TERRACES AND VALLEYS IN WESTERN PENNSYLVANIA 143 



grew until they formed huge dams 100 or more feet high, and that 

 they persisted until deposits over 100 feet thick accumulated above 

 them. In many cases these dams not only gave rise to terraces 

 but caused the rivers to abandon their old valleys and cut new 

 ones. 



In the Amity folio Frederick G. Clapp expresses the belief that 

 Professor White's theory — that of ponded waters throughout much 

 of western Pennsylvania — will best account for the phenomena. 

 He states that the upper limit of the stream deposits in all the val- 

 leys of southwestern Pennsylvania and parts of adjacent states 

 has a vertical range of but little over 100 feet, but since Mr. Clapp's 

 work was published the gravel has been found to lie at an elevation 

 of over 1,200 feet on Clarion River, making the vertical range more 

 than 200 feet. 



The data gathered by the present writer, instead of lending 

 support to any one of these views, seem rather to indicate that the 

 high terraces and abandoned channels on all the rivers developed 

 as a unit, through the overloading of the Allegheny in early glacial 

 time. 



The terraces may be divided into two groups, which have certain 

 essential differences. Those of the first group are capped with 

 glacial gravel, and are found along. the Allegheny and Ohio. Those 

 of the second bear material of local derivation, and are found on 

 streams tributary to the Allegheny and Ohio. There are other 

 differences which will be brought out later. In this connection 

 it should be stated that there are a few remnants of older gravels, 

 which lie at various elevations above the main high terrace forma- 

 tion, and in some places have been let down by erosion, so that they 

 seem to connect with the much more extensive deposit below, but 

 the older gravels have very slight extent. 



TERRACES OF THE ALLEGHENY AND OHIO 



The terraces of the Allegheny and Ohio are almost continuous 

 from the mouth of the Clarion to Pittsburgh, and on down the 

 Ohio. The gravel deposits on them are thin or absent where crossed 

 by lateral streams; in other words, where erosion has been most 

 severe; but enough remains to indicate clearly the position of the 



