Geology of Upper Illinois. 



135 



try, or one whose rapid descent might measurably compensate 

 for want of area in giving rise to alluvial deposits. Neither do 

 rivers of any magnitude find their outlet here, which, like the 

 St. Clair where it enters Lake St. Clair, might produce flat plains 

 of considerable extent. Its origin seems to have been connected 

 with a higher level of the lake, when its waters advanced inland 

 quite to the rolling prairie. Nor would this supposition be at all 

 satisfactory perhaps, except for the knowledge we possess of the 

 almost universal, rocky substratum which prevails over the wet 

 prairie, coming for the most part to within a few feet of the top 

 of the ground, — thus giving us the conditions of a hard bottom 

 as forming the shore of the lake, upon which the sediment and 

 wash of the coast was in the progress of ages spread out. The 

 deposit covering this rocky floor, is a horizontally stratified blue 

 clay, on top of which at Chicago, rests a yellowish clayey loam. 



Xa/u Mi/^Id^aTi 



On the subsidence of the lake to its present level, the beach- 

 line in the region of Chicago must have begun to form. For a 

 long distance up and down the lake, it is confined to one or two 

 embankments ; but on drawing near the head of the lake, by the 

 way of the road to Michigan city, we find the surface of the 

 prairie invaded far inland by a succession of ancient beaches, 

 formed with the utmost regularity as to width and height, as 

 well as conformity to the existing shore of the lake. I shall de- 

 scribe them as they came into view on the stage road, endeavor- 

 ing to render their character the more intelligible by means of the 

 above sketch, constructed from recollection. Leaving Chicago, the 



