PRE-MOENKOPI UNCONFORMITY OF COLORADO PLATEAU 65 



Box Mesa, the Moenkopi is about four hundred to five hundred 

 feet thick. Two miles north of the north end of the mesa, perhaps 

 four miles from the last point described, occur some smaller mesas, 

 also capped with Shinarump. Here the Moenkopi is not over 

 two hundred feet thick. The change may in part be due to the 

 post-Moenkopi, pre-Shinarump erosion, but is more probably 

 owing to the uneven surface of the Aubrey beds, on which the 

 Moenkopi was laid down. This seems the more probable, since 



Fig. 4. — Unconformity of Shinarump conglomerate on Moenkopi beds, west of 

 Wagon Box Mesa, Water Pocket Flexure, Utah. 



at this point the Moenkopi is again noted to rest on the limestone 

 member. In this vicinity the upper ledge of the limestone is itself 

 distinctly conglomeratic. 



The foregoing facts indicate a decided erosion of the Aubrey 

 before the Moenkopi was deposited, an erosion amounting in places 

 to probably two hundred feet or more, with the complete removal, 

 at certain points, of the limestone beds, allowing the shales to rest 

 directly on the cross-bedded sandstone member. 



Fruita. — At Fruita, about forty miles northwest of the Mule 

 Twist locality, the canyon of the Dirty Devil (Fremont) River 

 cuts deeply into gray limestones and sandstones below the "Red 



