TERRACES AND VALLEYS IN WESTERN PENNSYLVANIA 147 



becomes thinner and narrower upstream, and at a distance of 20 

 miles from the Allegheny the formation has the width, thinness, 

 and coarseness of an ordinary flood-plain deposit. 



These facts suggest at once that the Clarion terraces owe their 

 existence to conditions on the river into which that stream dis- 

 charged. When the Allegheny began to aggrade, the effect was that 

 of a gradually growing dam across the mouth of the Clarion. This 

 caused the latter stream to drop the coarsest part of its load. The 

 dam did not grow so rapidly as to produce a pond in the river above, 

 but aggradation kept pace with the growth of the dam. In other 

 words, at its mouth the Clarion built up as rapidly as the Allegheny. 

 This is shown by the even downstream dip of the Clarion gravels 

 and by their coarseness. If any ponded stage had existed, the 

 deposit would have been coarse only at the upper end of the pond 

 and would have taken the form of a delta. 



But of course the dam did not affect the Clarion throughout 

 its length. On the contrary, when the dam began to grow, its 

 influence was felt only in that part of the stream immediately 

 above. As it grew the area affected by it extended farther and 

 farther from the Allegheny and the river built up to a new gradient, 

 over which it was just able to carry its normal load. The coarser 

 part of the gravel was dropped where the gradient changed from 

 the old to the new. This point gradually moved upstream and the 

 extended coarse deposit became the basal coarse part of the forma- 

 tion. The Clarion then silted up because its master stream, the 

 Allegheny, was aggrading, and the elevation of its outlet was being 

 raised. The Allegheny aggraded because of great increase in load, 

 the Clarion because of decrease of gradient. The absolute load of 

 the latter stream has not changed materially since the dawn of 

 the Quaternary period. 



Space will not permit of complete description of all the high 

 terraces, but the work of the Clarion may be taken as a type of 

 the work of those streams which discharged into the overladen 

 Allegheny and Ohio. Redbank Creek, the Conemaugh, Kiski- 

 menitas, Youghiogheny, and Monongahela show similar characters. 

 On all except the smallest of the tributaries of the Allegheny, there 

 are deposits connecting with the early glacial valley train, such 



