift are sometimes found on higher ground than
their parent ledges. Thus bowlders have been left on the sides of
Mount Katahdin, Maine, which were plucked from limestone ledges
twelve miles distant and three thousand feet lower than their
resting place. In other cases stones have been carried over
mountain ranges, as in Vermont, where pebbles of Burlington red
sandstone were dragged over the Green Mountains, three thousand
feet in height, and left in the Connecticut valley sixty miles
away. No other geological agent than glacier ice could do this
work.

The bowlders of the drift are often large. Bowlders ten and twenty
feet in diameter are not uncommon, and some are known whose
diameter exceeds fifty feet. As a rule the average size of
bowlders decreases with increasing distance from their sources.
Why?

TILL PLAINS. The surface of the drift, where left in its initial
state, also displays clear proof of its glacial origin. Over large
areas it is spread in level plains of till, perhaps bowlder-
dotted, similar to the plains of stony clay left in Spitzbergen by
the recent retreat of some of the glaciers of that island. In
places the unstratified drift is heaped in hills of various kinds,
which we will now describe.

DRUMLINS. Drumlins are smooth, rounded hills composed of till,
elliptical in base, and having their longer axes parallel to the
movement of the ice as shown by glacial scorings. They crowd
certain districts in central New York and in southern Wisconsin,
where they may be counted by the thousands. Among the numerous
drumlins about Boston is historic Bunker Hill.

Drumlins are made of ground moraine. They were accumulated and
given shape beneath the overriding ice, much as are sand bars in a
river, or in some instances were carved, like roches moutonnees,
by an ice sheet out of the till left by an earlier ice invasion.

TERMINAL MORAINES. The glaciated area is crossed by belts of
thickened drift, often a mile or two, and sometimes even ten miles
and more, in breadth, which lie transverse to the movement of the
ice and clearly are the terminal moraines of ancient ice sheets,
marking either the limit of their farthest advance or pauses in
their general retreat.

The surface of these moraines is a jumble of elevations and
depressions, which vary from low, gentle swells and shallow sags
to sharp hills, a hundred feet or so in height, and deep, steep-
sided hollows. Such tumultuous hills and hummocks, set with
depressions of all shapes, which usually are without outlet and
are often occupied by marshes, ponds, and lakes, surely cannot be
the work of running water. The hills are heaps of drift, lodged
beneath the ice edge or piled along its front. The basins were
left among the tangle of morainic knolls and ridges as the margin
of the ice moved back and forth. Some bowl-shaped basins were made
by the melting of a mass of ice left behind by the retreating
glacier and buried in its debris.

THE STRATIFIED DRIFT. Like modern glaciers the ice sheets of the
Pleistocene were ever being converted into water about their
margins. Their limits on the land were the lines where their
onward flow was just balanced by melting and evaporation. On the
surface of the ice along the marginal zone, rivulets no doubt
flowed in summer, and found their way through crevasses to the
interior of the glacier or to the ground. Subglacial streams, like
those of the Malaspina glacier, issued from tunnels in the ice,
and water ran along the melting ice front as it is seen to do
about the glacier tongues of Greenland. All these glacier waters
flowed away down the chief drainage channels in swollen rivers
loaded with glacial waste.

It is not unexpected therefore that there are found, over all the
country where the melting ice retreated, deposits made of the same
materials as the till, but sorted and stratified by running water.
Some of these were deposited behind the ice front in ice-walled
channels, some at the edge of the glaciers by issuing streams, and
others were spread to long distances in front of the ice edge by
glacial waters as they flowed away.

ESKERS are narrow, winding ridges of stratified sand and gravel
whose general course lies parallel with the movement of the
glacier. These ridges, though evidently laid by running water, do
not follow lines of continuous descent, but may be found to cross
river valleys and ascend their sides. Hence the streams by which
eskers were laid did not flow unconfined upon the surface of the
ground. We may infer that eskers were deposited in the tunnels and
ice-walled gorges of glacial streams before they issued from the
ice front.

KAMES are sand and gravel knolls, associated for the most part
with terminal moraines, and heaped by glacial waters along the
margin of the ice.

KAME TERRACES are hummocky embankments of stratified drift
sometimes found in rugged regions along the sides of valleys. In
these valleys long tongues of glacier ice lay slowly melting.
Glacial waters took their way between the edges of the glaciers
and the hillside, and here deposited sand and gravel in rude
terraces.

Outwash plains are plains of sand and gravel which frequently
border terminal moraines on their outward face, and were spread
evidently by outwash from the melting ice. Outwash plains are
sometimes pitted by bowl-shaped basins where ice blocks were left
buried in the sand by the retreating glacier.

Valley trains are deposits of stratified drift with which river
valleys have been aggraded. Valleys leading outward from the ice
front were flooded by glacial waters and were filled often to
great depths with trains of stream-swept drift. Since the
disappearance of the ice these glacial flood plains have been
dissected by the shrunken rivers of recent times and left on
either side the valley in high terraces. Valley trains head in
morainic plains, and their material grows finer down valley and
coarser toward their sources. Their gradient is commonly greater
than that of the present rivers.

THE EXTENT OF THE DRIFT. The extent of the drift of North America
and its southern limits are best seen in Figure 359. Its area is
reckoned at about four million square miles. The ice fields which
once covered so much of our continent were all together ten times
as large as the inland ice of Greenland, and about equal to the
enormous ice cap which now covers the antartic regions.

The ice field of Europe was much smaller, measuring about seven
hundred and seventy thousand square miles.

CENTERS OF DISPERSION. The direction of the movement of the ice is
recorded plainly in the scorings of the rock surface, in the
shapes of glaciated hills, in the axes of drumlins and eskers, and
in trains of bowlders, when the ledges from which they were
plucked can be discovered. In these ways it has been proved that
in North America there were three centers where ice gathered to
the greatest depth, and from which it flowed in all directions
outward. There were thus three vast ice fields,--one the
Cordilleran, which lay upon the Cordilleras of British America;
one the Keewatin, which flowed out from the province of Keewatin,
west of Hudson Bay; and one the LABRADOR ice field, whose center
of dispersion was on the highlands of the peninsula of Labrador.
As shown in Figure 359, the western ice field extended but a short
way beyond the eastern foothills of the Rocky Mountains, where
perhaps it met the far-traveled ice from the great central field.
The Keewatin and the Labrador ice fields flowed farthest toward
the south, and in the Mississippi valley the one reached the mouth
of the Missouri and the other nearly to the mouth of the Ohio. In
Minnesota and Wisconsin and northward they merged in one vast
field.

The thickness of the ice was so great that it buried the highest
mountains of eastern North America, as is proved by the
transported bowlders which have been found upon their summits. If
the land then stood at its present height above sea level, and if
the average slope of the ice were no more than ten feet to the
mile,--a slope so gentle that the eye could not detect it and less
than half the slope of the interior of the inland ice of
Greenland,--the ice plateaus about Hudson Bay must have reached a
thickness of at least ten thousand feet.

In Europe the Scandinavian plateau was the chief center of
dispersion. At the time of greatest glaciation a continuous field
of ice extended from the Ural Mountains to the Atlantic, where,
off the coasts of Norway and the British Isles, it met the sea in
an unbroken ice wall. On the south it reached to southern England,
Belgium, and central Germany, and deployed on the eastern plains
in wide lobes over Poland and central Russia (Fig. 360).

At the same time the Alps supported giant glaciers many times the
size of the surviving glaciers of to-day, and a piedmont glacier
covered the plains of northern Switzerland.

THE THICKNESS OF THE DRIFT. The drift is far from uniform in
thickness. It is comparatively thin and scanty over the Laurentian
highlands and the rugged regions of New England, while from
southern New York and Ontario westward over the Mississippi
valley, and on the great western plains of Canada, it exceeds an
average of one hundred feet over wide areas, and in places has
five and six times that thickness. It was to this marginal belt
that the ice sheets brought their loads, while northwards, nearer
the centers of dispersion, erosion was excessive and deposition
slight.

SUCCESSIVE ICE INVASIONS AND THEIR DRIFT SHEETS. Recent studies of
the drift prove that it does not consist of one indivisible
formation, but includes a number of distinct drift sheets, each
with its own peculiar features. The Pleistocene epoch consisted,
therefore, of several glacial stages,--during each of which the
ice advanced far southward,--together with the intervening
interglacial stages when, under a milder climate, the ice melted
back toward its sources or wholly disappeared.

The evidences of such interglacial stages, and the means by which
the different drift sheets are told apart, are illustrated in
Figure 361. Here the country from N to S is wholly covered by
drift, but the drift from N to m is so unlike that from m to S
that we may believe it the product of a distinct ice invasion and
deposited during another and far later glacial stage. The former
drift is very young, for its drainage is as yet immature, and
there are many lakes and marshes upon its surface; the latter is
far older, for its surface has been thoroughly dissected by its
streams. The former is but slightly weathered, while the latter is
so old that it is deeply reddened by oxidation and is leached of
its soluble ingredients such as lime. The younger drift is
bordered by a distinct terminal moraine, while the margin of the
older drift is not thus marked. Moreover, the two drift sheets are
somewhat unlike in composition, and the different proportion of
pebbles of the various kinds of rocks which they contain shows
that their respective glaciers followed different tracks and
gathered their loads from different regions. Again, in places
beneath the younger drift there is found the buried land surface
of an older drift with old soils, forest grounds, and vegetable
deposits, containing the remains of animals and plants, which tell
of the climate of the interglacial stage in which they lived.

By such differences as these the following drift sheets have been
made out in America, and similar subdivisions have been recognized
in Europe.

5 The Wisconsin formation
4 The Iowan formation
3 The Illinoian formation
2 The Kansan formation
1 The pre-Kansan or Jerseyan formation

In New Jersey and Pennsylvania the edge of a deeply weathered and
eroded drift sheet, the Jerseyan, extends beyond the limits of a
much younger overlying drift. It may be the equivalent of a deep-
buried basal drift sheet found in the Mississippi valley beneath
the Kansan and parted from it by peat, old soil, and gravel beds.

The two succeeding stages mark the greatest snowfall of the
Glacial epoch. In Kansan times the Keewatin ice field slowly grew
southward until it reached fifteen hundred miles from its center
of dispersion and extended from the Arctic Ocean to northeastern
Kansas. In the Illinoian stage the Labrador ice field stretched
from Hudson Straits nearly to the Ohio River in Illinois. In the
Iowan and the Wisconsin, the closing stages of the Glacial epoch,
the readvancing ice fields fell far short of their former limits
in the Mississippi valley, but in the eastern states the Labrador
ice field during Wisconsin times overrode for the most part all
earlier deposits, and, covering New England, probably met the
ocean in a continuous wall of ice which set its bergs afloat from
Massachusetts to northern Labrador.

We select for detailed description the Kansan and the Wisconsin
formations as representatives, the one of the older and the other
of the younger drift sheets.

THE KANSAN FORMATION. The Kansan drift consists for the most part
of a sheet of clayey till carrying smaller bowlders than the later
drift. Few traces of drumlins, kames, or terminal moraines are
found upon the Kansan drift, and where thick enough to mask the
preexisting surface, it seems to have been spread originally in
level plains of till.

The initial Kansan plain has been worn by running water until
there are now left only isolated patches and the narrow strips and
crests of the divides, which still rise to the ancient level. The
valleys of the larger streams have been opened wide. Their well-
developed tributaries have carved nearly the entire plain to
valley slopes (Figs. 50 B, and 59). The lakes and marshes which
once marked the infancy of the region have long since been
effaced. The drift is also deeply weathered. The till, originally
blue in color, has been yellowed by oxidation to a depth of ten
and twenty feet and even more, and its surface is sometimes rusted
to terra-cotta red. To a somewhat less depth it has been leached
of its lime and other soluble ingredients. In the weathered zone
its pebbles, especially where the till is loose in texture, are
sometimes so rotted that granites may be crumbled with the
fingers. The Kansan drift is therefore old.

THE WISCONSIN FORMATION. The Wisconsin drift sheet is but little
weathered and eroded, and therefore is extremely young. Oxidation
has effected it but slightly, and lime and other soluble plant
foods remain undissolved even at the grass roots. Its river
systems are still in their infancy (Fig. 50, A). Swamps and peat
bogs are abundant on its undrained surface, and to this drift
sheet belong the lake lands of our northern states and of the
Laurentian peneplain of Canada.

The lake basins of the Wisconsin drift are of several different
classes. Many are shallow sags in the ground moraine. Still more
numerous are the lakes set in hollows among the hills of the
terminal moraines; such as the thousands of lakelets of eastern
Massachusetts. Indeed, the terminal moraines of the Wisconsin
drift may often be roughly traced on maps by means of belts of
lakes and ponds. Some lakes are due to the blockade of ancient
valleys by morainic delms, and this class includes many of the
lakes of the Adirondacks, the mountain regions of New England,
and the Laurentian area. Still other lakes rest in rock basins
scooped out by glaciers. In many cases lakes are due to more than
one cause, as where preglacial valleys have both been basined by
the ice and blockaded by its moraines. The Finger lakes of New
York, for example, occupy such glacial troughs.

Massive TERMINAL MORAINES, which mark the farthest limits to which
the Wisconsin ice advanced, have been traced from Cape Cod and the
islands south of New England, across the Appalachians and the
Mississippi valley, through the Dakotas, and far to the north over
the plains of British America. Where the ice halted for a time in
its general retreat, it left RECESSIONAL MORAINES, as this variety
of the terminal moraine is called. The moraines of the Wisconsin
drift lie upon the country like great festoons, each series of
concentric loops marking the utmost advance of broad lobes of the
ice margin and the various pauses in their recession.

Behind the terminal moraines lie wide till plains, in places
studded thickly with drumlins, or ridged with an occasional esker.
Great outwash plains of sand and gravel lie in front of the
moraine belts, and long valley trains of coarse gravels tell of
the swift and powerful rivers of the time.

THE LOESS OF THE MISSISSIPPI VALLEY. A yellow earth, quite like
the loess of China, is laid broadly as a surface deposit over the
Mississippi valley from eastern Nebraska to Ohio outside the
boundaries of the Iowan and the Wisconsin drift. Much of the loess
was deposited in Iowan times. It is younger than the earlier drift
sheets, for it overlies their weathered and eroded surfaces. It
thickens to the Iowan drift border, but is not found upon that
drift. It is older than the Wisconsin, for in many places it
passes underneath the Wisconsin terminal moraines. In part the
loess seems to have been washed from glacial waste and spread in
sluggish glacial waters, and in part to have been distributed by
the wind from plains of aggrading glacial streams.

THE EFFECTS OF THE ICE INVASIONS ON RIVERS. The repeated ice
invasions of the Pleistocene profoundly disarranged the drainage
systems of our northern states. In some regions the ancient
valleys were completely filled with drift. On the withdrawal of
the ice the streams were compelled to find their way, as best they
could, over a fresh land surface, where we now find them flowing
on the drift in young, narrow channels. But hundreds of feet below
the ground the well driller and the prospector for coal and oil
discover deep, wide, buried valleys cut in rock,--the channels of
preglacial and interglacial streams. In places the ancient valleys
were filled with drift to a depth of a hundred feet, and sometimes
even to a depth of four hundred and five hundred feet. In such
valleys, rivers now flow high above their ancient beds of rock on
floors of valley drift. Many of the valleys of our present rivers
are but patchworks of preglacial, interglacial, and postglacial
courses (Fig. 366). Here the river winds along an ancient valley
with gently sloping sides and a wide alluvial floor perhaps a mile
or so in width, and there it enters a young, rock-walled gorge,
whose rocky bed may be crossed by ledges over which the river
plunges in waterfalls and rapids.

In such cases it is possible that the river was pushed to one side
of its former valley by a lobe of ice, and compelled to cut a new
channel in the adjacent uplands. A section of the valley may have
been blockaded with morainic waste, and the lake formed behind the
barrier may have found outlet over the country to one side of the
ancient drift-filled valley. In some instances it would seem that
during the waning of the ice sheets, glacial streams, while
confined within walls of stagnant ice, cut down through the ice
and incised their channels on the underlying country, in some
cases being let down on old river courses, and in other cases
excavating gorges in adjacent uplands.

PLEISTOCENE LAKES. Temporary lakes were formed wherever the ice
front dammed the natural drainage of the region. Some, held in the
minor valleys crossed by ice lobes, were small, and no doubt many
were too short-lived to leave lasting records. Others, long held
against the northward sloping country by the retreating ice edge,
left in their beaches their clayey beds, and their outlet channels
permanent evidences of their area and depth. Some of these glacial
lakes are thus known to have been larger than any present lake.

Lake Agassiz, named in honor of the author of the theory of
continental glaciation, is supposed to have been held by the
united front of the Keewatin and the Labrador ice fields as they
finally retreated down the valley of the Red River of the North
and the drainage basin of Lake Winnipeg. From first to last Lake
Agassiz covered a hundred and ten thousand square miles in
Manitoba and the adjacent parts of Minnesota and North Dakota,--an
area larger than all the Great Lakes combined. It discharged its
waters across the divide which held it on the south, and thus
excavated the valley of the Minnesota River. The lake bed--a plain
of till--was spread smooth and level as a floor with lacustrine
silts. Since Lake Agassiz vanished with the melting back of the
ice beyond the outlet by the Nelson River into Hudson Bay, there
has gathered on its floor a deep humus, rich in the nitrogenous
elements so needful for the growth of plants, and it is to this
soil that the region owes its well-known fertility.

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.

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