394 THE ILLINOIS GLACIAL LOBE. 



bowlders are composed very largely of pre-Cambrian crystalline rocks of 

 Canadian derivation, rocks from the Paleozoic formations being compara- 

 tively rare. But the till contains a large number of local rocks, and its rock 

 constituents vary from place to place, following in a general way the 

 changes in the underlying rock formations. Large bowlders of sandstone 

 were found on Covert Ridge in western Allegan County similar to those 

 noted on the Valparaiso moraine in Allegan and Van Buren counties. A 

 bowlder of red jaspery conglomerate, apparently from the Huronian out- 

 crops north of Georgian Bay, was found on the Covert Ridge, a short 

 distance northeast of East Saugatuck. The ice movement which formed 

 the ridge can scarcely be supposed to have brought the bowlder from the 

 parent ledge, for the line of axial movement was southward through the 

 Lake Michigan Basin, and the radial movementwas southeastward toward this 

 ridge. The presence of this bowlder is probably to be accounted for through 

 an earlier ice movement which crossed Michigan and the Lake Michigan 

 Basin in a southwestward course, strewing bowlders along its path to be 

 taken up by the later movements and redeposited in the later till sheets. 



An excellent section of the structure of Covert Ridge is found on 

 the shore of Lake Michigan a few miles north of New Buffalo, in sec. 25, 

 T. 7 S., R. 21 W. The lake bluff here is about 95 feet in height. At the 

 surface there is a coating of sand in places 6 or 8 feet in depth, but in other 

 places only a few inches. Beneath it there is a brownish-yellow till which 

 graduates into blue-gray till within 8 or 10 feet of the surface. For a 

 depth of about 60 feet the till shows distinct lamination and has thin part- 

 ings of sand and laminated clay. Below this depth it is somewhat harder 

 and the lamination is very imperfect. It is 1 probable that the lower 30 feet 

 of the exposure is independent of the upper 60 feet. The latter alone 

 appears to be referable to the stage of glaciation at which the ridges under 

 discussion were formed. About one-half mile southwest from the point 

 just described the lake bluff presents the following series: 



Section of Covert Ridge, near New Buffalo, Michigan. 



Feet. 

 Beach gravels - • 8 



Brown laminated clay, slightly pebbly, with thin partings of sand H 



Blue laminated clay, slightly pebbly, with thin partings of sand 14 



Coarse water-bearing sand 2 



Blue-cray till, very stony, interbedded -with thin layers of sand in horizontal beds, extending 

 down beneath the level of the lake, exposed 30 



Total 55i 



