Page:Popular Science Monthly Volume 46.djvu/240

228 bluffs here vary from seventy-five to a hundred feet in height, being frequently cut back by ravines, as already described. The level of the Illinois River at this point is a little under four hundred and fifty feet. The floor of the old channel stands at four hundred and seventy feet. Its descent from Summit Station is therefore one hundred and twenty feet in about seventy miles; but it must be remembered that part of this slope may be due to post-glacial elevation of the land to the northward.

The entrance of the Fox River from the north at Ottawa was one of the special features that I wished to see. It runs near the western base of the Marseilles morainic belt, and its trench below the general upland is as deep as the old channel; but it is narrow and steep-sided, like the many side ravines of the old-channel bluffs, although in volume the Fox is at least half as great as the Illinois. It has a flood plain of slight width where its banks are of fire clay, as at Dayton, three miles from Ottawa; but farther up and down stream, where it is inclosed by sandstones, the rocks rise directly from the water's edge, and steep bluffs rise above the rocks to the upland. The descent of the river bed is relatively rapid, amounting to about sixty feet in the first ten miles above its mouth.

The other rivers that enter the old channel present the same peculiarity as the Fox, but as they come in over lower ground their valleys are less deep, and therefore less noticeable. The Desplaines has already been described as flowing in a narrow trench in the plain west of Chicago, until it abruptly enters the swampy bed of the old channel. The Kankakee has a similar narrow valley when it joins the Desplaines, from the southeast, in the Morris basin, the two rivers forming the Illinois. Next is the Fox, and below this is the Vermilion, with a steep-sided, narrow valley like those of its fellows. The contrast between the narrow valleys of the side streams and the broad channel followed by the Illinois is strongly marked.

This is all plain enough on the ground; it is distinctly shown on the maps; but it should also be represented by photographs. If any readers of this article happen to have views illustrative of the district here described, I beg that they will communicate with me. There should, indeed, be a photographic exchange established by a union of our professional and amateur photographers, in which good views might be selected under certain conditions for purchase or exchange. At present it is a very difficult matter to find views of the simpler landscapes of our country, however well represented the greater mountains may be.

When all these features are considered together, there is good warrant in the old belief of the southwestward overflow of Lake Michigan. The considerable breadth of the old channel, in which