THE LIFE OF GLACIAL LAKE CHICAGO 77 



flooded the lands laid bare by the lowering of the lake, causing the extinction 

 of the biota in the lake basin, burying it under a heavy deposit of sand and 

 gravel. 



C. THE CALUMET STAGE 



Following the period of low water, the lake again rose until it stood 35 

 to 40 feet above its present level (it may have risen to the Glenwood level and 

 then fallen to the Calumet level). The Calumet beaches are usually strongly 

 marked, showing that the lake must have remained at this height (35 feet) 

 for a considerable period. This uniform level was probably made possible by 

 a "sill" of bed rock near Lockport, which offered such resistance that a con- 

 siderable time was needed to cut thru this material. 25 Goldthwait 26 attributes 

 the bold character of the Calumet beach to a sudden drop from the 55 foot 

 level, caused by cutting thru the valley train in the Desplaines 27 Valley, but 

 it was more likely formed (viewed in the light of the post-Glenwood low water 

 stage) by the rising of the water level which cut cliffs and built up terraces. 

 The sand overlying the post-Glenwood silt and peat beds was evidently largely 

 deposited during the rising of the lake, as it is so widely distributed over the 

 lake plain; it varies in thickness from 2 to 19 inches, and in character from fine 

 sand to coarse gravel. 



At this time the shore of Lake Chicago (Plate XXXV) extended from west 

 of Wilmette south to Jefferson Park, Austin, Riverside and Lyons, and thence 

 down the Desplaines Valley, below the line of the Glenwood beach. On the 

 south side' the beach follows pretty closely the Glenwood line east to a point 

 just south of Palos Springs, thence southeast to near Thornton, from which 

 point it runs east and northeast into northwestern Indiana and southwestern 

 Michigan. Mt. Forest island and Blue Island were joined at this stage, form- 

 ing a rather long, narrow piece of land, bent in the form of a bow. Washington 

 Heights marks its eastern limit and Summit its northern limit. A hook and 

 small bar were formed southeast of the former village. Extending from the 

 northern point of this island, in a northwesterly direction, was a large bar which 

 terminated in a strong hook (composed of three overlapping branches) bent 

 to the south. This bar evidently partly enclosed a small bay 23 or lagoon, with 

 marshy borders, which was of considerable extent and varied in depth from a 

 few inches to ten feet. The crest of Lane's Island was just above the water, 

 which rushed thru the Sag outlet in a stream from one to over two miles in 

 width. 



25 See Goldthwait, Records of Extinct Lakes, p. 61, for an explanation of this "stoping" 

 process. 



28 Op. oil., p. 60. 



27 Goldthwait, The Desplaines Valley, pp. 54, 55. 



21 For convenience this may be called Summit Bay. 



