TERRACES AND VALLEYS IN WESTERN PENNSYLVANIA 149 



To sum up, the inferred history of the terraces reads about as 

 follows. The Allegheny was overloaded at a certain point. The 

 material was spread out evenly from the place of overloading. 

 On each tributary stream deposits accumulated, first at the point 

 of junction with the overloaded one, then farther and farther 

 upstream. These processes continued until the load and gradient 

 of the Allegheny were so adjusted that the river was able to carry 

 its load. Later, probably on account of an elevation of the land, 

 the stream has cut through its deposit and 200 feet below the level 

 of the old rock floor. 



"abandoned channels" 



In close association with the high terraces are the many so-called 

 abandoned channels or side tracks to the main lines of drainage. 

 Examples are found not only in western Pennsylvania, but along 

 the Ohio, Mississippi, and a large number of tributary streams. 

 Genetically these features seem to be similar, though some devel- 

 oped early in the Quaternary period and others later. 



The abandoned part of the Monongahela valley at Carmichaels, 

 Pa., referred to on p. 142, has been described as containing evidence 

 of a huge local dam of ice, but to the present writer the evidence 

 did not seem to indicate a local barrier for the following reasons: 

 (1) The deposit thins at the position of the supposed dam not 

 abruptly, but irregularly, and a mile or more below considerable 

 thicknesses are found. (2) Just below the place of thinning, the 

 formation extends up the valley side to the altitude of the upper 

 limit of gravel, and a little farther away are extensive bodies of the 

 deposits, fully 100 feet thick. (3) The thinner parts are found 

 at a place where erosion has been very severe — where the gravel 

 has been dissected by a good-sized tributary. It appears, therefore, 

 that the thin part of the deposit is simply a result of irregular clear- 

 ing-out of the old valley by the tributary and is a feature to be 

 expected. The stream seems to have cut down quickly through the 

 silt and gravel, but when it came to hard rock it hesitated, mean- 

 dered a little, and then cut down farther, leaving the shelf covered 

 with pebbles and bowlders concentrated from the original deposit. 

 The fact that just below the site of the dam the formation is found 



