Popular Science Monthly/Volume 24/April 1884/Recent Geological Changes in Western Michigan
|RECENT GEOLOGICAL CHANGES IN WESTERN MICHIGAN.|
By C. W. WOOLDRIDGE, B. S., M. D.
WESTERN Michigan is a region noted for its lumber, its peaches, and its sand. It has other claims, however, to the attention of those who are interested in the workings of Nature, that are not nearly so well known as they deserve to be, for it bears the marks of very extensive geological changes in recent times, which are even yet in progress, but have not attracted the attention that their importance merits, and have been overlooked altogether by some geological writers, whose observations might be expected to cover their field. Let us take a look at this region and examine briefly some of the marks in which Nature has written its history. We find it in the main a sandy plain, wooded with white-oak, beech, maple, hemlock, and pine, varying in the proportion which they bear to one another, and interspersed with other trees and undergrowth in all the variety which the prolific flora of that region affords. In places the land sinks so low as to constitute a timbered swamp, and in others it rises to a moderate height above ground-water; often it appears, as indicated by the vegetation it bears, to be very fertile, but occasionally it is almost naked in its barrenness. Taken as a whole, it is of a lower degree of fertility than the heavier soils found in the more central and southern parts of the State, and for this reason it is less generally under cultivation than it otherwise would be.
This plain is, however, interspersed with tracts of land of a very different character. These consist mostly of a clayey loam, containing bowlders, as the sandy levels generally do not, having a more rolling and irregular surface, and, so far as it has been the writer's privilege to observe them, lying at a higher level than the sandy plain by which they are surrounded.
On scanning the map of Michigan, it will strike one as a peculiar feature of this west side of the State, that nearly every stream, large or small, that flows into Lake Michigan, expands into a small lake near its mouth, a fact that may have given rise to a query in some as to why such a peculiar feature should exist, especially in a country all of whose features are post-glacial—carved, indeed, out of the glacial drift or built upon it everywhere; and it is with the hope to throw some light on this matter, as well as other peculiarities of the region, that this article is written.
Our principal field of observation is the country near Whitehall, Muskegon County, Michigan, where the writer began to reside in the summer of 1878. This village is situated at the head of White Lake, on White River, which opens into Lake Michigan some six miles to the southwest.
In this river-lake one may see in many places an old water-line on piling, which at that time was at an elevation of three feet or more above the water. The fact that this line was continuous at a uniform level on lines of piling that were apparently undisturbed rendered the theory of uplifting by ice that was often given in explanation of it exceedingly unsatisfactory; and when old residents of the neighborhood were heard to speak, as they often did, of schooners loading and unloading, a few years before, in places where there was not at this time water enough to float a raft, it left very little room for doubt that this old water-line recorded a real change of relative level between the water and the land.
On passing along the borders of the lake, however, another phenomenon was observed that seemed to contradict this hypothesis, or to indicate that the change of level had been the other way. This was the existence of the stumps of large trees, evidently in the position where they had grown, but at this time standing in the water. And again, a living witness was found to corroborate the testimony of the stumps, in the person of an old resident who tells of the willows growing far out on what is now a shallow in the lake, and forming a haunt that the deer used to frequent in the years when this country was first settled.
That summer (1878), a new trestle was built across the head of the lake for the Chicago and West Michigan Railroad. In building it piles were driven and sawed off beneath the surface of the water for the bottom sill of each bent to rest upon, but before the next spring these piles began to lift their heads out of the water, and, before the summer of 1879 had passed, the sills that rested on them were lifted from ten to fourteen inches above the water-level. During the summer of 1879 an iron swing-bridge was built across the mouth of White River, at the head of the lake, and, as a foundation for the turn-table, a bed of piling was driven in the center of the channel, which was sawed off at a considerable depth below the surface of the water. On this piling a platform of lumber was built, so that its surface, when completed, was at a depth of some six inches below the surface of the water, and on that a tower of stone-work was built for the turn-table to rest on. As the lake was considered to be at a low level at this time, it was supposed that this platform would be perpetually under water; but the bridge was not yet completed when it began to rise above the surface, and, by the next spring, it was some eight inches above the water-level. At this point, however, the water again began to rise, and at present this platform is again under water.
Another matter must now claim our attention, that speaks of a time somewhat more remote; but first, perhaps, it will be as well to glance briefly at the immediate border of Lake Michigan. Here, along the border of the low-lying, sandy country, there is generally a strip, varying from a few rods to half a mile or more in width, on which the sand has been piled up by the wind into dunes. Here the surface of the ground is fantastically irregular. Sharp crests, gorges, valleys, and crater-like depressions abound everywhere, and the whole is generally covered with forest and filled in with a rank undergrowth. In places, however, especially at the foot of the river-lakes, the sand is yet without vegetation, except here and there, on some sheltered slope, a few bunches of beach-grass or a stunted shrub; white and shining, its surface rippled by the wind, and traced at times with the strangely varied tracks of insects, birds, small creatures from the neighboring woods, turtles from the water, and, most numerous of all, 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 waves was disturbed by the bottom near the edge of the deep water marking the ancient boundary of the lake, sand-bars would be deposited there as we find them, and these would stretch across the mouths of the submerged river-valleys, and on further uplifting they would separate the waters occupying them from those of the great lake, which, meanwhile, would go on adding more sand to them from without. This is the condition of things existing at present. The changes of level that have brought it about have not been uniform and constant; they may have consisted of a single sinking and rising, but more probably there were many. Even yet we see that the solid seeming earth is sinking and swelling there in a most capricious manner. It is hard to tell to what the present movements are tending even—whether for a long period the land is to remain substantially at its present level, whether it is to rise until the river-lakes are drained and the Western Michigan lake-ports are left stranded inland, or whether the country is to be again submerged. We see, within the memory of those now living there, a variation of level to the extent of six feet at least, and in both directions. Forty years ago the land seems to have been at a higher level than it is at present, and to have continued so long enough to permit the growth of large trees on land since submerged. Then there was a subsidence to an extent of several feet, then an uplift until the waters were below their present level, and at last accounts another subsidence seemed to be in progress. Who can tell us its limits, either as to time of continuance, rapidity, or extent? What is the nature of this movement? There are difficulties in the way of accounting for it that would not exist if Lake Michigan were the ocean. A rising and falling of the land as a whole would include the bed of the lake, and would not produce these changes of relative level. To lift the bed of Lake Michigan, might pour out a part of its contents, and so cause an enormous increase in the volume of the St. Clair, Detroit, and St. Lawrence Rivers, with a corresponding diminution when a subsidence was taking place, the rivers rising as the lake was going down, and falling as the waters of the lake were rising; but this, we believe, has not taken place. Is it a shrinking and swelling of the upper strata of Western Michigan, leaving the deeper strata in which the bed of the lake rests comparatively undisturbed? Is it a rocking of the lake-bed from side to side, one part sinking as another rises? What is the extent of the country through which these movements are felt? These questions, and others relating to the matter, would seem to be of interest. Perhaps, if the Government would take the subject in hand and cause a record to be kept of the waterlevel at all light-houses and life-saving stations, a few years might throw light upon it.