GEOLOGICAL AND NATURAL HISTORY SURVEY. 6*7 



minus, perhaps one mile from the point where the terrace departs 

 from the present shore of the lake. From the terminus of this 

 ridge a bar was built out southwestward five or six miles, ter- 

 minating about a mile east of Chicago River, in the western part of 

 T. 41, R. 13, E. The bar sends out two prominent spurs to the 

 west, a distance of nearly one mile. These probably mark the 

 termini in its earlier stages. The average width of this bar is about 

 one-fourth mile, and it was built up to a height of lo to 20 feet 

 above the bay back of it. It consists largely of gravel, but has a 

 liberal admixture of sand. The bay back of this bar extended to the 

 valley of the Chicago River and had a width of two to three miles. 

 The northern end finds a narrow extension northward, in Skokie 

 marsh. The site of this old bay is now largely under cultivation, 

 though some portions are still marshy. 



The question naturally arises whether this accumulation of 

 gravel and sand was formed by the lake currents and waves, inde- 

 pendent of the Chicago River, or was largely formed as a delta 

 from that stream. This deposit is not in the form of a delta built 

 up at the debouchure of the river into the lake, but lies some dis- 

 tance to the east of the river valley. Moreover, to make it still 

 more evident that it was the lake and not the river which con- 

 tributed the great bulk of the beach deposit, it is found that the 

 river valley above the point where it entered the old lake has very 

 little assorted material, such as would accumulate above a delta. 



The beach appears on the west side of the Chicago River, 

 Sec. 19, T. 41, R. 13 E., about a mile northwest from the terminus 

 of the bar. From this point southward to Oak Park the shore is 

 usually a cut terrace (see Fig. 8), with a bank ranging from 6 to 25 

 feet in height, with occasional deposits of beach gravel and sand 

 along its front. At Oak Park there is an extension of gravel down 

 the east side of the Des Plaines River similar to that of the bar east 

 of the Chicago River, noted above. A ridge or bar 20 to 40 rods in 

 width and 10 feet or more in height extends from Oak Park south 

 about two miles to the south part of Sees. 13 and 14, T. 39, R. 12 

 E., and there terminates abruptly with a level nearly 20 feet above 

 the plain on its immediate borders. 



Passing to the west side of the Des Plaines River, the beach 

 appears about a mile above the southern end of the bar just de- 

 scribed and passes in a curving course westward through 

 the south edge of May wood, in Sees. 14, 22, and 16, T. 

 39, R. 12 E. This portion of the beach is only two 

 to four feet in height, and at the west it fades out completely. 



