EVIDENCES AND THEORIES OF AN ICE AGE.
There is abundant and unassailable evidence that at one time, ages ago,
a vast ice sheet covered the whole of the northern part of North
America, extending south in Illinois to a point between latitudes 37 and
38. This is the most southerly point to which the ice sheet reached.
From this point the line of extreme flow runs off in a northeasterly and
northwesterly direction. The northeasterly line is through southeastern
Ohio and Pennsylvania, striking the Atlantic Ocean about at New York,
thence through Long Island and up the coast of Massachusetts.
Northwesterly it follows the Mississippi River to its junction with the
Missouri, which it crosses at a point some miles west of this junction,
following the general course of this river a little south of it through
the States of Missouri, Nebraska, Dakota, and Montana. The lines,
especially the northeasterly one, are very irregular, shooting out into
curves and then receding. This line of extreme ice flow is marked by
glacial drift so prominently that no one who has studied glacial action
can doubt for a moment what was the cause of these deposits. The line
is called the "terminal moraine." By examining a map of North America
and tracing the line of the moraine as we have described it, it will be
seen that about two-thirds of North America was at one time covered with
ice to a greater or less depth. How deep, is simply a matter of
conjecture, but in the central portions of the great glacier, where was
the bulk of snowfall, it must have reached a depth of several miles to
account for the enormous pressure that would be required to carry the
ice so far southward.
But let us go back and define what is meant by a moraine. A moraine is a
name given to the deposits that are of stone, gravel, and earth that
have been carried along by the movement of the glaciers and deposited at
their margins, sometimes piled up to great depths. The composition of
these moraines is determined of course by the nature of the country over
which the stream of ice is flowing. Bowlders of enormous size have been
carried for hundreds of miles, and the experienced geologist is able to
examine any one of them and tell us where its home was before the
glacial period. Moraines are divided into different classes according to
their position and constitution. The moraine found at the extreme limit
of ice-flow is called the "terminal" moraine, as before mentioned. Those
that are found inside of this line and between two flows are called
"medial" moraines. There is a subdivision called "kettle" or "gravel"
moraines, which are very prominent in northern Illinois and southern
Wisconsin, and may be said to culminate in the vicinity of Madison. This
moraine is a great deposit of gravelly soil. Where this moraine exists
the face of the country is covered with "kettle holes" of all sizes and
shapes, and in some of them there are small lakes, while others are dry.
The great chain of inland lakes that are found in southern Wisconsin and
northern Illinois were formed by deposits of ice that had been covered
by glacial drift, gravel and otherwise, brought down and deposited upon
these masses of ice which gradually melted away, leaving a depression at
the points where they lay, while the drift that was piled around them
loomed up and became the shores of the lake. This is substantially Dr.
Wright's theory, who studied the formation of these "kettle holes" at
the mouth of the Muir glacier. This enthusiastic glacialist has spent
many summers tracing the terminal moraine with its fringe along the
lines heretofore indicated. He is, therefore, entitled to speak with
authority on matters of glacial action.
The part of the country that has been plowed over by these glaciers is
called the glaciated area and the rest the unglaciated. The whole of
North America north of the line of the terminal moraine that we have
traced is a glacial region, with the exception of a few hundred square
miles chiefly in Wisconsin, where the ice seemed to have parted and
passed around this area, coming together again on the south side of it.
The ice probably did not reach the extreme limit that shows glacial
deposit, but undoubtedly the effects of it are seen for some distance to
the south, owing to the fact that during the time it was melting great
quantities of water flowed away from the extreme edge of the ice,
carrying with it more or less of the glacial drift, which was deposited
for some distance to the south. When the ice receded it undoubtedly
paused at different points, where it remained stationary for a long
period of time. I mean stationary at its edges, for the flow of ice was
continually moving, but in its progress southward it came to a point
where the heat was sufficient to melt the ice as fast as it arrived at
that point. The on-moving ice was continually bringing with it the
debris that it had gathered up at different points on its journey, so
that it is easy to see how these moraines could accumulate to a greater
or less depth at the margin of the ice flow, which would be determined
by the duration of the period it remained stationary. This, however, is
only one factor, as the surface of the earth in some parts of the
country would be more easily picked up and carried than in others;
therefore, the drift accumulated much more rapidly in some sections than
in others.
Another factor that was active in the more rapid accumulation at certain
points was the speed at which the ice moved, and this would be
determined by the pressure that was behind it, and there would always be
lines of unequal pressure existing in such a great glacier as must have
existed when these moraines were formed.
As an instance of the difference in the glacial deposits that are made
in different periods during the time of the melting of the great ice
sheet we may compare the Kettle Moraines of Wisconsin with the clay
deposit mixed with broken gravel that we find along the west coast of
Lake Michigan. Those whose homes are situated between Winnetka and
Waukegan on the lake shore have the foundations of their houses set in
glacial drift that was shoved into position by the ice during the
glacial period.
Anyone who makes an examination of the bluffs along the shore of this
lake will notice that there is no stratification whatever to the deposit
such as will always be found in an unglaciated region. Going west from
the bluff a few miles we come down to the prairie level, where we find
the soil of an entirely different nature. The soil of the prairies of
Illinois and Iowa is probably to a great extent a water deposit. It is
the kind we find in the bottom of a pond that has stood for many years,
and it would seem that at some period all this prairie country with the
black soil was the bottom of a great lake.
The facts of a glacial period are beyond question, but when it occurred,
and how it occurred are questions that many have tried to answer. So
far, all that we can say of them is that some of them are shrewd
guesses. The evidences adduced for determining the time, are the erosion
caused by rivers and streams since the ice subsided. Some of the rivers
and outlets of lakes had their courses changed by the action of the ice,
so that when it subsided new water courses were formed, and the erosion
that they have produced from that time to the present furnishes the data
for determining the time since the subsidence of the ice at any
particular point. For instance, Niagara Falls was undoubtedly at one
time situated at Queenstown, a number of miles below its present
position. And the time that it has taken to grind out the great gorge
that exists between that point and the present falls is approximately a
measure of the time that has elapsed since the subsidence of the ice at
that point. Various estimates have been made to determinate the rate of
erosion. The earlier ones put the time at about 35,000 years. But there
are later investigators who make the time much shorter, not over 10,000
years.
So much for the time; but you ask What about the occasion, or cause?
This is a question that many have attempted to answer, there having been
eight or ten theories promulgated with regard to the cause of the
glacial period, but no one of them is entirely satisfactory, and only
two or three of them are deserving of much discussion. It is always
interesting to know what people think, however, even if we do not agree
with them.
The first theory named is that the glacial period is due to the decrease
of the original heat in our climate. This theory can be dismissed by
saying that the planet was cooling at the time and has been cooling ever
since, and that the reasons for an ice age are greater now than then, on
that theory. Another theory assumes that at some former period there was
a greater amount of moisture in the atmosphere; while this of course
would be the occasion for greater precipitation of snow, it does not
account for the changing conditions that would produce the ice effect.
That there was a preglacial period there is abundant evidence, in buried
forests, the filling up and changing of river beds, and other evidences
that will be referred to further on. This theory, unmodified and stated
broadly, is not satisfactory. Another way of accounting for the glacial
period is the change in the distribution of land and water, which is
supposed to affect the distribution of heat over the earth's surface.
There is much in this theory that commends itself as plausible. Another
theory supposes that the land in northern Europe and America was
elevated to a higher level at that time than it is now. Others attribute
it to variation of temperature in space and of the amount of heat
radiated by the sun. The final theory for accounting for the ice age is
attributed to what is termed the precession of the equinoxes. In short,
the precession of the equinoxes means that the division between summer
and winter is changing gradually, so that during a period of 10,500
years the summers are growing longer in the northern hemisphere and the
winters shorter. We are now in the period of long summers, but in
another 10,000 years we shall be in the period of short summers and long
winters. This difference of time between the winters and the summers is
supposed to be sufficient to change the thermal conditions sufficiently
to produce an ice age.
It is true that the conditions now are very evenly balanced, so much so
that in Switzerland the glaciers will increase for some years together,
when the conditions will change, causing them to gradually recede.
Several of the theories that have been advanced present evidences that
are entitled to careful consideration, but none of them can be said to
be entirely satisfactory. It is well known that the chief factors in the
production of glaciers are moisture and cold. Cold alone is not
sufficient; neither is moisture, unless we can precipitate it in the
form of snow. Cold is opposed to the production of moisture, and this is
a flaw in the argument presented by the last theory, unless we can
couple with it another set of conditions which we will discuss later.
The solution, if it is ever reached, is perhaps more likely to be found
in the realm of meteorology than geology.
It is unnecessary to change the conditions of temperature or the amount
of moisture now existing in order to produce the great glacier again,
provided this moisture could be precipitated, enough of it, in the right
place as snow. For instance, if in Switzerland, where the conditions are
nearly balanced, the annual precipitation could be slightly increased we
should have a condition that would precipitate more snow in winter than
would melt in summer. And the glaciers would gradually accumulate in
size until they would fill the valleys and gorges to the same extent as
formerly prevailed. There only needs to be such a change in the
meteorological conditions as will cause a greater precipitation in that
part of the globe favorable to glaciers, as, for instance, in the
northern part of North America toward Alaska. This might be produced by
a change in the conditions of the equatorial current, so that
evaporation would be more rapid in the northern Pacific than it now is.
When we consider that evaporation increases in proportion as the heat
increases, we can see that heat is just as important a factor in the
production of glaciers as cold. If evaporation could be increased in the
Pacific Ocean west of Alaska, which would be carried by the wind over
the mountains upon the land, and precipitated as snow, the great
glaciers in that region would begin to grow instead of gradually
receding, as is the case at present, and this without any change in the
temperature of the world as a whole or in the amount of heat received
from the sun. One can readily see how changes in the elevation of the
bottom of the ocean would have such an effect upon the tropical stream
as would either increase or decrease the temperature of the thermal
river that flows up the western coast of Alaska.
Whatever may have been the cause that created the great ice age in North
America, so that a sheet of ice covered considerably more than half of
the continent, there is no doubt in regard to the fact of the existence
of such an age, and it will be interesting to study some of the physical
changes that have been made by the ice at that period on the surface of
the glaciated area.