70 LIFE OF THE PLEISTOCENE 



data are supplied by a bed of very fine silt and peat, 10 to 18 inches in thickness, 

 which overlies the thin bed of Glenwood sand or lies directly upon the boulder 

 clay. This low water stage is affirmed by Leverett, 8 Alden, 9 and Andrews, 10 

 but is questioned by Goldthwait, 11 who refers the peat deposit underlying the 

 Calumet beach to the Toleston stage. Leverett lla has recently questioned the 

 age of the deposits found below sand and gravel at Bowmanville, and believes 

 that further evidence of the supposed low water stage should be found in the 

 valleys draining into Lake Chicago, suggesting that the remains of the^Calumet 

 beach should be found across these valleys which should have cut thru these 

 valley floors to the level of the low water stage. It is probable, however, that 

 in the vicinity of Chicago, at least on the North Shore, the destruction of the 

 shore by Lake Michigan in recent times has destroyed much of this evidence. 

 The lake waters may also have cut the outlet down to the Calumet level before 

 the low water stage. The suggestion is of value in this connection and should 

 be followed when opportunity permits. Alden 10a later questions the low water 

 stage between the Glenwood and Calumet stages, accepting the conclusion of 

 Goldthwait that " the peat is merely a lacustrine deposit formed in quiet water 

 behind a barrier during the Calumet stage and buried by shoreward advance 

 of the bar." The presence of this extensive and wide-spread silt deposit, 

 with its abundance of shallow water life, is, however, strong evidence of the 

 truth of Dr. Andrews' early contention of a post-Glenwood low water stage. 

 This deposit extends as far north as Devon Avenue, to the 590 foot contour. 



The humus soil mentioned by Andrews, Leverett, and Alden, possibly repre- 

 sents a land surface correlative with the post-Glenwood stage, altho it might 

 represent a marsh formation deposited in shallow water behind a bar to the 

 eastward, the evidence of which, however, has been lost by the destruction of 

 the shore in later years. From Devon Avenue to the lake, along the line of 

 the north shore channel, there is a deposit of water-laid clay, of varying thick- 

 ness, which underlies Calumet gravel and sand, and which contains a mass of 

 roots and rootlets, giving (apparently) conclusive evidence of a post-Glenwood 

 land surface (see figure 1). 



If we assume that the depth of water did not exceed ten feet, the outline 

 of the submerged area would appear about as shown in plate XXXIII. Mt. 

 Forest and adjacent territory would have been a large island, with Lane's 

 Island to the south and Stony Island to the east. The outlet formed two 

 broad, shallow rivers. The area covered by water extended from about Wil- 



* Pleistocene Features, p. 71 ; 111. Glacial Lobe, p. 440. 



9 Geog. of Chicago, pp. 32, 40, Chicago Folio, p. 9. 



10 Trans. Chi. Acad. Sci., II, p. 15. 



108 Professional Paper 106, U. S. G. S., pp .332-333. 



11 Records of Extinct Lakes, p. 61. 



118 U. S. Geol. Surv., Mon. LIII, pp. 356, 357. 



