EROSION AL HISTORY OF DRIFTLESS AREA 43 



peneplain reconstructed varies in altitude above sea, that 

 it has on its surface stream deposits distributed without 

 reference to the variations in altitude, that the variations 

 are due to flexures rather than to erosional irregularities, 

 and that streams have antecedent courses at right angles 

 or oblique to the flexures. 



Warping may have occurred in regions where there is 

 no evidence of movement of any kind. There may also be 

 suggestions of warping where uplift was uniform. There 

 are doubtless regions, however, such as the Appalachian 

 mountains, where warping movements have taken place and 

 where, by application of the principles outlined above, -uch 

 movements can be proven to have taken place. These prin- 

 ciples, therefore, are usable, but their use is attended with 

 difficulty and may result in uncertainty. 



Uplift with Faulting: One of the best known illustra- 

 tions of a cycle of erosion having been interrupted by 

 faulting is found in the mountains and valleys of eastern 

 California. Here an ancient erosion surface which is 

 characterized by mid-Tertiary stream gravels which lie on 

 the remnants of the old surface in many places, slopes up 

 from low levels on the west flanks of the Sierras and reaches 

 altitudes of more than 14,000 feet at the crest of the range 

 where it is broken by the great fault whose scarp forms the 

 oast slope of the mountains. East of this line the surface 

 seems to be buried under the late Tertiary and Pleistocene 

 sediments of Owens Valley below altitudes of 2,000 feet. 

 The surface and its gravel deposits appear again in the 

 Inyo mountains east of Owens Valley, reaching altitudes 

 close to 10,000 feet, where the surface is broken by another 

 fault on the east side of these mountains. The evidence of 

 faulting in this case is a series of tilted blocks, each one of 

 the series being sharply set off from the adjacent one by a 

 fault scarp. 



It seems that cases of uplift with faulting could be cer- 

 tainly interpreted only where the old erosion surface is dis- 

 tinguishable in spite of great relief within short distances, 

 where the separate blocks are distinct and where difl'erences 



