Open main menu

Page:Popular Science Monthly Volume 24.djvu/849

This page has been proofread, but needs to be validated.
829
GEOLOGICAL CHANGES IN MICHIGAN.

the mimic tracks made by light objects that are moved along by the wind, such a scene is in itself a study for a naturalist.

In some places, Lake Michigan is year by year building out the land with fresh deposits of sand, but oftener it is cutting it away with every storm. A reach of coast, extending perhaps a mile and a half southward from the foot of White Lake, is particularly interesting to one who wishes to study the structure of the country. Here, of late years, the lake has been eating away the land. The bluff facing the water is from fifty to one hundred and fifty feet high; sometimes its face is covered from top to bottom with earth that has slid down so as to conceal its structure, at other times this is all swept away and the strata are revealed. At such times an old surface-line of vegetable mold may be seen through the entire extent of the section at a height of from ten to twenty feet above the lake. Above this line all is sand, below it all is a heavy solid earth, of which clay forms the principal part. In the depressions of this line, where channels of drainage in this ancient line of surface may be supposed to be cut across, springs flow out. In one such depression there is a bed of peat, marking the site of an ancient swamp, and near each edge of this bed it is full of timber that has fallen into it when a swamp and there been preserved. Some of this wood seems to be but little changed, while other pieces have almost the color and texture of charcoal. Here we have found elm, oak, and black-ash, the species of which might be recognized as easily as if just from the forest. Some branches had been charred by fire, and altogether the deposit is exactly what we might expect to find in the edge of a Michigan swamp of the present day, with the difference that this has been compacted and hardened by time and pressure and drainage. The clay soil in which this old swamp was situated seems to underlie the sand everywhere in this region at varying depths, but on excavating to it we do not everywhere find the vegetable mold that here marks its surface. From these facts the conviction has grown that here in Western Michigan the condition of things has varied somewhat like this: First succeeding the introduction of the present order of things at the close of the last Glacial epoch, the entire country was at an elevation above Lake Michigan much greater than at present, great enough to drain the bottoms of all these river-lakes which, it should be noticed, are deepest near the great lake, and generally terminate in a swamp at their head, and each of which is elongated in the same general direction as the valley the foot of which it occupies. This condition of things lasted until the configuration of the land had become substantially what it is at present; then a subsidence took place, until all the lower levels of the country were beneath the waters of Lake Michigan. Again the country began to rise, and as the submerged lands were lifted above the water they were covered with sand, exactly as the lake now deposits sand on a retreating coast. When this uplifting reached such a degree that the action of the