PERMIAN-TRIASSIC ? 433 



appear. A widespread sheet of limestone alioiit 2 feet thick lies from 100 

 to 150 feet below the top of the formation, and on the east bank of Big- 

 horn river, 3 miles north of Thermopolis, it yielded many fossils. At a 

 few localities, especially near the head of Mnd creek and at the south end 

 of Bighorn canyon, the upper portion of the formation contains a number 

 of beds of gray to biiif sandstone. To the west there is a prominent bed 

 of orange-colored, massive sandstone overlain by a thick bed of gypsum, 

 the top of which is about 100 feet below the top of the formation. This 

 gypsum bed is 30 feet thick south of Embar and on Eed creek southeast 

 of Thermopolis and 40 feet in extensive exposures on ]\Iuddy creek. Near 

 the middle of the formation there is usually a massive bed of pale red 

 sandstone of fine, uniform grain. 



Rattlesnalce mountains. — The Chugwater outcrop extends for about 13 

 miles along the northeast slope of Eattlesnake mountain. Tlie formation 

 is aboiit 1,200 feet thick. At the top are 200 feet of red shales and sand- 

 stones lying on an 8-foot bed of limestone which caps a red wall. Some 

 thin limestone beds occur in the top member and also toward the base of 

 the underlying red sandstones and shale. No large deposits of gypsum 

 appear. 



Casper-Douglas-Medicine Bow region. — In the vicinity of Casper 

 mountain and the other ridges at the north end of the Laramie uplift the 

 Chugwater formation presents its usual features, but near Douglas and 

 Big Muddy the Minnekahta limestone of the Black Hills and Hartville 

 region appears in its lower part with thickness of 20 to 25 feet. The 

 underlying red shales (Opeche) are 80 feet thick and the overlying red 

 sandstones and shales are 450 feet or more. Exposures are extensive on 

 Muddy creek and its branches in the syncline south of Casper mountain, 

 and in the high red wall surrounding the Haystack range. The Minne- 

 kahta limestone is characteristically thin-bedded and of purplish tint. 

 Eighty feet higher is a 15-foot bed of cherty limestone, and near the top 

 of the formation is a 20-foot limestone member, which gives rise to a 

 prominent "wall" in the steep dipping beds on Muddy creek at the east 

 end of Casper mountain. Overlying this limestone are 20 feet or more 

 of white to red sandstone; then a few feet of buff sandstones and shale 

 with an 8-foot bed of gypsum. East of Muddy creek the outcrops are 

 continuous to the Tertiary overlap south of Glen Eock. The formation 

 appears again for a short distance on La Prele creek Just below the Nat- 

 ural bridge. In this region 20 feet of Minnekahta limestone lie on 80 

 feet of red shale (Opeche) which rests on Tensleep sandstone. East of 

 La Prele creek the red beds are covered by Chadron formation, but they 

 reappear south of Douglas and extend up the valleys of Wagonhound and 



