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.