on the Navajo Reservation. 107 



that this intrusion is similar in composition to that of the dikes 

 and necks of Monument Valley 20 to 50 miles to the west. 



Along the axis of the Moses Rock field the Moenkopi 

 strata are faulted as indicated by Woodruff.* The amount 

 of displacement is difficult to determine because of the ab- 

 sence of distinctive beds within the 500 or 600 feet of strata 

 exposed. At the north end of the field the downthrow on 

 the east appears to be less than ten feet ; further south the 

 displacement is somewhat greater but probably does not exceed 

 200 feet, the maximum assigned by "Woodruff, and may be con- 

 siderably less. 



North of Moses Rock terraces along the Chinle and its west- 

 ern tributaries are heavily floored with gravel consisting of 

 monzonite, quartzite, and sandstone. These materials find 

 their origin in the Carrizo Mountains, about fifty miles distant, 

 as measured along the streams, and are unrelated to the "drift" 

 under discussion, from which they differ both in composition 

 and origin. The erratics of the Moses Rock field are believed 

 to have no connection with Pleistocene or Mesozoic glaciation, 

 but to owe their origin to a dike or possibly a group of dikes 

 intruded into Pennsylvanian and Permian strata. 



The Garnet Ridge Field. 



Character of the "drift? — In the lower reaches of the 

 Chinle Valley the JS r avajo sandstone is laid bare over wide 

 areas along the eastern limb of the Comb Monocline. At one 

 locality about three miles south of the Utah line overlying 

 strata remain in the form of a terraced ridge which terminates 

 in a series of rounded buttes. This area is the principal source 

 of the Arizona garnets of commerce. f The remarkable feature 

 of the district is the presence of fields of bowlders, hills capped 

 by gravel of an unusual aspect, and beds and lenses of conglom- 

 erate of a character not observed elsewhere. This erratic con- 

 glomerate covers the east end of Garnet Ridge, forms a talus 

 on its south and west flanks, mantles the adjoining buttes, and 

 forms a thin disconnected cover of the wind-swept Navajo 

 sandstone extending from Garnet Ridge southward toward 

 Tyende Creek and eastward nearly to the Chinle. The bowl- 

 ders are most numerous on top of the ridge and buttes and 

 along their southern slopes and upon the bare rock floor at 

 their bases. At one point forty bowlders exceeding two feet 

 in diameter are in view and smaller erratics may be counted 



*Loc. cit., pp. 88-89. 

 f Sterrett, loc. cit. 

 Gregory, H. E. : Garnet Deposits of the Navajo Eeservation, to be pub- 

 lished in Economic Geology. 



