74 LIFE OF THE PLEISTOCENE 



c. Cause of Low Water Stage 



Just what may have caused the level of the water of Lake Chicago to fall 

 is not definitely known. The uncovering of a lower outlet to the northeast 

 is suggested by Leverett 13 and Alden. 14 The readvance of the ice mentioned 

 by Alden^ and Goldthwait 16 may have succeeded this low water stage. The red 

 clays, which are buried under Calumet beach deposits, appear to have been 

 left in a retiring water body. 17 Goldthwait 18 has suggested that the peat 

 deposits mentioned by Andrews, Leverett, and Alden, might be "a lacustrine 

 deposit, formed in quiet water behind the barrier during the Calumet stage, 

 and buried by shoreward advance of the reef. " This seems scarcely possible, 

 in view of the data at hand, as these deposits are widespread and have been 

 found in most places where excavations have been made, and, as recorded by 

 Andrews, underlie both the Calumet and Toleston beaches. 



Leverett says 19 "The peat not only underlies the bar under discussion (the 

 Rose Hill bar), but extends eastward across the interval between it and the 

 third beach.** There seems no escape from the conclusion that the lake stood 

 at a lower stage than the level of the second beach before that beach and the bar 

 under discussion were formed" (See figure 1). Leverett's statement is sub- 

 stantiated by the character of the deposits found in the north shore channel, 

 and the post-Glenwood low water stage must, therefore, be admitted as having 

 existed. 



Leverett's and Alden's suggestion of an outlet to the northeast is not 

 tenable. Until the coalescence of the waters of the Michigan basin with those 

 of the Superior and Huron basins to form Lake Algonquin, the history of 

 Lake Chicago (as far as known) was quite distinct from that of the northern 

 and eastern basins. The Algonquin stage occurred long after the low water 

 stage under discussion. The cause of the variation in level of Lake Chicago 

 must, therefore, be looked for in the fluctuation of the glacial lobe in the 

 Michigan basin. It is known that the ice margin retreated and advanced 

 several times in the Huron-Erie-Ontario basins, these fluctuations opening 

 new outlets and affecting the level of the water in these basins. Corresponding 

 changes occurred in the Michigan basin, and it is probable that to these we 

 must ascribe the early changes of level of Lake Chicago. The ice of the 

 Michigan lobe left two moraines which indicate as many advances of the ice 



13 Pleistocene Features, p. 71. 



14 Chicago Folio, p. 9; Delavan Lobe, p. 70. 



16 Milwaukee Special Folio; Science, XXIX, p. 557, 1909. 



16 Abandoned Shore Lines, pp. 2, 40. 



17 Alden and Leverett. 



13 Records of Extinct Lakes, p. 61. 



18 Pleistocene Features, p. 74. 



