454 N. H. DARTON PALEOZOIC AND MESOZOIC OF WYOMING 



are two sandstones. At the top are shales, probably representing the 

 Niobrara. Near Clarkson the upper sandstone is conspicuous and near 

 the month of Poison Spider creek there are 4 beds, the lowest 40 feet 

 thick with ironstone concretions at its top. These sandstones give rise 

 to prominent ridges which extend along two anticlines from North Platte 

 river nearly to Powder river. Near Platte river, southwest of Casper, 

 the lower shales contain a 40-foot bed of buff sandstone. The Mowry 

 beds are 100 feet thick and overlain by 100 feet of black shale, followed 

 by a succession of hard gray to buff sandstone and shales extending to the 

 Niobrara limy shale, which outcrops at several localities. The lower 

 Benton shales on Poison Spider creek, 8 miles east of Efell, contain a bed 

 of sandstone saturated with petroleum, notably at the "oil spring." This 

 sandstone lies about 250 feet below the base of the Mowry beds. It ap- 

 pears in the vicinity of Alcova, where it is about 8 feet thick, and lies 

 midway between Mowry beds and Cloverly sandstone. The upper sand- 

 stone forms extensive ridges west of Preeland, and the chalky deposits of 

 the Niobrara appear a short distance above. In Muddy valle}^, south of 

 Big Muddy station, the Mowry beds lie on 500 feet of dark shales with 

 thin beds of rusty sandstone. They are hard shales, 50 feet thick, whicli 

 weather to light gray color and rise in low but sharp ridges bearing scat- 

 tered pines. Next above are 500 feet of dark shales, capped by a very 

 conspicuous sandstone 20 feet thick, partly massive and partly thin- 

 bedded, which contains many upper Benton fossils. This sandstone is 

 overlain by 200 feet of dark shales with some thin layers of sandstone 

 and horizons of concretions; then follows Niobrara, which is about 100 

 feet thick and extends oast to Boxelder creek. In the Medicine Bow re- 

 gion the upper sliale of the Benton is overlain by chalky deposits of Nio- 

 brara, about 200 feet thick, which outcrop prominently in Chalk bluff 

 east of Eock Eiver, in banks along Little Medicine creek, in the river bank 

 just north of Medicine Bow, and at intervals along the flanks of the 

 Freezeout and Shirley uplifts. 



Section of Benton Formation, 2 Miles NortMvest of Medicine Bow, Wyominy 



Feet 



Shale, gray 200 



Sandstone, gray, partly slabby 40 



Shale, gray 800 



Shales, hax'd, and fine-grained slabby sandstones (Mowry 



beds) 75 



Shales, mostly dark, weathering brownish ; thin sandstones 



in lower part 250 



Cloverly sandstone. 



Total 1,365 



