49 J: II. E. Gregory — Reconnaissance of a Portion of the 



this late Tertiary erosion surface are preserved at Black Point* 

 beneath a lava flow, and also at points east of the Little Colorado. 

 Uplift and tilting reestablished the Little Colorado, whose 

 subsequent history is to be read chiefly in terms of structural 

 control exerted by eastward-dipping sedimentaries of Triassic 

 and Carboniferous (Permian ? and Pennsylvanian) age. The 

 ungraded state of the Little Colorado reflects the relative 

 erodibility of shales, sandstones, conglomerates and limestone. 

 Both master-stream and tributaries present valleys of youthful 

 features, — flat-floored, precipitous-walled canyons of steep 

 gradients determined by structural slopes. 



The normal course of physiographic development has been 

 interrupted by volcanic activity. Patches of basalt overlying 

 dissected strata of both Moencopie formation and Shinaruinp 

 conglomerate were noted along Cedar Wash. At Tappan 

 Springs and at the month of Cedar Wash these remnant flows 

 attain a thickness of 30 feet ; and near the mouth of the Moen- 

 copie the canyon of the Little Colorado is rimmed on both 

 sides by palisaded basalt cliffs 30 to 40 feet high. About five 

 miles below Tanner's Crossing the west bank of the river 

 consists of a narrow lava terrace of undetermined length. The 

 thickness of these flows and their lack of connection with any 

 of the lavas of San Francisco Mountain or with Pogue Buttes,f a 

 volcanic cone five miles north of this area, suggests a local 

 origin . 



No large faults were noted in this area, but several faults of 

 a few inches to a few feet displacement were mapped, and at a 

 point about three miles below Tanner's Crossing a fault trend- 

 ing N. 70° E. with a throw of 70-80 feet was noted by Mr. 

 Heald. 



A view of a typical portion of this area is reproduced in 

 fig. 2. 



Stratigraphy. 



The formations represented in the vicinity of Tanner' 8 

 Crossing are Kaibab (Pennsylvanian) limestone, Moencopie 

 formation (Permian?) shales and sandstone, Slrinarump con- 

 glomerate (Triassic), and to a limited extent the LTpper Triassic 

 shales including the Leroux of Ward. The Kaibab lime- 

 stone forms the east face of the Coconino Plateau, the floor 

 of the upper parts of several streams leading northeastward 

 from Coconino Point, and the base of the canyon walls of 

 the Little Colorado beginning about three miles north of 



* For a detailed description of this occurrence see Robinson, H. H. : The 

 Tertiary Peneplain of the Plateau District and Adjacent Country in Arizona 

 and New Mexico. This Journal, xxiv, pp. 109-129, 1907. 



f Named for Dr. J. E. Pogue. 



