80 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



200 feet or more, that it is a difficult matter to locate the precise position of 

 the old divides. The term " through-cut valleys " has been applied to them, 

 but this seems an inappropriate name for such as have rock floors sloping in 

 opposite directions from a concealed divide. They differ from abandoned 

 channels, which are of somewhat frequent occui'rence in the portions of the 

 region where the drift is heavy, for in these chamiels the rock floor has a 

 slope in but one direction, there being no old divides buried along their 

 courses. Several of the abandoned channels will be discussed below. 



The valleys of this hilly country pi-esent marked diftei'ences in topog- 

 raphy. In some valleys the slopes from top to bottom have a mature 

 aspect, while in others the upper part of the slope is mature but the lower 

 part is gorge-like and youthful in appearance. The phenomena suggest at 

 once that some valleys have remained below the level of stream cutting 

 while others have been undergoing a marked trenching. In those which 

 have been deepened the old valley bottoms are traceable as terraces along 

 the brow of the rock gorges or canyon valleys, for the old valleys are 

 generally broader than the new ones. In some cases, however, the new 

 valleys occupy the full width of the bottoms of the old ones, and there is 

 only the change in the angle of slope of the valley bluff to mark the depth 

 reached by the old valley. There is in some valleys a series of complex 

 terraces or rock shelves, of which one set or system stands at the brow or 

 border of the canyon valley and the others at higher altitudes. There are 

 also in some cases rock shelves inside the trenches of the canyon valleys. 

 The set of terraces standing at the brow of the canyon valley is, however, 

 a far more persistent feature than an}^ of the others, and it is this set which 

 receives chief attention in the ensuing discussion of drainage systems. It 

 seems to mark a true gradation plain, formed when the stream was in a con- 

 dition between degrading and aggrading its bed.^ 



Let us turn to this region for a few general illustrations of the above 

 statements. It may be noted that in western New York and the northwest 

 corner of Peimsylvania the gradation plains have been buried beneath the 

 heavy accumulations of drift, so that the streams are now flowing at levels 

 far above them. The same is true of much of the glaciated portion of the 

 hilly region in Ohio. On the middle portion of the Allegheny drainage 



' For use of terms see W. M. Davis: Jour. Geol., Vol. II, 1894, p. 77; also Chaniberliii and Leverett 

 Am. Jour. Sci., 3d series, Vol. XL VII, 1894, p. 255. 



