1921] Smiley: Flora of the Sierra Nevada of California 11 



Sierran structure is determined by faulting and a number of well 

 defined fault lines have been detected and traced for many miles, while 

 other lines of displacement are suspected to exist. The fault lines 

 trend northwest to southeast with displacement of varying amount. 

 West of Owens Valley the total movement is thought to be not less than 

 10 — 11,000 feet ;^^ the displacement is less in the north, amounting to 

 some 3,000 feet in Plumas County,-^ In the Tahoe region there are 

 three main lines of faulting, the two east and west of the Carson Range 

 and the one on the west side of Lake Tahoe.^^ In addition to the great 

 fault lines, there are within the range topographic details interpreted 

 as the result of minor displacements.^^ Sierran faulting is of normal 

 type, with small hade, large throw, and bold escarpments. Since, in 

 addition to faulting, the region has also been revolved slightly on its 

 long axis, the areas west of the fault plane are now tipped to the west 

 and we find, almost without exception, that the west and southwest 

 sides of elevations are of easy ascent compared to the more pre- 

 cipitous east and northeast sides. This structure and accompanying 

 topography affect the vegetation, particularly that of the highest 

 mountains, for this tilting of the range brings the surfaces near the 

 crest line into the position most favorable for insolation, with its 

 accompanying higher temperatures of both air and soil, and also for 

 rainfall (compare rainfall of Fordyce and Tahoe on opposite sides 

 of the Tahoe fault). It is true that not faulting only is responsible 

 for the relatively low gradients of west and southwest as compared 

 to east and northeast sides of elevations. These steeper sides unques- 

 tionably owe something of their greater abruptness to the fact that, 

 as the leeward sides, the snow of winter drifts more deeply there while 

 the opposite sides may be swept nearly bare. This deeper accumula- 

 tion of snow, coupled with less melting because of lessened insolation, 

 produced larger glaciers which cut their cirques deeper into the lee 

 side of the ridges and summits. Today the only Sierran glaciers are 

 on these protected slopes. 



All the high mountain region has been more or less profoundly 

 modified by the Pleistocene glaciation. Within that region the evi- 

 dences of ice action are everywhere present. In the early period of 

 the study of Sierran geology it was generally believed that this 

 glaciation was much more severe than later investigations have shown 

 to be the case. In the extreme north Diller^^ and Turner^* found but 

 slight evidences of glacial ice, as might be inferred from the relatively 

 low altitude of the Sierra in Plumas County. Widespread evidences 



