576 Emery — Green River Desert Section, Utah. 



is placed at the base of the Salt Wash member of this 

 formation because there is at this horizon a distinct 

 lithologic and color break, which with the presence of a 

 thick series of coarse sandstone is indicative of a time 

 break and unconformity. The upper limit of the 

 McElmo is the Dakota sandstone, or Mancos shale where 

 the Dakota is absent. As so defined the McElmo appears 

 to be in accord with the description of the McElmo at the 

 type locality along the creek of that name in southwestern 

 Colorado where it consists principally of marl-like varie- 

 gated shales. 



Dakota Sandstone (Cretaceous). 



The Dakota sandstone is exposed near the Greenriver- 

 Hanksville road in the vicinity of Greenriver where it 

 reaches a maximum thickness of about 40 feet but is 

 absent elsewhere in the northern part of Green River 

 Desert. It consists of loosely cemented and friable, 

 coarse sandstone layers and lenticular beds of coarse 

 conglomerate containing rather well rounded translu- 

 cent, gray, black, and red quartz pebbles. On fresh frac- 

 ture the color is buff but the rock commonly weathers to 

 a rusty dark brown. No coal was observed in the Dakota 

 in the Greenriver region nor were any fossils collected 

 in the rather cursory examination of the formation, but 

 Richardson 27 who studied the Dakota in this vicinity as 

 well as over a large area to the east and west found 

 characteristic Dakota plants (Cretaceous) in it near 

 Elgin, and therefore has correlated it with the Dakota of 

 the Rocky Mountain region. It is unconformable on 

 the underlying variegated shales and sandstone of the 

 McElmo. 



Mancos Shale (Cretaceous). 



The Mancos shale overlies the Dakota sandstone and 

 is broadly exposed in the vicinity of Greenriver and 

 along the base of the Book Cliffs for many miles east and 

 west of that town. It comprises a series of rather uni- 

 form drab to dark gray clay shales, which are calcareous 

 throughout, in places to such an extent as to be almost 

 limestones. About 200 feet above the base of the forma- 

 tion and forming a prominent escarpment is 15 to 20 feet 



27 Kichardson, G. B., Reconnaissance of the Book Cliffs Coal field, U. S. 

 Geol. Survey, Bull. 371, p. 14, 1909. 



