210 GEOLOGY OF OHIO. 



long after the bowlder clay, when water filled the lake basin, and they 

 are largely due to the action of shore waves and of the streams which 

 drained the high lands back from the Lake, and brought down sand and 

 gravel from their sources. 



The bowlders which are scattered abundantly over the county must 

 have been transported from the Canadian highlands by icebergs, as I 

 have shown elsewhere (Vol. I., Part I., p. 183). 



GEOLOGICAL STRUCTURE. 



The rocks which immediately underlie the surface in Lorain county 

 are, with the exception of a single exposure on the lake shore, portions 

 of the great Carboniferous system, and belong to the Lower Carbonif- 

 erous, or Waverly group. They include all the members of the Waverly 

 group, and nearly the entire thickness of the formation. The succession 

 of rocks in the county is as follows, beginning with the highest and 

 descending to the surface of the Lake : 



1. Cuyahoga shale, average thickness, 150 feet . 



2. Bereagrit, " 



3. Bedford shaJe, " 



4. Cleveland shale, " 



5. Erie shale, " 



6. Huron shale, exposed 



60 " 



70 " 



50 " 



100 " 



50 " J 



Waverly. 



Y Devonian. 



The lower two elements in the above section represent the summit of 

 the Devonian system; the others are all Waverly. The rocks enume- 

 rated form sheets which have a general dip in the State toward the south 

 and east, but within the limits of Lorain county this dip is reversed or 

 replaced by several local folds. It is not easy to say precisely what the 

 north and south dip of the rocks is, as the exposures are only superficial 

 in the southern part of the county. Taking the Berea grit, however, as 

 a guide, we find it in Brownhelm, within a quarter of a mile of the 

 Lake, where its base has an altitude of less than 100 feet above the 

 Lake. In Amherst it lies 140 feet above the Lake, while in the valley 

 of Black River, at Elyria, it is but 65 feet. Toward the eastern margin 

 of the county it rises again, reaching an altitude of 140 feet. This latter 

 arch is strongly marked on the lake shore, where the strata are seen 

 rising westward from Rocky River to Avon Point and dipping again to 

 the west, half way between Avon Point and Black River. 



Cuyahoga Shale. — All the southern half of the county is underlain by 

 the Cuyahoga shale, the uppermost member of the Waverly group. This 

 formation consists of blue or gray argillaceous shale — frequently called 



