328 ADJUSTMENT OF RIVERS 



waters of the smaller main stream above the point where its capt- 

 ured tributary entered it. Or, a tributary of the larger main stream 

 may push its way up a longitudinal valley until it taps and diverts 

 the smaller transverse stream without the intermediation of any 

 tributary of the latter. Examples of both of these varieties of 

 capture may be found among the Appalachian rivers ; an excellent 

 illustration of the latter method is given by the Potomac and 

 Shenandoah. 



When the Potomac was beginning to cut its gap through the 

 Blue Ridge at Harper's Ferry, a smaller stream, Beaverdam Creek, 

 was cutting a similar gorge through the same ridge a few miles to 

 the south. The Shenandoah was then a young and short tributary 

 of the Potomac, which it entered from the south, flowing through the 

 longitudinal valley which was opening along the strike of the softer 

 strata to the west of the Blue Ridge. As the Potomac is much 

 larger than Beaverdam Creek, it cut its gap much more rapidly, 

 thus giving a steep and swift course to the Shenandoah. The 

 latter pushed its way up the longitudinal valley until it tapped 

 Beaverdam Creek and captured its upper course, diverting its 

 waters to the Potomac. Beaverdam Creek no longer flowed 

 through the gorge which it had cut in the Blue Ridge and which 

 was thus abandoned and became a "wind gap," the beheaded 

 Beaverdam now rising to the eastward of the abandoned gorge. 

 This gorge is known as Snickers Gap. The great number of wind 

 gaps in the Appalachian ridges shows how frequently the capture 

 and diversion of smaller streams by larger ones has been accom- 

 plished among those mountains. 



Figures 134 and 135 show two stages in the evolution of a river 

 system. Figure 134 represents the first stage, in which several 

 transverse streams, a, c, <?,/, g, are breaching the escarpments indi- 

 cated by shaded lines. Of these streams c carries the most water, 

 and will therefore deepen its gorges through the hard ridges more 

 rapidly than the others, and give its tributaries the advantage of 

 a greater fall. In the second stage (Fig. 135), c has captured the 

 upper courses of all the other streams except g, which has not yet 

 been reached. The branch / has captured a, beheading it, divert- 



