THE GLACIAL LAKE CHICAGO. 437 



and Rapid River the altitude is 75 to 90 feet above Lake Michigan. The 

 portion south from the Kalamazoo River ranges in altitude from 70 feet to 

 fully 100 feet above the lake, being highest on the south border. The sand 

 ranges in depth from a foot or two up to fully 50 feet and appears to be 

 thickest on the borders of the Kalamazoo River. If we except the southern 

 border, the sand deposition seems largely referable to a delta accumulation 

 made by Kalamazoo and Rapid rivers, beginning, perhaps, while the ice 

 sheet was forming Covert Ridge and continuing through the formation of 

 the upper beach of Lake Chicago. The altitude of much of the sand 

 plain is such as to correspond somewhat closely with that of the upper 

 beach in neighboring portions of Van Buren Count)', being about 70 feet 

 above the lake. The water level could not well have stood higher than 90 

 feet for any prolonged period, for basins were observed on the east border 

 of the sand plain near Swan Creek, at 90 to 100 feet above Lake Michigan. 

 Probably a portion of the sand, especially that on the somewhat elevated 

 south border, was deposited in connection with the withdrawal of the ice 

 sheet from the Valparaiso morainic system. A small portion also may have 

 been brought in as a wash from neighboring portions of the Valparaiso 

 morainic system, which here consists of a sandy till which might easilv 

 furnish a large amount of sand upon erosion. The bulk of the deposit, 

 however, appears to be referable to the Kalamazoo River and the small 

 neighboring stream, Rapid River. This being the case, the lake level may 

 be confidently placed at fully 70 feet and possibly may have been 90 feet 

 in western Allegan County. The latter elevation would fall in well with 

 the elevation of the wave-washed surface in southern Ottawa County, 100 

 feet above the lake. 



Reviewing the preceding statements concerning the altitude of the 

 upper beach, it appears that there is little variation in altitude from northern 

 Cook County, Illinois, around the head of Lake Michigan, to St. Joseph, 

 Michigan, the elevation being usually between 55 and 60 feet above the 

 lake. Upon passing northward from St. Joseph, the evidence from river 

 deltas seems to support the view that there has been a slight differential 

 northward uplift, but is not considered conclusive. The western shore of 

 the lake has not been examined sufficiently to afford a basis for comparison. 

 One observation near Waukegan shows what appears to be wave action at 



