THE ANCIENT OUTLET OF LAKE MICHIGAN. 225 



Lake Michigan is inclosed at Chicago by the contour line of 

 five hundred and eighty-five feet above sea level ; and the lacus- 

 trine plain on which Chicago is built lies mostly under the con- 

 tour 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 men- 

 tioned 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 condi- 

 tions 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 far- 

 ther 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 



VOL. XL VI. 18 



