244 G. F. WitlGHT ABANDONED BEACHES ABOUT LAKE MICHIGAN 



southern extremity of Chicago, and at Evanston, north of the city, where 

 the new diversion channel is in process of construction. A photograph of 

 a section of the deposit is here shown (see figure 3). The significance of" 

 this with reference to the raised beaches under consideration- is easily 

 seen. The few feet of clay muck and'yellow loam covering the surface is 

 evidently a deposit connected with the Tolleston beach or the Calumet 

 beach, both deposited after the Glenwood stage. The thick deposit of' 

 soft blue clay underlying it must have taken place in the deep water 

 which prevailed during the formation of the Glenwood beach. More 

 careful and extensive study of these deposits is required* to detennine the 

 length of time required for their deposition. As near as we could esti- 

 mate, there are 100 of these clay laminse to the foot in the upper deposit. 



Provisioxal Estimates of Glacial Time afforded in this Area 



Assuming that each of the laminae represents an annual deposit, which, 

 however, is by no means certain, that would give 4,000 years for the con- 

 tinuance of the Glenwood stage of water. This calculation is roughly 

 approximate to that made from deposits in the bottom of Lake Maumee, 

 in the Lake Erie basin, where 35 feet of fine sediment had accumulated 

 in laminae which numbered 8 to the inch. These calculations, also, 

 roughly approximate those made by Doctor Andrews concerning the time 

 required for the accumulation of the dunes at the south end of Lake 

 Michigan, which he estimates to be 6,000 years, though later authorities 

 have, however, questioned the correctness of some of his data. 



All this, however, precedes the time during which Lake Michigan has 

 occupied its present level, determined by the eastward drainage of its 

 waters. The most promising method of estimating this time is presented 

 in the estimated amount and rate of the erosion of its western banks by 

 the waters of the lake. The extent of this erosion is revealed by the- 

 width of the shallow shelf covered by about 60 feet of water, which ex- 

 tends out to the deeper depths of the lake. Assuming that the margin 

 of these 60-foot soundings follow the old shoreline, which is now at an 

 average distance of 2.72 miles from the present shore, and that the rate 

 of erosion (3.33 feet) determined by the Wisconsin Sun^ey is correct, 

 the age of the present lake would be only 4,708 3^ears. Granting that 

 these estimates are at the best only a rude approximation, ^Ir. Leverett 

 concedes that they "have much value in their bearing on the length of 

 postglacial time," and quotes with approval Doctor Andrews's remark 



"that they are useful in showing that it is impossible to allow, even on the 

 most liberal estimates, any such postglacial antiquity as 100,000 years, which 

 has often been claimed." ® 



« Illinois glacial lobe, p. 459. 



