6i4 JOHN LYON RICH 



large areas may be accomplished by any one of three agencies: 

 marine planation, subaerial denudation under conditions of moist 

 climate, or denudation under arid conditions by wind scour. In 

 the case at hand the first of these, marine planation, is thrown out 

 of the question by the broad irregularities of the peneplain and its 

 gently undulating nature, as well as by the improbability of such 

 planation having occurred so far inland in late Tertiary times. 



The second explanation, normal peneplanation under a moist 

 climate, accords well with the facts. We find a smooth, yet gently 

 undulating surface with differences in relief amounting to 700 or 

 800 feet at points twenty-five miles or more apart. This indicates 

 slopes of from ten to thirty feet per mile. These conditions are 

 what should be expected if the peneplain is the result of normal 

 denudation under moist conditions. 



In the third place there is the possibility of planation under 

 arid conditions by wind scour. This agency in an arid climate 

 is worthy of serious consideration, for, under favorable conditions, 

 it seems capable of playing a very important role in denudation. 

 Passarge has shown that under conditions of long-continued 

 aridity in an inclosed basin remarkably even plains may be 

 developed over large areas by this agency combined with the 

 action of occasional rains, which by washing loose material into 

 the hollows tend to counteract any tendency on the part of 

 the wind toward the formation of basins in the areas of the more 

 easily eroded rocks. Plains of such an origin with harder rock- 

 masses standing above them as "Inselberge" are described from 

 South Africa. In so far as the smooth plain condition and the 

 occurrence of monadnocks projecting above it is concerned the 

 conditions observed in the Rock Springs region agree with the 

 wind-erosion hypothesis. Another condition which is in harmony 

 with this interpretation is that the rock floor of the peneplain 

 underneath the gravels, wherever observed, was found to be fresh 

 and undecayed. There was no evidence of an old soil underlying 

 the gravels such as might be expected if the planation were per- 

 formed under moist conditions. It is possible however that any 

 old soils which may have been formed were removed during an 

 arid period just preceding the deposition of the gravels. 



