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, an