188 lupton: geology of san rafael swell, tjtah 



Henry Mountains, but probably should be classified with the 

 Flaming Gorge formation of Powell. Sixty to one hundred feet 

 of grayish sandstone (which, near Cedar Mountain, is replaced by 

 conglomerate) and sandy shale with thin streaks of coal at the 

 top and base, unconformably (?) overlie the conglomeratic strata 

 just described and probably represent the Dakota sandstone. 

 This sandstone corresponds to the uppermost part of Gilbert's 

 Henrys Fork formation. The Mancos shale, about 4000 feet 

 thick and consisting of three members, rests upon the Dakota (?) 

 in apparent conformity. In places, however, a thin bed of con- 

 glomerate separates the Dakota(?) sandstone from the overlying 

 shale. The Mancos outcrops in a monoclinal valley, the west 

 part of which is known as Castle Valley. The lowest member 

 consists of about 600 feet of bluish drab shale which is sandy in 

 its lower and upper portions. Overlying this is a sandstone mem- 

 ber approximately 500 feet thick containing coal beds near the 

 top. This sandstone probably is equivalent to the Bluegate sand- 

 stone of Gilbert's Henry Mountains section and is represented at 

 the north end of the Swell in the vicinity of Sunnyside Junction 

 by a thin concretionary sandy formation. Overlying the sand- 

 stone is about 3000 feet of grayish drab shale which is sandy in 

 its lower and upper parts. Conformably overlying the Mancos 

 shale is the Mesaverde formation 1100 feet thick (in the vicinity 

 of Emery) which caps the east scarp of the Wasatch Plateau. 

 This formation consists mainly of sandstone with beds of sandy 

 shale and coal intercalated. 



Sills and dikes of basalt, which were noted at several localities 

 near the south end of the Swell, extend as far north as Muddy or 

 Curtis Creek. 



