156 EUGENE WESLEY SHAW 



SUMMARY 



Summarizing, the method of development of the high terraces 

 and abandoned parts of valleys of western Pennsylvania seems 

 to be as follows: (1) The development of a valley train over 100 

 feet thick, along the Allegheny and Ohio; (2) from the beginning 

 the aggradation of this stream produced an effect felt on every 

 tributary, and a portion of each, beginning at its mouth and extend- 

 ing gradually upstream, became silted up. (The lower end of 

 each tributary valley thus took on a form resembling the half- 

 filled character of the valley of the master stream.) (3) As the 

 rivers built up they found themselves flowing at the height of one 

 after another of the lowest places in near-by divides, and at such 

 times and places the currents were divided and the cols were occu- 

 pied. This overloaded condition of the streams lasted a long time 

 and there were many fluctuations, for at some places, as at Pitts- 

 burgh and Belle Vernon, there are two or three well-developed 

 valleys side by side. (4) When final redissection began, the 

 rivers chose the channels momentarily most desirable. In most 

 cases the short route was the principal factor in the choice, but 

 in others the largest current at the time and other comparatively 

 trivial conditions determined the courses of the streams. As in 

 all cases of superimposition, the resistance of underlying rock 

 played no part in their location, and at many places the rivers 

 soon found themselves sawing into hard rock where near by were 

 courses through unconsolidated materials. 



