180 EEPOKT UNITED STATES GEOLOGICAL SUEVEY. 



The line separating the Jurassic from the overlying Dakota is indef- 

 inite. The discussion in regard to it will be found in the reports for 

 1875. 



CRETACEOUS. 



Area A. — Of the 1,000 square miles included in the area, about 850 

 are covered with rocks of Cretaceous age. Of this 850 square miles, 

 about 70 have the Colorado group as the surface formation, the rest 

 being mainly Dakota Group. 



The Dakota group for the most part is horizontal, and the conditions 

 of the strata are very uniform. It forms the tops of the bluffs on all the 

 caiious. It is a moderately compact, yellowish, silicious sandstone. 

 Beneath are greenish shales and bands of sandstone. The latter have 

 been referred to the Lower Dakota. The total thickness is about COO 

 or 700 feet, about 200 feet being referable to the upper Dakota. 



The massive sandstone at the top in most places appears to have 

 formed a barrier to erosion, the overlying shales having been swept 

 away. Eemnants are still seen in Basin Plateau, and in the angle of 

 the San Miguel opposite the mouth of Naturita Creek. Farther south, 

 however, near the slopes of Lone Cone, the shales are more persistent, 

 forming mesas extending northward. The eruption of the trachytic 

 masses seems to have had some influence in protecting them in this 

 locality. These remarks are merely intended to sui^plement the Ee- 

 port for 1875. 



Area B. — As already stated, the whole of Grand Elver Valley, and a 

 considerable portion of the cliffs bordering it, are occupied by beds of 

 Cretaceous age. Of this, the Colorado shales occupy over 1,000 square 

 miles in the space between the cliffs and the hogbacks on Grand Eiver. 

 They are so eroded and covered by d6bris that but little can be predi- 

 cated of them. They form low bluffs back of the hogbacks. The total 

 thickness of the group is about 1,500 feet. The beds consist of calca- 

 reous and argillaceous shales with sandstone and limestone bands. 



The sandstone of the Dakota group here, as at so many other locali- 

 ties, seems to have presented a decided resistance to erosion, and as a 

 result we find it forming the summit of the hogbacks. 



It is somewhat difficult to determine the exact thickness of the Dakota 

 group, as we do not know exactly what its lower limit is. We find the 

 same difficulty in dividing the Dakota Cretaceous from the Jurassic that 

 is always found where the deposition of sediments has been continuous 

 from age to age. 



The following is the average section of the Dakota group in the hog- 

 backs on Grand Eiver : 



Top. Thickness in feet. 



1. Massive yellow, siliceous sandstone, with faint impressions") 



of dicotyledonous stems, at some localities i gQ_200 



2. Variegated marls, with bands of reddish sandstones. These | 

 beds have the characteristic weathering of Bad Lands j 



3. Dark-colored sandstones, with bands of marls and shales. ^ 



Near the base is a layer containing nodules in which there > 50-100 

 are jaspers '. ) 



Total thickness ' 140-300 



The greatest thickness is noted as we go to the southwest, beyond the 

 bend of the Grand. 



