GLACIAL AND PREGLACIAL LAKES AND RIVERS.
Since the recession of the ice, preglacial lakes have been filled up and
are now dry land, and river beds have been changed so that new channels
have been cut and new lakes have been formed. Even the imagination, that
wonderful architect, with all its tendencies to exaggeration, palls in
its attempt to give expression in measured quantities to the mighty
power exerted by the great glacier or combination of glaciers that
existed in comparatively recent times. I say recent times, because even
10,000 years is only a mere point of time when compared with the actual
age of our globe.
Some years ago, in company with Dr. Wright, author of the "Ice Age in
North America," I visited Devil's Lake near Baraboo, Wis. At this point
are striking evidences of the work of the ice age. Before the glacial
period the Wisconsin River made a detour some miles west of its present
channel through the high hills in the region of Baraboo. The hills on
each side of Devil's Lake are very precipitous and are formed almost
entirely of rocks. The river at that point passed between two of these
hills. When the ice flowed down it surrounded these hills, yet did not
sweep over their tops, but left great piles of glacial drift, both at
the points where the river channel entered the hills and where it
emerges from them. The channel between the hills was protected and not
filled with the debris. Therefore a deep basin was left, which is kept
filled by the watershed furnished by the surrounding hills. This lake
recedes many feet during the summer, but it is again filled up by the
rains and snows of winter. There is no considerable stream either
flowing into or out from it. It is a lake formed by the glaciers, but in
a different way from those in the gravel deposits at other parts of
southern Wisconsin and northern Illinois.
There are hundreds and perhaps thousands of lakes that have been formed
in one way or another through the power of glacial action. These smaller
inland lakes, so many of which are seen in northern Illinois, southern
Wisconsin, and Minnesota, are due almost entirely to the great deposits
of glacial drift that have been transported with the ice. Wherever these
"kettle holes" are found large bodies of ice have become anchored,
while the ice behind it has carried the drift until it is covered over
and piled up at the sides. When these ice mountains melted away
depressions were left which in some cases have resulted in lakes, and in
others simply dry kettle holes. This process has been hinted at in a
former chapter, but we give it here as one of the kinds of lakes formed
during the glacial period. They are found everywhere that glacial action
has prevailed. They are found in great abundance in some parts of New
England on the margin of the terminal moraine. These lakes, however, are
comparatively insignificant as compared with the great inland seas like
Lake Superior and Lake Michigan, that undoubtedly owe their origin
largely to the ice age.
There are other factors, however, that enter into the formation of the
great chain of lakes on the northern boundary of the United States
besides those mentioned, that have brought into existence the smaller
inland lakes.
Glacial lakes may be divided into three classes. Those found in the
"kettle holes" of the terminal or medial moraines, and those that are
formed by the deposition of the glacial drift, as, for instance, Devil's
Lake, and those that are caused by ice forming dams across the valley of
a river that lasted only during the ice age. In some lakes of the
second class erosion undoubtedly entered into their formation as well as
the piling up of glacial drift.
In order, however, that we may understand more fully the formation of
these greater lakes it will be necessary for us to go back and examine
the conditions that seem to have existed before the glacial period.
It is a fact well known that continents have periods of elevation and
depression. There is abundant evidence that the northern portion of the
North American continent was elevated to a much higher level in
preglacial times than it occupies now. This is evidenced in very many
ways by sounding the depths of old river beds now filled with glacial
debris. The old beds show unmistakable evidences of having been worn
down to their present level by the action of running water. They also
prove to be many feet below the present sea-level. This fact seems to be
sufficient to prove the theory of a higher elevation of the North
American continent in preglacial times. It should be said here that
undoubtedly the constant filling up of the ocean with the drift carried
down by the rivers has somewhat raised its level, but hardly to the
extent indicated by the old river beds. The question naturally arises,
Where did all the dirt come from to fill up these great river beds and
change the whole topography of the northern half of the continent? Dr.
Wright estimates that there is not less than 1,000,000 square miles of
territory in North America covered with glacial debris to an average
depth of 50 feet. Of course, the depth varies in different places from a
few inches to several hundred feet. Of the carrying power of these great
glaciers we will speak more fully in a future chapter. In preglacial
times the watershed of the Mississippi and of the great rivers east of
the Alleghany Mountains, the Susquehanna and Hudson, extended probably
farther north than it does to-day. The larger portion of the drainage
area that now finds an outlet through the River St. Lawrence at one time
undoubtedly drained off through the Mississippi Valley into the Gulf and
the Valley of the Mohawk into that of the Hudson.
It is supposed by those who have made this branch of geology a study
that prior to the glacial period a river flowed down through Lake
Superior, which connected with Lake Michigan at a point near its present
outlet at Sault Ste. Marie, the channel of the river passing down
through what is now the bottom of Lake Michigan, which had an outlet at
the head of the lake near Chicago and flowed off into the Mississippi
River. All of the lake bottoms of this great chain, with the exception
of Lake Erie, are now below sea-level. The reason for this exception
will appear further on. Before the ice age there was supposed to be no
connection between Lake Michigan and Lake Huron, as there is now,
through the Straits of Mackinac.
Another preglacial river had its rise in the region of Lake Huron and
flowed through an old river bed extending from the Georgian Bay in a
southeasterly direction through the province of Ontario, and emptied
into the present Lake Ontario. From Lake Ontario there is an old river
bed running through the Valley of the Mohawk which empties into the
Hudson at Troy. Neither of these two rivers, having their sources in the
north, found an outlet through the present St. Lawrence River. During
the time of the glacial period there is evidence that there was more
than one center of snow and ice accumulation and each of these great
centers probably had several subcenters. This theory has color given to
it by the directions of movement shown by the glacial drift.
The rounded appearance of bowlders was caused by the grinding action of
the ice. These bowlders, when they were first torn from their rocky beds
by the irresistible power of ice pressure, were rough and jagged in
shape, the same as any rock would be, torn from a quarry by a blast.
They have been smoothed and rounded by rubbing against the moving ice
and against each other in the progress of their long journey from their
original homes. Where their home was the geologist can immediately tell
upon examination. It is only necessary then to examine the bowlders of
any particular locality to determine the direction of the ice flow at
that point.
There seem to have existed centers of ice accumulation to the north of
all of the great lakes. And when they had grown to a sufficient height
they joined at their edges, making one grand glacier, the movements of
which were the resultant of the combined pressure exerted by these great
centers of power, so that all of North America north of the line of the
terminal moraine, with the exception of a small area (heretofore noted)
chiefly in Wisconsin, became covered with one vast sheet of ice.
The glacier north of Lake Superior widened out the old river bed by a
process of erosion to its present width.
There may have existed something of a lake in preglacial times, through
which the river ran, but it undoubtedly owes its present width to the
grinding action of the irresistible icebergs and the piling up of debris
on the shores. The river bed was filled up by a glacial drift at the
point of its present outlet until the lake was raised in its level much
higher than that of Lake Michigan. Another glacier plowed down through
Lake Michigan, widening it out to its present dimensions, while the
glacial drift was deposited at what is now the head of the lake, filling
up the old outlet and thus making a great dam. The damming up of these
great water courses was another cause for increasing the width of these
lakes. In a similar way Lake Erie was formed. It is supposed, however,
that this lake is entirely the product of glacial action, as there is no
evidence of an old river bed in its bottom; besides, it is much
shallower than the other lakes. The same action that formed Lake Erie
filled up the old river bed running through the province of Ontario, so
that when the ice receded Lake Erie became the new channel for the old
river. The same process filled up the Valley of the Mohawk to more than
100 feet in depth and also raised the Valley of the Hudson. This caused
the new channel to be made through the Niagara River and a new route to
the ocean for the drainage of all the chain of lakes through the St.
Lawrence. It will be seen that the bottoms of all of these great lakes
to a certain extent were worn out by the action of running water, except
Erie. The great glaciers widened them out, and in the case of Lake Erie
scooped it out. At the same time it built great dams across the outlets
which raised the surface of the water to a much higher level and caused
them to form new outlets, thus changing the whole face of the country
over which the ice drifted.
The glaciated region of North America is among the most productive in
the world, and in many respects presents a most pleasing landscape.
Other lakes besides these mentioned have been formed during the ice
period through blocking the course of a river by the ice itself. Dr.
Wright, during the time he traced out the line of the terminal moraine,
discovered that the ice sheet crossed the Ohio River at a point near
Cincinnati, where there is a great bend to the northward in the river.
With the exception of this point and perhaps another point below, the
edge of the great ice sheet kept a little north of the Ohio River. At
this point, however, the ice seems to have filled the valley from hill
to hill, which very naturally would form a great dam or lake in the Ohio
Valley. Of course such a lake could not be permanent, because, when the
ice melted away, it again opened the channel and allowed the water to
flow off.
Some years before this discovery was made there were terraces found
along the banks of the Ohio River and its tributaries that had been the
subject of much speculation. It is well known that by the action of
water from rainfall, earth, gravel, and other debris will wash down the
side of a hill or mountain until it strikes a water level, and there it
will build out a terrace near the level of the water surface. The width
of these terraces will be determined by the time the water has stood at
that level and the extent and nature of the soil from which the debris
comes. The evidences that are cited, pro and con, would fill a small
volume, but it is sufficient to say here that the sum of the evidence
goes to show that there was an ice dam formed at a point near Cincinnati
and that it was maintained for a considerable period of time. Terraces
were formed running up the Ohio and its tributaries corresponding to the
level that the water must have risen to if the valley were filled up
with ice. These facts, taken with the greater fact that the ice sheet
actually did cross the Ohio Valley into Kentucky, as is shown by the
terminal moraine, seems to prove conclusively the existence of such a
lake during the period that the ice rested at its extreme limit. The
fact that in some places successive terraces are found does not disprove
the theory, because it is more than likely that when the ice receded it
did so in successive stages, remaining at different positions for a
considerable length of time. There is abundant proof of this in the
successive moraines and also in the formation of successive terraces.
Some of these terraces could have been formed from other causes.
It does not require any great stretch of the imagination to understand
how numerous lakes, much larger than any at the present day, may have
extended over large portions of the West and Northwest during the period
that the ice was receding. The ice did not stand with an even thickness
over the surface of the glaciated area, but at some points it moved down
in great lobes, which marked the lines of greatest pressure as well as
the greatest accumulation. As the ice melted away, the thick bodies of
ice might be many, many years in melting, and they might block the
outlet to a very extensive drainage area and thus form a great inland
sea from the vast amounts of water that would come from the melting ice.
All of the region about Winnipeg, in the Red River country, covering
great areas of hundreds of miles in extent, is a level plain only
lacking the coloring to give to one passing through it the effect of a
great unruffled sea. There is no doubt but that all of this region was
the bottom of a great lake at some period when the ice was receding. And
this accounts for the great depth of black soil that we find in this and
other regions. The soil was a water deposit, such as may be found in the
bottom of any shallow lake or pond to-day, and thus many thousand years
ago provision was made for the fertile areas which to-day are feeding
the world with wheat.
We can imagine that during this period the water that flowed off
through the great Mississippi must have been of enormous volume as
compared to the present time. A large portion of the delta of the
Mississippi which now is a part of the States of Louisiana and
Mississippi was carried down during the ice-melting period. Dr.
Wright--as we have before stated--has estimated that there are a million
square miles of country that has been covered to an average depth of
fifty feet with glacial drift. A very large amount of the earth that was
spread over the northern portion of the United States by leveling down
hills and mountains in the northern country and scooping out the great
lakes has been carried much farther than to the margin of the ice sheet.
And I have no doubt but that a great portion of Louisiana and western
Mississippi is made of earth carried down largely during the period of
melting ice and deposited in this great delta.
Imagine the effect that would be produced by the giving way of an ice
dam or a great number of them at different periods, that would allow a
body of water as large or larger than Lake Michigan to be drained off in
a comparatively short time. When we think of it in this light the great
delta of the Mississippi is easily accounted for.
There are evidences of a great lake in the Red River country of the
Northwest that is much larger than any of our greatest lakes. The
shores of this lake--the bed of which is now dry land and the heart of a
great agricultural region--are well defined and have been surveyed and
mapped out. When this great body of water was released it was to the
northward. For this reason it was undoubtedly held for a much longer
time than some of the lakes to the southward where the ice melted
sooner.