Man and the Glacial Period

by G. Frederick Wright

[BE] See [map] at the beginning of the chapter.

In Illinois President Chamberlin’s original determination of the moraine fixed it near the southern end of Lake Michigan, as shown upon our map, but Mr. Frank Leverett has subsequently demonstrated that there is a concentric series of moraines south of this, reaching across the State, (but somewhat obscured by superficial accumulations of loess referred to) and extending nearly to the latitude of St. Louis.

West of Wisconsin President Chamberlin’s “terminal moraine of the second Glacial epoch” bends southward through eastern Minnesota, and, sweeping down through central Iowa, forms, near the middle of the northern part of that State, a loop, having its southern extremity in the vicinity of Des Moines. The western arm of this loop runs through Minnesota in a northwesterly direction nearly parallel with the upper portion of the valley of the Minnesota, until reaching the latitude of the head-waters of that river, where, in the vicinity of the Sisseton Agency, in Dakota, it turns to the south by an acute angle, and makes a loop in that State, extending to the vicinity of Yankton, and with the valley of the James River as its axis. The western arm of this loop follows pretty closely the line of the eastern edge of the trough of the Missouri River, constituting what is called the “Missouri Coteau,” which continues on as a well-marked line of hills running in a northwesterly direction far up into the Dominion of Canada.

One of the most puzzling glacial phenomena in the Mississippi Valley is the driftless area which occupies the southeastern portion of Minnesota, the southwestern part of Wisconsin, and the northwestern corner of Iowa, as delineated upon our map. This is an area which, while being surrounded on every side by all the characteristic marks of glaciation, is itself conspicuous for their entire absence. Its rocks preserve no glacial scratches and are covered by no deposits of till, while northern boulders avoided it in their journey to more southern latitudes.

The reason for all this is not evident in the topography of the region. The land is not higher than that to the north of it, nor is there any manifest protection to it by the highlands south of Lake Superior. Nor yet is there any reason to suppose that any extensive changes of level in former times seriously affected its relations to the surrounding country. Professor Dana, however, has called attention to the fact that even now it is in a region of comparatively light precipitation, suggesting that the snow-fall over it may always have been insignificant in amount. But this could scarcely account for the failure of the great ice-wave of the north to overrun it. We are indebted again to the sagacity of President Chamberlin in suggesting the true explanation.

By referring to the map it will be noticed that this area sustains a peculiar relation to the troughs of Lake Michigan and Lake Superior, while from the arrangements of the moraines in front of these lakes it will be seen that these lake basins were prominent factors in determining the direction of the movement of the surplus ice from the north. It is the more natural that they should do so because of their great depth, their bottoms being in both cases several hundred feet below the present water-level, reaching even below the level of the sea.

These broad, deep channels seem to have furnished the readiest outlet for the surplus ice of the North, and so to have carried both currents of ice beyond this driftless area before they became again confluent. The slight elevation south of Lake Superior served to protect the area on account of the feebleness of direct movement made possible by the strength of these diverging lateral ice-currents. The phenomenon is almost exactly what occurs where a slight obstruction in a river causes an eddy and preserves a low portion of land below it from submergence. A glance at the map will make it easily credible that an ice-movement south of Manitoba, becoming confluent with one from Lake Superior, pushed far down into the Missouri Valley and spread eastward to the Mississippi River, south of the unglaciated driftless area, and there became confluent with a similar movement which had been directed by the valleys of Lake Michigan and Lake Erie. There can be little doubt that President Chamberlin’s explanation is in the main correct, and we have in this another illustration of the analogy between the behaviour of moving ice and that of moving water.

Fig. 34.—Section of the east-and-west glacial furrows, on Kelly’s Island, preserved by Mr. Younglove. Fine sediment rests immediately on the rock, with washed pebbles at the surface.