526 Noble — Geology of the Grand Canyon, Arizona. 



such as is deposited in the Indus delta to-daj from a similar 

 source. Occasional lenses of fine conglomerate within some 

 of the strata suggest scoured and filled stream channels. 



The upper member of the Unkar again bears all the marks 

 of shallow water origin : mud-cracks, ripple-marks, and cross- 

 bedding characterize the entire thickness. The addition of 

 micaceous material and of some feldspar gives a slightly arkose 

 character to the rock ; possibly a crustal movement rejuve- 

 nating the land mass supplying the sediments was responsible 

 for the change in character. Here again are marks of aridity, 

 seen in the vermilion color and the vast development of 

 mud-cracks. 



All subsequent Unkar and Chuar deposits have been removed 

 by the truncation of the pre-Cambrian structure by the plane 

 of the base-leveled surface of erosion beneath the Tonto 

 Sandstone. 



In summary it may be said that the evidence obtainable 

 from the lithologic record of the Unkar sediments in the 

 Shinumo Area points probably to an arid climate, and almost 

 surely to deposition in shallow waters ; first in a permanent 

 body of water and later in deltas or on flood plains. Which 

 of the latter conditions prevailed the writer is not competent to 



The predominance of clastic sediments instead of limestones 

 in the basal portion of Walcott's section, 30 miles east, sug- 

 gests that that area was nearer to the shore line of the early 

 shallow sea. The close correspondence of the stratigraphic 

 succession and lithology in all higher members in the two 

 areas suggests uniform conditions over at least that distance. 



The next event which can be read from the geologic record 

 is the invasion of the Unkar strata by a thick sill of diabase 

 in the lower members, and by four thin sills of basaltic rock in 

 the upper part of the section. 



Following this came an orographic movement of block 

 faulting and tilting, accompanied or succeeded by elevation, 

 breaking the strata of the Grand Canyon series into great 

 crustal blocks, and throwing them into high ranges of moun- 

 tains which in character and aspect were probably not unlike 

 the faulted ranges of the Great Basin or the desert ranges of 

 Arizona. 



Then began a second vast interval of erosion, gnawing 

 slowly but surely into these faulted mountains, reducing them 

 in slow process of time through stages of youth, maturity, and 

 old age, and finally planing away all except the very hardest 

 strata of their cores to form the broad monotonous expanse of 

 a base-leveled surface. The monotony of this surface was 

 broken only by an occasional monadnock of hard Unkar quartz- 



