ARRANGEMENT OF ROCKS 313 



Topography conditioned by the Arrangement of Rocks 



While the final effect of the subaerial denuding agencies is to 

 sweep away all relief, and to cut the land surface down to low- 

 lying base-levels or peneplains, yet in the process great irregu- 

 larities are produced by the more rapid removal of some parts 

 than of others. The topographical forms generated by this dif- 

 ferential erosion vary much according to circumstances. We 

 have already considered some of these differences with regard 

 to the agencies which have produced them. Now we have to 

 examine the differences with a view of learning how topographi- 

 cal forms are determined by the character and arrangement of 

 the rocks which are undergoing degradation. 



When a peneplain or plain of marine denudation is lifted high 

 above sea-level, without folding or steep tilting of the strata, 

 streams are soon established upon the new land, and proceed 

 to cut deep trenches across the plateau, which are gradually 

 widened out under the influence of weathering, and the arrange- 

 ment of hard and soft rocks finds expression in the resulting 

 forms. If the surface layers resist weathering, the plateau will be 

 gradually dissected into flat-topped mesas and table-mountains ; 

 while if the whole mass of rocks be easily destructible, they 

 weather down into dome-shaped and rounded hills, which are 

 smallest at the top, the part longest exposed to weathering. The 

 wild and grotesque scenery of the Western bad lands, with their 

 chaos of peaks, ridges, mesas, and buttes, is merely the result of 

 the differential weathering of horizontal strata, some beds and 

 parts of beds yielding more readily than others. If a series of 

 more resistant beds underlies a mass of softer strata, a change in 

 the topographical forms will occur when the underlying harder 

 rocks are partially exposed. In the soft rocks the valley sides 

 have gentle slopes, but when the harder mass is penetrated, the 

 slopes become steep, or even vertical. When hard and soft 

 strata alternate in a valley wall, the harder beds form cliffs. This 

 is accomplished by cutting away the softer beds and thus under- 

 mining the harder ones, until the latter can no longer support 



