THE CHICAGO OUTLET. 421 



County, and the immediate borders of the Chicago Outlet along the Des 

 Plaines River. This has not been published, being merely a study map. 

 The topographic work carried on by the United States Geological Survey 

 in this region is largely published. The peculiar features of the upper por- 

 tion of the outlet are brought out in an effective manner by the following 

 sheets, viz, the Chicago, Riverside, Calumet, Des Plaines, Joliet, Wilming- 

 ton, Moms, Ottawa, Marseilles, Lasalle, Hennepin, and Lacon sheets. 

 These sheets cover something over 100 miles of the former lake outlet, or 

 nearly one-third the distance from the head of the outlet to the Mississippi. 

 The remainder of the outlet is shown in Prof. C. W. Rolfe's map sheets, yet 

 unpublished. The reduced contour map (PI. Ill) accompanying this report 

 is based upon these several surveys. It serves to indicate the comparative 

 size of the valleys occupied by the outlet and of the main tributaries of the 

 Illinois. But to fully appreciate the features produced by the outlet, refer- 

 ence should be made to the large scale maps just mentioned. 



In the interpretation of these features from the maps, care must be 

 exercised in determining the condition of the valley at the time the outlet 

 first became operative. The portion of the Illinois below Hennepin, it will 

 be observed, is a preglacial valley, and was only partially filled by the glacial 

 deposits. This filling is preserved in terraces along the borders of the 

 valley. The glacial terraces seldom rise to a height of more than 100 feet 

 and in the lower 100 miles their average height scarcely exceeds 50 feet 

 above the present stream. In the portion of the valley above Hennepin 

 the stream is mainly in a glacial or postglacial course, but even here there 

 are complications which make it no easy matter to^ determine the amount 

 of erosion attributable to the outlet. Before the accession of the lake 

 waters this valley was the line of discharge for streams issuing from the 

 ice sheet, as possibly of interglacial streams, some evidence of which has 

 been gathered both by Professor Chamberlin and the writer. Although 

 the streams were generally so heavily charged with detritus as to build up 

 rather than erode their beds for some distance below the point of emergence 

 from the ice sheet, it seems scarcely probable that filling would have 

 exceeded erosion throughout the entire length of the Des Plaines and 

 Illinois valleys. The basin at the head of the Illinois, as noted above, was 

 apparently occupied by a lake at the Valparaiso substage of glaciation, and 

 this would have received the greater part of the detritus borne down by the 



