The Rivers and Valleys of Pennsylvania. 



209 



consumed in the lower part. The location of the successful 

 lateral stream on one or the other side of the syncline may be 



Fig. 11. 



Fig. 12. 



Fig. 13. 



determined by the dip of the beds, gaps being cut quicker on 

 steep than on gentle dips. If another hard bed is encountered 

 below the soft one, the process Avill be repeated ; and the mature 

 arrangement of the streams will be as in fig. 13 (on a smaller 

 scale than the preceding), running obliquely off the axis of the 

 fold where a hard bed of the syncline rises above baselevel, and 

 returning to the axis where the hard bed is below or at baselevel ; 

 a monoclinal stream wandering gradually from the axis along the 

 strike of the soft bed, AE, by which the side-valley is located 

 and returning abruptly to the axis by a cataclinal* stream in a 



* See the terminology suggested by Powell. Expl. Col. R. of the "West, 

 1875, 160. Thi& terminology is applicable only to the most detailed 

 study of our rivers, by reason of their crossing so many folds, and 

 changing so often from longitudinal to transverse courses. 



