TERRACES AND VALLEYS LN WESTERN PENNSYLVANIA 155 



was, at one place, a little less than 100 feet above the river-valley 

 floor. As the river rose it dropped some coarse materials in the 

 ends of the tributary valleys, but a mile away the ponded water 

 was quiet and the deposit fine. This process continued until the 

 Allegheny reached the elevation of the lowest point in the divide 

 between the streams, about 3 miles away. Then the river current 

 was separated and a part flowed slowly up one tributary and down 

 the other, carrying some coarse and much fine material (varying 

 from season to season) and shaping the col into the form of a 

 valley. Finally the river cut down again, abandoning the course 

 which it had occupied temporarily for the much shorter original 

 course. 



A part of the history of the loop is reflected also in its present 

 drainage. When the river left it, the run-off was naturally in the 

 direction in which the river had flowed, out one arm and back 

 down the other. But a new small tributary in the position of the 

 upper one of the old ones is now cutting back into the upper end 

 of the abandoned valley, driving the head of the other stream back 

 and annexing a part of its unnatural drainage basin. 



All of the other abandoned parts of the valleys of this region 

 have been examined carefully and seem to have been developed 

 in the same way — by silting up and redissection — and the process 

 is the same whether the case be on the Allegheny or on one of the 

 tributary streams. In many cases the new courses made available 

 by the silting-up of the old channels were about as direct as the old, 

 and in certain of such cases the stream cut down in its old course, 

 while in others it assumed a new course. Thus some of the aban- 

 doned valleys mark courses temporarily occupied by the rivers, 

 while others show old and long-used courses. It is a significant fact 

 that the changes were, in nearly every case, from a longer route 

 to a shorter one. This would scarcely have been true had the 

 rivers been driven from their courses by ice dams. There are some 

 cols which stand just a few feet higher than the highest gravel, 

 and these were, of course, never crossed by the rivers. Indeed, 

 if aggradation had proceeded 50 or 100 per cent farther there would 

 have been an amazing network of long and devious "abandoned" 

 valleys. 



