Vol. 6] Beid: The Geomorphogeny of the Sierra Nevada. 107 



on the north is terminated by another low ridge connecting the 

 same two ranges. 



The third large topographic division is composed of the 

 western flanks of the Virginia and Pine Nut ranges, and is of 

 little importance in the present paper. These two ranges are 

 separated by the gorge of the Carson River and the low area 

 adjacent. 



To the south of the Carson quadrangle the Pine Nut Range 

 joins the Sierra Nevada and is therefore genetically a portion 

 of the large Sierra. 



GEOMOKPHY. 



The geomorphic features of the area mapped are in the 

 main clear and sharply defined. Most prominent are the 

 many fault-scarps in the mountains, descending from the 

 highest ridge crests to the valleys. The level valleys themselves, 

 meeting the steeply rising mountains at sharp angles, provoke 

 comment from those unaccustomed to the region. But the third 

 type of physiographic form is not seen unless one knows where 

 and how to look. Prom low points in the valleys there is a 

 wealth of suggestion to the physiographer in the flat-topped ap- 

 pearance of many of the highest ridges (pi. 22a). This sug- 

 gestiveness merges into a reality if one ascends the steep fault- 

 slopes to emerge at the summit upon a land of topographic old 

 age. The transition in some places from the sheer fault-scarps 

 to the flat ridge tops is so abrupt as to be almost startling. There 

 are thus three distinct geomorphic zones, each very different 

 from the other. Upon many of the ridge tops throughout the 

 region mapped occur small areas a mile or two in diameter that 

 present the appearance of an old eroded surface, a peneplain. 

 Plate 22b shows that one near Marlett Peak. In a few places 

 traces of the same surface are found little above the level of the 

 Nevada valleys. Each separate level area caps a fault-block, 

 and therefore, as each block has moved differently with respect 

 to those adjoining, there are marked differences of elevation be- 

 tween the residuals of what was undoubtedly a continuous pene- 

 plain before the uplift of the Sierra in Pliocene time. The old 

 surface may now be regarded as a broken and dislocated plateau. 



