There are many other great glaciers in the mountains of the Pacific coast. Some years ago I saw one of these immense glaciers in British Columbia, from a point called Glacier Station, in the Selkirk Mountains, on the Canadian Pacific Railroad. It was during the month of August, when all of the region was pervaded by a dense smoke occasioned by burning forests. This glacier is a very showy one, owing to the steepness of the side of the mountain and its great breadth. All the glaciers that exist to-day are gradually receding, and are destined eventually to entirely disappear, unless there is a change in meteorological conditions, which some scientists claim will be the case if we only wait long enough, when again all this northern country will be covered with a great ice sheet. There is no doubt in regard to the facts concerning a glacial period that must have existed in the ages past. To anyone who has made a study of the subject there is not wanting abundant evidence to prove that this northern country was at one time enveloped with a great ice sheet of enormous thickness. The conditions that existed to bring about such a state of things have been the subject of much speculation by philosophers, but no one, as yet, has arrived at any very satisfactory conclusion. Many theories have been advanced, some of them not worth considering, while others have many things that give them a show of plausibility. But all of them have what is said of the Darwinian theory, "a missing link." It will be interesting, however, and also instructive, to know what can be said in favor of a set of conditions that would produce such momentous results.
CHAPTER XXVI.
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 débris 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.