TENSLEEP SANDSTONE AND OHUGWATER FORMATION 397 



limestones have afforded a few fossils, which may possibly be Upper 

 Carboniferous forms, but they are not decisive. 



TENSLEEP SANDSTONE 



There extends along the flanks of the Bighorn range a sheet of sand- 

 stone which probably represents the upper sandstone of the Minnelusa 

 formation of the Black hills. Its thickness averages 200 feet for many 

 miles, but near the Montana line it gradually thins to about 50 feet. 

 Its white ledges are conspicuous features rising abruptly out of the red 

 valley and extending for some distance up the mountain slopes, as shown 

 in plate 31. No fossils have been discovered in it, but it is thought to 

 represent some portion of the Upper Carboniferous. 



The name is derived from the extensive exposures in the walls of the 

 lower canyon of Tensleep creek. 



CHUGWATER FORMATION ' 



This name is proposed for the series of red beds extending along the 

 foot of the Bighorn range southward through Wyoming and Colorado. 

 In the Black Hills region the red beds are divided near their bottom by 

 a limestone designated the Minnekahta, and although there appears to 

 be a continuous representative of this limestone in the Bighorn uplift, a 

 definite correlation can not be ventured, so that a name is required for 

 the undivided red beds as a whole. The thickness also is greater than 

 in the Black hills, the amount averaging about 1,250 feet. 



The rocks are mainly bright red sandstones and sandy shales contain- 

 ing gypsum and near their bottom and top; thin beds of limestone. 

 There are two beds of limestone near the base of the formation, one 

 about 20 feet about the Tensleep sandstone, which varies in thickness 

 from 2 to 4 feet, and consists of thinly laminated, pinkish limestone 

 apparently unfossiliferous and strongly suggesting the Minnekahta lime- 

 stone of the Black hills. About 40 feet higher there is another bed of im- 

 pure limestone, usually of porous texture, averaging about 2 feet in 

 thickness, but more extensivel} 7- developed on the west side of the uplift. 

 At the top of the formation there are usually several thin beds of lime- 

 stone intercalated among the red, sandy shales in a series having in all a 

 thickness of from 120 to 250 feet, the limestones varying from 2 to 10 

 feet in thickness. One of these beds is shown in plate 31. These lime- 

 stones contain a few fossils, but they are not sufficiently distinct for 

 accurate determination, as to whether they are of late Paleozoic or 

 early Mesozoic age. 



The name Chugwateris derived from Chugwater creek, on which there 



