UPPER CRETACEOUS. 



679 



Generalized columnar section in Yampa coalfield, Colorado. 



Cretaceous: ~ Feet. 



Laramie formation — sandstone and shale, with coal 1,000 



Lewis shale — dark shale, calcareous layers 1,000-2,000 



Mesaverde formation — sandstone, shale, and coal seams 2,500-3,500 



Mancos shale — dark shale, containing limestone and sandstone layers 2,000-2,500± 



Dakota formation — conglomerate and shale 200± 



Jurassic-Triassic (?) "Red Beds" (Triassic to Carboniferous?). 



Archean (ancient crystalline and metamorphic rocks). 



The Dakota and Mancos present no special peculiarities. The Mesaverde, 

 being the coal-bearing formation, was studied in detail and yielded the following 

 generalized section: 



Generalized section of the Mesaverde formation in Yampa coalfield, Colorado. 



Sandstones (occasionally massive) and shales, containing coal, especially in upper part (upper Feet. 



coal group) 400 



Twentymile sandstone member 75 



Weak sandstones and shales, with frequently a prominent ledge-making yellow sandstone about 



250 feet from the top 600 



Sandstones (occasionally massive), shales, and coals of the middle group , 400 



Trout Creek sandstone member 75 



Shales and weak sandstones, with few massive beds, containing in the lower part the upper 



seams of the lower group of coals 400 



Massive sandstone, with subordinate shaly sandstones and shales, containing near the top the 



lower seams of the lower group of coals 750 



Slabby or shaly sandstones with some shales, frequently having greater strength at two or three 



horizons, on erosion giving rise to hogbacks 750 



3,450 



Above the coal-bearing Mesaverde lies the Lewis shale, a marine formation 

 between 1,000 and 2,000 feet thick, which grades into the sandstones below and 

 above by thin sandy strata. The Lewis is followed by the "Laramie," a terrane 

 characterized by sandstones and coal beds, with shales, which occupies the strati- 

 graphic position of the Laramie of adjacent areas but from which in this district 

 no fossils have been obtained. A part of the "Laramie" is shown to have a 

 thickness of 312 feet, but this is presumably but a small part of the thick terrane. 



North of the Yampa field the Cretaceous outcrops range into Wyoming, on 

 the southern boundary Une of which Ball ^^* determined the following section : 



Section of Cretaceous rocJcs in the western portion of the Little SnaJce River coal field, Wyoming. 



