PREGLACIAL VALLEY OF THE ROCKY RIVER. 333 



The latter valley, if we could ascertain its average rate of cutting, would 

 yield a measure of the Postglacial period ; but this problem is not here 

 considered. Indeed, nearly the same conditions may be said to be pre- 

 sented not less conveniently by many other streams of this region, from 

 one of which, in Oberlin, Ohio, Professor G. F.Wright computes the dura- 

 tion of this period to have been between five and ten thousand years. 



■ SIZE AND COURSE OF THE PREGLACIAL VALLEY. 



Distinguished by its cliff section of drift uniform in height with the 

 shale cHffs forming the lake shore on each side, the mouth of the pre- 

 glacial Rocky river valley has a width slightly exceeding one mile, from 

 about three-fourths of a mile to nearl}^ two miles west of the present 

 river's mouth, and between 83 and 9j miles west of the Public Square at 

 the center of (Cleveland. The course of this old valley has been carefully 

 studied out and mapped, from well records and rock outcrops on either 

 side, along all its extent through Cuyahoga county, by Dr D. T. Gould, 

 of Berea, Ohio.''"^ The preglacial river course, supposed to average about 

 a mile in width, and in its last five miles probably reaching 200 feet or 

 more below the level of lake Erie, is crossed by the present river about 

 four miles above its mouth. Thence, in going up the Rocky river as it 

 now flows, one travels at a distance of 1 to 2 miles west of the old val- 

 ley along the next 10 miles to the south ; but at a distance of about 

 fourteen miles south of the lake Dr Gould believes that the preglacial and 

 postglacial valleys coincide, their further upward extent for at least sev- 

 eral miles to the southeast and south being the same. 



COMPLETE FILLING OF THE PREGLACIAL VALLEY^ WITH DRIFT. 



The Rocky river is a shorter and smaller stream than the Cuyahoga, 

 their ratio as to area of drainage and volume of water being approxi- 

 mately the same as that of their preglacial valleys. How the ice-sheet 

 acted to amass its drift in great depth, filling the old valley of Rocky 

 river to its brim, so markedly in contrast with the glacial drift of the 

 Cuyahoga valley, seems a difficult question. Tlie drift so deposited in 

 the mainly northwardly trending Rocky river valley is chiefly till or 

 boulder-cla}^ and it was doubtless formed gradually as a subglacial de- 

 posit, excepting its upper part, as much, but probably not all, of the 

 drift cliff on the lake shore. The trough-like valley appears to have 

 slowly caught subglacial drift until it became filled ; for the engiacial 

 and at last superglacial drift would be a rather uniform sheet, and the 



* Tract 70, in the Publications of the Western Reserve Historical Society, Cleveland, vol. ii, pp. 

 479-490, with map, Feb. G, 1886. 



