Lake Michigan is inclosed at Chicago by the contour line of five hundred and eighty-five feet above sea level; and the lacustrine plain on which Chicago is built lies mostly under the contour of six hundred and ten, the western suburbs rising to six hundred and thirty feet. In the southern part of the city, near the World's Fair grounds, several faint sandy lake ridges may be traced, rising three or four feet above the dark soil of the plain. The Desplaines River comes from the north on the western part of this plain, ten miles from the lake; its narrow channel, with comparatively steep banks to the water's edge, being sunk ten or twenty feet below the plain. Opposite Summit Station, on the Chicago and Alton Railroad, the river enters a broad, swampy trough, which it follows to the southwest. It is this trough that was briefly described as the lake outlet by Bannister twenty-six years ago; and from his rather casual mention of it I infer that its meaning was then generally appreciated by those who were familiar with the ground. Since then it has been frequently mentioned in a general way in geological literature. The trough is about a mile wide and lies just below the contour of five hundred and ninety feet. Its banks become better defined as it enters the rising ground farther west. From the entrance of the Desplaines eastward to the lake there is no perceptible divide. The canal by which the South Branch of the Chicago River is connected with the Desplaines joins the channel in the western part of the city; and it is by deepening the river channel farther down stream (southwest) that a sufficient volume of lake water is to be diverted through Chicago, thus returning, in a measure, to glacial conditions of drainage, and purifying what is now a very turbid stream.
In addition to this chief passage across the flat divide, there is another one, also mentioned by Bannister, about twelve miles farther south at the village of Blue Island, on the southern end of Washington Heights (six hundred and fifty feet). Here is a long, shallow, swampy trough, again at a level of five hundred and ninety feet, running west from the lacustrine plain through the rising land, and joining the Desplaines at Sag Station, Chicago and Alton Railroad, nine miles below Summit. The Calumet River runs toward the trough from the southeast, but turns abruptly eastward near Blue Island and flows to Lake Michigan. The western end of this southern trough is drained into the Desplaines by a little stream called the Feeder. The swampy part of the trough, between the Calumet and the Feeder, is probably inclosed by faint alluvial fans, swept down by brooks from the higher ground on the south; indeed, it is quite possible that the abrupt turn in the course of the Calumet at Blue Island results from the obstruction of a former westward course in this manner.
The plain west of Lake Michigan, for a distance of twelve or