UPPER CRETACEOUS. 



671 



Book Cliffs field, the largest and most important in the State. The southern Utah coal belongs 

 to the same group as that in the Weber River field. 



The Cretaceous strata which form the escarpment of the Roan or Book CUffs 

 extend south of the fortieth parallel across the basin of Green River and around that 

 of Grand River from central Utah to western-central Colorado. They comprise the 

 Dakota gandstone, the Mancos shale, and the Mesaverde formation, and are uncon- 

 formably succeeded by the Eocene. The following section is summarized from 

 Richardson:®" 



Eocene. 



Unconformity. 



Meaaverde: Alternating buff sandstone and drab or carbonaceous shale, with beds of coal in lower part; shale prevails in 

 the lower part, and sandstone in the upper; beds of sandstone lenticular; sections variable; thickness 1,200 to 2,200 

 feet, decreasing westward ; fossils consist of plants and invertebrates of fresh and brackish water. Transition marked 

 by increase of sands upward and appearance of brackish and fresh water, in place of marine conditions. 



Mancos: Clay shale, black or blue gray, with local lenses of limestone and near the top thin beds of buff sandstone, con- 

 tains Montana fossils in the upper part and Colorado fossils near the base; thickness about 3,000 feet. 



Dakota: Quartzitic buff sandstone, commonly conglomeratic, with layers of carbonaceous shale and low-grade coal; 

 thickness 2.5 to 200 feet, but formation sometimes wanting on account of irregularities of surface on which it was 

 deposited. 



Unconformity. 



The following section is given by Lee ^^° in an account of the Grand Mesa coal 



field: 



Generalized section of [Cretaceous] roclcs in the Grand Mesa coalfield, Colorado. 



