Vol. 6] Baker: Cenozoic History of the Mohave Desert. 



wind action has removed or covered their traces. These surfaces 

 are sparsely strewn with hard, polished pebbles. The action of 

 the wind, which carries away the finer particles of clay, sand, 

 and chemical deposits, is not able to remove the larger pebbles 

 which tend to accumulate on the surface because of the removal 

 of the finer materials. 



Origin of Materials and Processes of Formation. — The pro- 

 cesses of alluviation can be seen in constant operation through- 

 out the desert. In the neighborhood of the bedrock slopes of 

 the mountains ordinary talus slopes are forming of disintegrated 

 rocks which lie in a heterogeneous dump at the foot of the slopes 

 where they have fallen and rolled under the force of gravity. 

 Farther out from the bedrock slopes the material has been de- 

 posited with rude stratification by sheet and slope wash, and 

 down the gullies and canons by streams existing only during 

 times of rainfall, which in this region often attains the dignity 

 of torrential cloudbursts. The boulders and pebbles brought 

 down the canons, arroyos, and gullies have their edges more or 

 less rounded by attrition with their neighbors and the bedrock. 

 Those which have been brought down by sheet or slope wash 

 or have rolled or fallen into their present position preserve 

 almost all of their original angular form and rough edges. All 

 of the coarser fragments "shell off" upon exposure, the thin 

 shells disaggregating to form the finer matrices, while the still 

 finer particles are swept away by the wind. The wind, which 

 by its selective action in removing the finer material in which 

 the larger fragments would ultimately become wholly or in part 

 imbedded, tends to keep the latter as projecting boulders and 

 pebbles strewn over the surfaces of the slopes and therefore 

 longer exposed to the disintegrating action. Only the finest, 

 particles are carried by water and wind to the playas in the- 

 midst of the basins where the soluble materials derived from the 

 encircling rocks is deposited upon the evaporation of the thin 

 sheets of water occasionally submerging the playas. The natural 

 baking of the playa surfaces by the heat of the sun and the 

 cementing of the sediments by aid of the water and chemical 

 precipitates serves to protect a portion of the fine sediments 

 and chemical precipitates from the erosive force of the wind,. 



