60 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



formations. The entire group here consists almost wholly of shale, there 

 being- few layers of hard rock so much as a foot in thickness. Because 

 of this absence of hard rock the slopes are worn down to regularity and do 

 not present the benches or abrupt changes characteristic of the eastward 

 extension of the upper members of the group. 



The area of outcrop extends southward well toward the head of Grand 

 River Basin in northern Trumbull County, but returns on the west side of 

 that basin to within 10 miles of Lake Erie. It extends up Chagrin Valley 

 to about Chagrin Falls, 20 miles from the Lake, and up the Cuyahoga 

 Valley about 25 miles, or nearly to Peninsula. Aside from these exten- 

 sions its outcrop from the Graiid River Basin westward to the Huron 

 River is but 5 to 15 miles wide, with an average of perhaps 10 miles. It 

 extends northward beneath Lake Erie to an undetermined distance. The 

 basin in which Lake Erie lies no doubt owes its existence chiefly to the soft 

 and friable character of the rocks of this group. The thickness of the group 

 near the south border of its outcrop south of Lake Erie is found to be 

 about 950 feet at Elyria and 1,300 feet at Cleveland.^ It apparently 

 becomes much thicker in passing eastward from Cleveland into Pennsyl- 

 vania and New York. 



From near the mouth of Huron River, in Erie County, the belt of out- 

 crop of this group tm-ns abruptly southward, passing west of Norwalk, 

 Gallon, and Mount Gilead, and east of Delaware and Columbus. Below 

 Columbus it occupies the Scioto Valley to the vicinity of Chillicothe and 

 also for a few miles below Waverly. It appears also in valleys or narrow 

 strips of lowland to the west of the Scioto, to and beyond the Ohio Valley. 

 Its thickness, as reported hj Ortou, decreases from about 900 feet on the 

 borders of Lake Erie to 450 feet in Crawford County, to 350 feet in Ross 

 County, and to about 250 feet in Highland County. 



Throughout the entire line of outcrop from the Ohio River nortliward 

 to Lake Erie, as well as eastward into Pennsylvania, this group consists 

 almost entirely of soft shales. These have not resisted erosion as well as 

 the overlying beds, and hence are found mainly in basins or at the base of 

 hills. In this respect they are in contrast with the New York portion of the 

 outcrop, which, as above noted, occupies some of the highest ridges and 

 hills of the western part of that State. 



^See Orton: Geology of Ohio, Vol. VII, p. 24. 



