834 W. UPHAM — PKEGLACIAL AND POSTGLACIAL VALLFA'S. 



drift above the shale on each side of the old channel has often no greater 

 thickness than from 3 to 10 feet. 



THE POSTGLACIAL ROCKY RIVER VALLEY. 



Along its last five miles the present Rocky river flows in a gorge-like 

 valley of postglacial erosion, from a sixth to a third of a mile wide and 

 mostly 80 to 90 feet deep. On account of the lakeward slope of tlie gen- 

 eral plain through which this gorge is channeled, its depth where it ends 

 in tlie escarpment forming the south shore of lake P]rie is reduced to 50 

 or 60 feet. It is cut down somewhat below the lake level along its last 

 three-fourths of a mile, giving evidence that its erosion was far advanced 

 before the relations of the land and the lake became as now. Another 

 proof of this is the absence of a delta at the mouth, showing that the 

 large mass of alluvium derived from the channeling; of the vallev was 

 mostly borne forward by the stream flowing on what was formerly a land 

 area to the central parts of the ])resent lake bed. 



At Scenic park, a pleasure resort on the bottomland of this postglacial 

 valley in the southwest corner of section 23, Rock port, the east blufl* is 

 well covered by talus and trees ; but the west bluff, at the base of which 

 the river runs, is vertical, about 90 feet high, consisting wliolly of shale, 

 excepting 3 to 5 feet of till on its top. This locality is about three- 

 fourths of a mile from the mouth of the river. The shale is a soft and 

 easily eroded rock, scarceh' more resistant to water-wearing and weath- 

 ering than the usual boulder-clay or till; but the amount which the 

 river has carried away is very impressive. At first thought it might seem 

 to imply a geologically long postglacial period ; but when the probable 

 erosion of each year is made a divisor, the length of time re(iuired for 

 the whole work is found to be only a few thousand years. During the 

 recent time, comprising i)robably the greater part of this period, while 

 the lake has held its i)resent level, eroding the cliffs of all its south shore, 

 as along the front of Cuyahoga county, the eastward shore currents and 

 powerful waves of storms have swept away the delta tribute of both the 

 Cuyahoga and Rocky rivers and the detritus furnished by the lake cliff 

 erosion, bearing the alluvium and the detritus of the wave-cutting east- 

 ward and outward to be laid down on the shallow lake bottom. 



Drift Sections. 



lake erie cliff crossing the preglacial rocky river valley. 



The most interesting drift section found in the vicinity of Cleveland is 

 the lake cliff along its extent of a little more than a mile where it crosses 

 the drift-filled preglacial valley of the Rocky river, because it testifies of 



