504 Noble — Geology of the Grand Canyon, Arizona. 



is tilted about 10° N.E. The section of the basal members of 

 the group was measured in this locality, where the sequence is 

 unbroken by faulting. 



Detailed Section. 



First Member. — Basal Conglomerate. The surface rep- 

 resented by the upper unconformity that separates the Unkar 

 group from the basal Cambrian is a striking enough example 

 of a base-leveled surface, although monadnocks rise in places to 

 a height of 700 feet above the base of the Tonto sandstone. 

 But the surface represented by the lower unconformity that 

 separates the Unkar group from the Yishnu is an almost per- 

 fect plane : nowhere in the seven linear miles exposed in the 

 Shinumo area can a difference in relief be observed that 

 exceeds 20 feet. The depth of weathering below this surface 

 appears to be slight, in spite of the enormous amount of rock 

 that has been removed, and the weathering appears to be the 

 result of physical disintegration rather than of chemical 

 decomposition. 



The basal conglomerate is an arkose conglomerate varying 

 in thickness from 1 to 6 feet in the Shinumo area. It is com- 

 posed of angular or subangular fragments of the rocks of the 

 underlying Yishnu series, cemented by a matrix of red arkose 

 mud which usually contains small fragments of pink feldspar. 

 Occasionally the matrix contains small rounded grains of 

 quartz. 



The degree of induration of this conglomerate presents all 

 variations from a hard, dense, siliceous rock, which fractures 

 across matrix and enclosed rock fragments alike, to an easily 

 disintegrated rock in which the matrix crumbles away from 

 the enclosed fragments. This phenomenon, however, does not 

 depend upon original cementation, but upon metamorphic 

 effects produced by the diabase sill that is intruded in the over- 

 lying rocks, the degree of induration depending upon how far 

 the conglomerate lies below the contact of the sill. 



The matrix is usually of the same composition everywhere 

 in the area. The character of the enclosed fragments, how- 

 ever, is sharply localized by the character of the underlying 

 rock. The rock which underlies the conglomerate in the East 

 Wash is the quartz-diorite of the batholith previously 

 described. For three feet below the conglomerate the diorite 

 is weathered along the joints into roughly angular blocks. 

 These joints are filled with the red arkose material of the 

 matrix. Above follows a layer of the conglomerate one foot 

 in thickness, composed of weathered fragments of the diorite 

 cemented with the red arkose material. Then follows a layer 

 six inches thick composed of small rounded quartz pebbles and 



