i 5 4 E UGENE WESLE Y SHA W 



sheet. The abandonment took place before the stream began 

 again to cut down, for deposits are found around the loop almost 

 as high as the highest gravel. The broad valley around the oxbow 

 was cut previous to this time. One can only conjecture how long 

 a period of time was necessary for this. 



There is some evidence that the rock floor of the east end of the 

 loop is higher than the Parker strath. If this be true the oxbow 

 must have been developed either in pre-Kansan time, before the 

 stream had cut as low as the Parker strath, or after the Allegheny 

 had aggraded until it was high enough to take this route. How- 

 ever this may be, the close association of the abandoned channel 

 with the high terrace, and the occurrence of Kansan material in the 

 channel, show that whenever it was formed, it was occupied and 

 abandoned in Kansan time. 



Third: The length, depth, and narrowness of the rock channel 

 through which the river now flows across the neck of the oxbow 

 suggests that the oxbow was not cut off in the way that streams 

 ordinarily cut off their meander, but points rather to superimposi- 

 tion. The present valley across the neck of the abandoned channel 

 is a narrow rock gorge over a mile long, and the top of the gorge 

 extends up to the level of the highest part of the old channel. 



Another abandoned valley which is thought to show the method 

 of development very well is found on the Allegheny, a few miles 

 northeast of Pittsburgh, and opposite Verona. The topography 

 suggests at once that this feature is a cut-off loop of the Allegheny, 

 and it is found on a level with the high terraces. The width is 

 nearly as great as that of the old valley of the Allegheny, and glacial 

 gravels are found in it. But on closer inspection it is found that 

 the width of the valley and the thickness of the deposit decrease 

 rapidly away from the present course of the river, and this through 

 a rise in the rock floor. Also there is an impressive amount of 

 fine material and a scarcity of bowlders. Finally, at the extreme 

 end of the loop the old valley, if such it be, is very narrow, and the 

 deposit but a few feet thick. 



The meaning of these features seems quite evident. At the time 

 the Allegheny began to aggrade, the position of this loop was occu- 

 pied by two small tributary streams. The divide between them 



