STREAM CAPTURE 327 



eroded rocks, while its rival is embarrassed by hard rocks and 

 ledges. Another favourable circumstance which may decide be- 

 tween streams otherwise equal is given by the attitude of the strata. 

 In regions of inclined strata, as we have already learned, the 

 escarpments formed by outcropping ledges of harder rocks tend 

 to migrate in the direction of the dip. As such escarpments 

 frequently form divides between minor streams, the stream towards 

 which the escarpment migrates will be at a disadvantage. This 

 shifting of divides is a very slow process, but after a long time of 

 insidious advance the actual capture and diversion of part of a 

 stream may be quite suddenly effected. 



Stream capture may be effected in a great variety of ways, but 

 a few examples must suffice. We may, in the first place, suppose 

 two neighbouring streams following roughly parallel courses, but, 

 owing to the original conformation of the region, flowing at 

 different levels. The stream that flows at the lower level will 

 allow greater fall to its tributaries, which will thus work upward 

 more rapidly. One of these tributaries will eventually work 

 its way through the divide and tap the rival stream, all of whose 

 waters above the point of tapping will be diverted to the main 

 stream which flows at the lower level. 



Another method of stream capture is well illustrated by the 

 Delaware, the Potomac, and other transverse rivers which have 

 cut deep gorges through the Appalachian ridges. Suppose two 

 parallel transverse streams flowing across a gently sloping pene- 

 plain which is composed of tilted rocks of different degrees of 

 hardness. In the manner already explained (p. 317) these streams 

 cut gorges through the ridges of hard rock, while longitudinal val- 

 leys are worn, out along the strike of softer strata, which valleys are 

 occupied by tributaries of the transverse streams. If one of the 

 two transverse streams be considerably larger than the other it will 

 saw its way through the hard ridges at a correspondingly faster rate 

 and establish a lower base-level for its ^tributaries. One of the 

 tributaries with its more rapid fall will be thus enabled to shift its 

 divide at the expense of a branch of the rival transverse stream, 

 capture it, and by reversing the direction of its flow draw off the 



