DESCRIPTION OF THE OLDEST DRIFT. 229 



a gravel. In valleys which discharged toward the ice margin and had their 

 lower courses obstructed by the ice sheet a large amount of silt or fine sand 

 is found under drift which was deposited directly by the ice sheet. This 

 is notably the case in the old valley that leads northward from the bend of 

 the Tionesta to the Allegheny. Carll reports that the filling to a depth of 

 over 200 feet is chiefly clay, and that only the surface carries large stones 

 of northern derivation.^ Several valleys lying within the limits of the Wis- 

 consin drift show a great amount of fine material under the stony drift, and 

 this, as in valleys outside the limits of the Wisconsin, was probably 

 deposited hefore the first ice invasion or contemporaneously with it. 



In the portion of the Allegheny Valley which was either covered or 

 closely bordered by the ice sheet, viz, that from Warren down to Kenner- 

 dell, stream action was probably interrupted or more or less intermittent. 

 As a result there is on the whole a less uniform and less clearly assorted 

 deposit than in the portion below Kennerdell, which was not obstructed 

 by the ice sheet. Portions of it, however, are as well assorted as in the 

 terraces outside the limits of glaciation. 



The bowlders found in this old drift are remarkably small, it being 

 rare to find one that exceeds 2 feet in diameter, Avhile the great majority 

 are less than 1 foot. Many of them are of a red granite which has become 

 so decayed that a single bloM^ with a hammer will knock the rock to pieces. 

 Both in size and in state of decay they are strikingly in contrast witli the 

 bowlders found on the Wisconsin drift, there being many large and fresh- 

 looking bowlders in that drift. The rocks contained in this old drift, and also 

 those in the Wisconsin drift of northwestern Pennsylvania, are very largely 

 derived from the Devonian and Carboniferous rocks of that region. Appar- 

 ently less than 1 per cent of the coarse rock ingredients has been derived 

 from the region beyond Lake Erie. 



The state of decay of the local as well as of the foreign stones in this 

 old drift, and also the great amount of erosion it has sustained, put it 

 in striking contrast with the fresh-looking and but slightly eroded Wis- 

 consin drift. Nearly all the pebbles found on the surface of the old di'ift 

 have become so deeply weathered that it is often necessary to break them 

 in order to obtain a sufiiciently fresh surface to warrant classification. On 

 the surface of the Wisconsin drift the same classes of pebbles are still so 



' Second Geol. Survey Pennsylvania, Rept. I*, p. 353. 



