FORMATIONS OF TPIE LAKAMIF RANGE 403 



the north, but the amount increases to over three times that amount in 

 southern Wyoming, where there are extensive intercalations of beds of 

 gray and reddish sandstones from 5 to 40 feet thick. Except in the 

 Casper Mountain area, they lie directly on the old schists and granites. 



TENSLEEP SANDSTONE 



This member, which I first differentiated in the Bighorn region, is a 

 conspicuous feature in portions of the Laramie range, especially to the 

 northward, though it appears to be traceable far to the southward. In 

 the Casper range it is a massive, coarse grained, porous sandstone, vary- 

 ing in color from light gray to dark gray and brown, and having a thick- 

 ness of 50 feet or more (see plate 30). It appears again from under the 

 Tertiary on Chugwater creek, near Iron Mountain station, and at inter- 

 vals southward, but is much less distinct in this portion of the region. 

 The formation underlying the typical gypsiferous Red beds west of Iron 

 Mountain station is 65 feet of brownish red, soft sandstones of massive 

 texture containing two thin beds of limestone. On Horse creek there is 

 a similar succession and thickness of sandstone, excepting that only one 

 5-foot bed of limestone is included. 



CHUGWATER FORMATION 



The " Red beds " are exposed on the south slope of Casper mountain 

 and in the valleys of La Prele, La Bonte, Sybylle, Chugwater, and Horse 

 creeks. In the main they present the usual characteristics of the gyp- 

 siferous red deposits of the Black Hills and Bighorn regions. Their 

 thickness is about 550 feet to the north and about 800 feet in the Chug- 

 water and Horse Creek valleys. To the northward the Minnekahta 

 limestone of the Black Hills series is represented with unmistakable 

 characteristics, lying about 80 feet above the Tensleep sandstone, the 

 intervening red sandy shales having the character of the Opeche forma- 

 tion. There are excellent exposures of these members in the gorge at 

 the east end of Casper mountain, on La Prele creek, and on the North 

 Platte river, 6 miles south of Douglas. The last locality is shown in 

 figure 2, plate 32. The Minnekahta limestone is mostly thin bedded 

 and of a purplish tint, and its thickness is from 20 to 25 feet. At the 

 east end of Casper mountain there is a 15-foot layer of cherty limestone, 

 80 feet above the Minnekahta limestone, a feature which is often seen 

 in the Bighorn section's. At the top of the Chugwater formation at this 

 locality there is a 20-foot limestone layer which probably represents the 

 alternations of limestone and red shales at the top of the same formation 

 in the Bighorn region. Local deposits of gypsum occur. 



