290 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



Pike and Scioto counties, about 30 miles below. The surface in that dis- 

 tance falls from fully 700 feet above tide to 650 feet or less, while the rock 

 shelf, or old valley floor, rises from below 600 feet to fully 625 feet. The 

 material in this tei'race varies from a well-assorted gravel of medium coarse- 

 ness to a sandy and clayey gravel very imperfectly assorted. As a rule, 

 the gravel contains a large amount of sand. The current was probably 

 moderate except, perhaps, on the immediate borders of the ice sheet. The 

 gravel, like the drift from which it was derived, contains only a few Cana- 

 dian rooks, yet they have been found as far down as Coopersville. 



In an old oxbow back of Lucasville, discussed in Chapter III, no 

 glacial deposits were noted, but pebbles were found on the inner slope up to 

 a level above the probable limits of the filling by glacial gravel. These 

 pebbles, as well as the material in the bottom of that oxbow, may be older 

 than the glacial gravel. The altitude of the bottom of the oxbow is fully 

 as great as the glacial gravel at Coopersville, and iio Canadian rocks were 

 found in it after prolonged search. 



MIDDLE AND LOWER OHIO VALLEY. 



The portion of the Ohio Valley which was covered by the ice sheet, 

 from near Maysville down to the vicinity of Louisville, Ky., received a 

 large amount of drift, some of which is evidently waterlaid. It is not, 

 however, so distinct an outwash from the ice sheet as that found in valleys 

 to the east, already discussed. Much of it was probably deposited as directly 

 by the ice sheet as the till of the bordering uplands. It has accordingly 

 been treated in connection with the drift sheet. By reference to these 

 descriptions, it will be seen that the filling varies greatly in constitution, as 

 is to be expected in a partially obstructed valley. 



The encroachment of the ice sheet on a part of the Lower Ohio at the 

 lUinoian stage of glaciation may seem to oppose the view that some of the 

 gravel carried down the Scioto was of Illinoian age. It seems probable, 

 however, that the drainage through the Ohio was obstructed for only a brief 

 time if at all. The glacial boundary at its farthest point extends but 10 or 

 12 nules beyond the Ohio, and generally but 2 to 5 miles. It is not certain 

 that so slight an extension could cause complete damming-. 



Furthermore, the ice sheet extended beyond the Ohio during only a 

 small part of the Illinoian stage of glaciation, thus leaving- the valley open 



