OUTWASH FROM THE OLDEST DRIFT. 237 



the rapid slopes of the outwash aprons. There seems to be a case of such 

 erosion by glacial floods in the trench above noted, which was cut in the 

 old di'ift at Clarendon to a depth of 100 feet and opened southward into 

 the Tionesta Valley. This is out of the reach of stream action in later 

 stages of glaciation and apparently seems referable only to the work of a 

 glacial stream connected with the earliest ice invasion. 



But conditions for erosion of this sort seem not to have been open to 

 any great extent on the Allegheny or its main tributaries at the time of the 

 first ice invasion. The trenching in general must have kept pace with the 

 slow deepening of the Lower Allegheny and Ohio valleys which followed 

 the filling with the gravel of this earliest glacial stage. The floods con- 

 nected with the earliest stage of glaciation would probably effect scarcely 

 more than the reduction of the upper part of the Allegheny to a gradient in 

 harmony with the contemporaneous gradation plain of the Lower Allegheny, 

 whose height after this gravel filling occurred was, as already shown, nearly 

 300 feet above the present stream. 



What propoi'tion of the erosion, both of the rock and of the drift, on 

 the Allegheny and its main tributaries is referable to glacial streams in 

 connection with later ice advances will be difficult to determine. It may 

 perhaps be determined by a careful comparison of work done by eastern 

 and southern tributaries which have not been aided by glacial floods with 

 that done in the glaciated tributaries which were thus aided. Such a com- 

 parison has not been attempted, but it is apparent from a merely casual 

 observation that the streams which have been unaided by glacial floods 

 have nevertheless opened valleys of sufficient size to warrant the inference 

 that glacial floods are responsible for only a minor part of the erosion dis- 

 played by the glaciated tributaries. 



CHARACTER OF THE OUTWASH. 



An outwash of gravel and sand appears in such portions of the Alle- 

 gheny River and some of the tributaries as were not obstructed by the 

 ice sheet, but were instead favorably situated for receiving it. It was dis- 

 tributed far down the valleys, reaching even into the upper part of the Ohio 

 Valley. The western tributaries of the Allegheny, as far down as the gla- 

 cial boundary near Kennerdell, were largely covered by tlie ice sheet, but 

 seem to have been utilized as lines of discharge for glacial waters as the 

 ice melted. 



