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 dewn 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 ......... .....0000 covers | 
2. Berea grit, “ it GOH ie seds sue Gee gee Re ees 
ly. 
3. Bedford shale, s rf AQ) Sl PRiaa tects ec ole epee | Waverly 
4, Cleveland shale, ‘“ <s BOE aren Gah SI toe anne J 
ae 66 66 és 
5. Erie shale, LOO er Ga eeu nen ene eenonnen Deora 
6. Huron shale, exposed ‘ BO 5k a icieetca sate acerca 
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 
