1921] Smiley: Flora of the Sierra Nevada of California 9 



Great Basin, the rivers were rejuvenated and deep caiions trenched 

 in the old plateau. The period of alpine glaciation in the Pleistocene 

 widened many of these valleys in the higher mountains, giving to them 

 the characteristic U-shape of glaciated valle3\s, and deepened the 

 heads of these valleys by cirque cutting into the divides. The details 

 of the surface now seen in the high Sierra Nevada are the results of 

 this process of weathering and erosion upon the several terranes 

 constituting the surface. The rock species weather differentially. 



The small areas yet remaining in the high mountains of the early 

 Mesozoic sedimentaries offer the most bizarre topographic fomis, 

 their minutely jointed and faulted condition making them peculiarly 

 subject to the alpine erosional factors. Since, for the most part, they 

 lie along or near the higher crests, many of the sharpest pinnacles and 

 thinnest aretes are formed of these vestigial metamorphosed sedi- 

 ments; Mt. Ritter (13,096 feet), Red-and- white Peak (12,840 feet), 

 the Palisades (1-4,000 feet), are conspicuous examples of many true 

 dents along the crest. About their bases are slopes of scarcely altered 

 rock debris of fairly uniform size, 2 to 3 inches in diameter. 



Vastly more important in the topography are the granodiorite and 

 granite areas. The latter rock disintegrates more abruptly than the 

 granodiorite because of the usually coarse macrocrystalline texture. 

 The rock becomes friable, crumbling to sand in which nearly fresh 

 crystals of quartz and feldspar may be found. The resulting sand- 

 slopes are a constant feature in the granite localities about Lake Tahoe 

 and above Yosemite Valley, and Knopf and Thelen report the same 

 thing for the Mineral King district in the southern Sierra. In cross- 

 ing these slopes one sinks deeplj". Since the fragments often lie at 

 the angle of repose for such material, it is easy to start small slides 

 which carry downward many of the plants found growing in such 

 places. The upper reaches of these slopes are excessively dry and 

 sterile, the large size of the constituent particles causing the pore space 

 to be too great for capillarity to overcome. Water falling on the 

 surface immediately sinks ; there is no run-off in even the hardest 

 showers. This absorptive quality of the upper reaches, with the 

 attendant leaching effect, causes the lower levels to support a rich 

 and varied flora of distinctly mesopli}i:ic shrubs and herbs, while at 

 the bases marsh conditions may prevail 



The principal rock species of the range, granodiorite, is differ- 

 entially affected by the forces of weathering accordingly as it is jointed 

 or massive. The jointed rock is easily attacked by the erosional agents 



