66 LIFE OF THE PLEISTOCENE 



consistency, to coarse gravel, approaching shingle. This stratum has been 

 encountered everywhere in the Chicago basin where excavations have been 

 made. On the beaches and bars the sand is piled up to a depth of 17 to 25 

 feet. The Fullerton Avenue conduit, which extends from the Chicago River 

 into the lake, cuts thru the lower beach (on Clark Street) giving an excellent 

 section of this formation. Of this conduit Leverett says, 11 "The sand has its 

 greatest thickness at about 1,700 feet from the lake, where it reaches 25 feet. 12 

 It decreases westward to 17 feet at 2,100 feet from the lake, and to 12 feet at 

 6,000 feet, 13 and entirely disappears before reaching the Chicago River valley. 

 Toward the lake shore it also decreases, being about 18 feet in depth for 1,400 

 feet west from the shore. At the waters edge the depth is about 10 feet. The 

 profile continues out 1,100 feet below the lake, and there is but 3 feet of sand 

 at its terminus." 



The deposits discussed above include the material which formed the Grace- 

 land bar and the Toleston beach and are not homologous with the sand and 

 gravel deposits discussed under this section, altho they probably include these 

 deposits in the lower part of the strata. Altho Leverett states that the sand 

 entirely disappears before reaching the Chicago River valley, the writer 

 observed sand and gravel deposits in various localities for a considerable dis- 

 tance down the valley on the banks of the river, near the water's edge. South 

 of Lawrence Avenue, they are mostly below the surface of the river. 



4. Resting on and in the upper part of the gravel deposit is a heavy bed of 

 Unios and other mollusks. This bed has been observed in many parts of 

 Chicago, and its presence is assumed to be as universal as are similar beds 

 which are found today in our large rivers. In the north shore channel, they 

 have been observed from Montrose Boulevard to Devon Avenue, a distance 

 of about two and one-half miles. The same deposit occurs at Austin Avenue 

 and Ogden ditch, and in the Calumet-Sag channel. North of Devon Avenue 

 this deposit does not occur, showing that the water was not deep enough for 

 the heavy Unios. 



5. Above the Unio beds occur deposits of silt, sand, peat, and marl beds, 

 varying from 19 to 59 inches in thickness. The lower deposit of silt or sandy- 

 silt is usually devoid of life; but the upper layers are filled with mollusks and 

 fish remains representing species which live in comparatively shallow water. 

 In several localities distinct marl beds occur which are solid masses of shells. 

 The upper part of the silt deposits is oxidized, giving indisputable evidence of 

 a former land surface. This stratum also contains the burrows of crayfish 

 and the remains of Potamogeton, Chara, and Najas, all evidences of shallow 

 water. North of Devon AvenUe the silt deposits above the gravel contain the 

 remains of mollusks which live in very shallow water or in swales and marshes. 



n Clark Street, the crest of the Toleston beach. 

 13 West of the St. Paul tracks. 



