TERRACES AND VALLEYS IN WESTERN PENNSYLVANIA 145 



clay, though in some places there is so little fine material that the 

 gravel is dug from pits and used without further washing. In 

 such places beds of gravel are separated by lenses of clay, but on 

 the whole the formation is homogeneous. 



That the deposit is of fluviatile and not lacustrine origin seems 

 to be shown decisively by the characters to which Chamberlin 

 has called attention: The deposit slopes regularly with the present 

 streams throughout their winding courses. A lake deposit would 

 be horizontal unless affected by crustal deformation, and in that 

 case the slope would not change direction at just the places where 

 the course of the river changes. Second, the material is distinctly 

 fluvial, consisting of irregularly bedded gravel which contains 

 lenticular masses of silt and clay. A lake deposit in a valley might 

 have deltas containing some coarse material, but in no way could 

 coarse glacial debris, poured into the end of a narrow lake 100 or 

 more miles long, be evenly distributed so that the resulting forma- 

 tion throughout its length would be homogeneous and of uniform 

 thickness. 



There seems to be good evidence also, as Leverett has pointed 

 out, that in pre-Glacial time the Clarion was the headwater portion 

 of the Allegheny, a divide crossing the present course of the latter 

 stream just above the mouth of the former, and that the glacier, 

 by cutting off the outlets of the drainage of the area to the north, 

 forced the water to cut across the divide to the old Lower Allegheny, 

 thus thrusting greatness upon the Allegheny basin. 



Through the new cut were discharged great volumes of glacial 

 outwash — too great for the Allegheny to transport — and the 

 coarsest part of the debris was spread along the bottom of the valley, 

 forming a typical valley train which had a nearly uniform thick- 

 ness throughout its length. The bodies of gravel on the high 

 terraces of the Allegheny and Ohio, then, are the remnants of this 

 old valley train. 



The overloaded condition of the Allegheny was probably due 

 to several causes, among which the following may be mentioned as 

 being more or less effective: First, an actual increase in load 

 derived from (a) material fed more or less directly to the streams 

 by the glacier; (b) debris from the cutting of new gorges across 



