106 Gregory — Igneous Origin of the " Glacial Deposits'''' 



country rock as a dike, and although no consolidated material 

 was found either at the surface or by digging shallow trenches, 

 the intrusive origin of the material is scarcely a matter of 

 doubt. It differs from other dikes of the region only in the 

 fact that the igneous paste constitutes probably less than five 

 per cent of the mass. It is essentially a pudding of sedimentary 

 fragments holding igneous plums. 



2. The largest accumulation of erratics occurs at a point 

 about two miles northwest of Moses Rock. At this locality 

 the dike-like band is replaced by mounds of broken strata 

 deeply covered with erratics and occupying an area about 1000 

 feet long and 500 feet wide. Four hills, highly irregular in 

 outline and arranged along a north-south axis, rise 100-200 feet 

 above the surrounding surface and are capped by chunks of 

 limestone and sandstone 10 X 30 X 6 feet to 2x2x4 feet, 

 arranged at various angles. Within this area both native and 

 foreign rocks are piled in confusion, while on and among them 

 are found the erratic gravels including fragments of rock of 

 various types. All about this area of jumbled blocks the 

 Moenkopi strata are undisturbed, displaying their normal east- 

 erly dip. "With the drift-covered knolls the IS". 10° W. direc- 

 tion of the belt of erratics abruptly ceases and the band of 

 irregular mounds and ridges made of broken strata tilted at 

 various angles extends westward for about one-half mile. This 

 second portion of the erratic belt is believed to have the same 

 origin as the narrow band at the north, from which it differs 

 mainly in width and in quantity and variety of foreign material. 



3. The southernmost portion of the Moses Rock field extends 

 southwest, south and southeast in a sweeping curve, nearly 

 four miles in length. At the north end, and especially at the 

 south end of this line, the erratics form a belt of low knobs, in 

 places merging into the sand-covered plain. For a distance of 



bout one mile the drift forms a ridge which stands 20 to 40 

 feet above the country at its base, — a ridge coated with ash- 

 grey to green-grey gravels, contrasting strongly with the dark 

 red Moenkopi strata upon which it appears to be resting. The 

 strata both east and west of the grey -green ridge are seemingly 

 in undisturbed position. The materials forming the crest 

 of the ridge and strewn over its flanks include many of the 

 erratics elsewhere noted in the Moses Rock area. Numerous 

 fragments of chert and chalcedony are present and there is a 

 large amount of mica-diabase or minette. Like other parts of 

 the Moses Rock belt, this southern portion is thought to be 

 located along a dike and the ridge is believed to owe its pres- 

 ence to remnants of the country rock made resistant by local 

 metamorphism. The presence of fragments of diabase or mi- 

 nette, in places constituting 15 per cent of the debris, suggests 



