146 



GEOLOGY. 



Adjustment of streams to rock structures. — Valleys (gullies) locate 

 themselves at the outset without immediate regard to the hardness 

 and softness of their beds. It is primarity the slope about the head 

 of a gully which determines its hne of growth, though relative hardness 

 often determines the details of slope, even in the early stages of an 

 erosion cycle. Once estabhshed, streams tend to hold their courses, 

 even if this involves the crossing of resistant layers. 



While a region where more and less resistant layers of rock come 

 to the surface is in a youthful stage of erosion, some of the valleys 

 (and therefore the streams) are likely to be located on the less resistant 

 rock, some on the more resistant, and some partly on the one and 

 partly on the other. The streams on the weaker rock will deepen 

 their valleys more rapidly than the others, and those which flow across 

 stronger and weaker rocks alternately will deepen their valleys more 



THE 

 KITTATINNY 

 PLAIN 



lENANDOAH 

 PLAIN 



Figs. 131, 132. — The capture of the head of Beaverdam Creek by the Shenandoah 



Va.-W. Va. (After Willis.) 



rapidly than those which run on hard rock all the time. The former 

 conclusion is self-evident. The latter appears from the fact that 

 rapids will be likely to develop at the crossing of each hard layer, thus 

 accelerating erosion at those points. Such a stream therefore not only 

 has less hard rock to erode than one which flows on resistant rock 

 all the time, but it erodes that which it does cross much faster. 



