348 



WALTER C. MENDENHALL 



on the southwest slopes of Black Mountain. The middle drainage 

 basin of Carrizo Creek is a bewildering bad-land area (Fig. 7) formed 

 by the sharp dissection of these soft clays. For many hundreds of 

 feet above the conglomerate, the shale beds contain only occasional 

 strata of thin brown nodular sandstone, hence they form smooth 

 clay hills. They are entirely destitute of vegetation because of the 

 aridity of the region, so the area in which they are found is desolate 

 in the extreme. Fresh outcrops of the shales are to be seen only 



i 



ViG. 6. — Fossil coral reef near the head of Barrett Creek. 



along the flood channels where there has been recent cutting. Ordi- 

 narily each shale hill is mantled by several feet of residual material, 

 dust much the greater part of the time, soft adhesive mud during the 

 occasional desert rains. This mantle is the result of the weathering 

 of the soft shales. Exposed to the air, they disintegrate completely and 

 rapidly. This action is probably aided by the efflorescence of cer- 

 tain of the alkah minerals which are abundant in the shales. Wher- 

 ever a thin sandstone is interstratified with them, it partially pre- 

 serves them from the rapid disintegration which ordinarily affects 

 them and so usually caps a hill which stands above the general level 

 of the unprotected shale. Such low structural monadnocks are 

 numerous in the neighborhood of Barrett's Well (Fig. 2). 



