58 CHICAGO ACADEMY OF SCIENCES. 



which covers nearly all of Cook County and the immediate borders 

 of the Chicago outlet along the Des Plaines River. This map has 

 not been published, being merely a study map. The topographic 

 work carried on by the U. S. Geological Survey in this region is 

 largely published. The peculiar features of the upper portion of 

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

 sheets, viz : The Chicago, Riverside, Calumet, Des Plaines, Joliet, 

 Wilmington, Morris, Ottawa, Marseilles, La Salle, Hennepin and 

 Lacon sheets. These sheets cover something over lOO 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 shown in Professor Rolfe's map sheets, yet unpublished. The re- 

 duced contour map (Plate 2) accompanying this paper is based 

 upon these several surveys. It serves to indicate the comparative 

 size of the valleys occupied by the outlet and of the main tribu- 

 taries of the Illinois. But to fully appreciate the features produced 

 by the outlet, reference should be made to the large scale maps 

 just mentioned. .• ."I 



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, which is only 

 partially filled by the glacial deposits. This filling is preserved in 

 terraces along the borders of the valley. The glacial terraces sel- 

 dom 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, or the two com- 

 bined, 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 

 by Professor ChamberHn. Although the streams Avere generally so 

 heavily charged with detritus as to build up rather than erode 

 their beds for some distance below the point of emergende 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 

 stage of glaciation and this would have received the greater part 

 of the detritus borne down by the glacial floods on the Des 



