Page:Popular Science Monthly Volume 46.djvu/238

226 fifteen miles, appears to have been under the lake waters during the greater time of the westward overflow. Its level surface, its fine, dark soil, and the occasional sandy ridges that traverse it have already been mentioned. Its western boundary, at the point where the Rock Island Railroad gradually ascends to higher ground, is rather distinctly defined by a low but definite bank, apparently an old shore line of the lake, the base being near the contour of six hundred and twenty feet.

Farther west there is a belt of higher ground, whose contours reach seven hundred or seven hundred and fifty feet. On the topographic maps this belt appears to be a plateau-like swell, well dissected by streams; but on the ground it has the appearance of a faintly marked moraine, and it is so represented on the soil map of Illinois, prepared by Mr. F. Leverett and exhibited in the Illinois State Building in the World's Fair. Its morainic form is indicated by numerous faint mounds and small hollows, and the railroad cuts show it to be composed largely of drift. The spurs and valleys, apparently of simple drainage development as indicated on the map, do not justly represent the expression of the surface at this point. A more appreciative drawing of the contour lines is required to express this faint morainic topography; but I do not think it should escape representation on a scale of 1:62,500. It would be interesting to compare a careful contouring of a small portion of this belt with its generalized portrayal on the survey sheets.

The old channel, now occupied by the Desplaines, crosses this belt of higher ground in a well-marked trough. The breadth of the flat bottom of the trough is almost constant at a measure of a mile; its depth below the immediately adjacent upland is about seventy-five feet. This is partly cut in horizontal Niagara limestone, and the descent into the flat-bottomed trough is accomplished on steep sloping bluffs, somewhat dissected by narrow, short, and steep-sided ravines. This may be called the Lemont channel, from the village of that name at its middle, where quarrying is now going on in the limestone in order to increase the westward discharge from Lake Michigan, as stated above.

The morainic belt has a width of fifteen or twenty miles on the Desplaines and Joliet sheets. Joliet lies near its western base. Farther west there is a second belt of higher country, also represented as a morainic belt on Leverett's map, of which further mention will be made below. Between the two belts there is a strip of lower country, about twenty-five miles wide, whose elevation at and below the junction of the Desplaines and Kankakee Rivers varies from six hundred to five hundred and fifty or less. This I shall call the Morris basin, from a town of that name near its middle. On entering the basin, the old channel is less