372 



University of California Publications. 



[Geology 



of the alluvium, where the rounded surfaces are not polished and 

 are rough and pitted, and probably owe their form to exfoliation. 

 The boulders strewing the surfaces of the fans have "shelled 

 off" under the action of sudden heating and cooling. True 

 water-worn pebbles and boulders that have preserved the effects 

 of polishing are found in the desert only in the beds and walls 

 of the canons and arroyos and the valley of Mohave River. The 

 finer materials of these debris slopes are almost wholly arkosic, 

 being rough, angular particles of quartz, feldspar, the ferro- 

 magnesian minerals, lava, metamorphic and crystalline rocks, 

 and the more indurated sedimentary rocks. Some boulders of 

 the agglomeratic materials of the more resistant portions of the 

 Rosamond Series are found near the outcrops of that series, in 

 which places doubtless much of the arkosic material of the 

 later alluvium J is only the redeposited or disintegrated arkoses 

 of the Rosamond. In many places much wind-blown sand is 

 mixed with the alluvial debris, derived from the low sand dunes 

 which are- a conspicuous feature of the desert basins and lower 

 mountain flanks. Close to the steeper bedrock slopes of the 

 mountains the debris is merely a pile of unassorted material of 

 heterogeneous texture, but on the middle and lower slopes of 

 the fans, as seen in the sections cut in them by the arroyos, there 

 is generally some assortment into poorly defined layers of more 

 homogeneous texture. The alluvium is locally indurated. . The 

 mean surface angles of the alluvial debris aprons average about 

 2°, being of course greater at their junction with the bedrock 

 of the mountains and gradually dying out to horizontality as a 

 nearer and nearer approach is made to the basin playas. 



The material of the playas is quite fine clay and chemical 

 deposits, mixed with some wind-blown sand, which has lodged 

 there during the very intermittent periods of their submergence. 

 There are also rather fine pebbles of resistant rocks stained dark 

 brown on the surface with iron and well polished, partly by 

 water action" and partly by attrition of grains of wind-blown 

 sand. Clay concretions of grotesque shapes are also found. The 

 smooth, hard, wind-swept surfaces of the playas exhibit sun 

 cracks,, rain prints, trails of plants and animals, and ripple- 

 marks after periods of precipitation and inundation and before 



