404 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



The Minnekahta limestone is massive in general appearance, but is 

 very thin bedded and weathers out in thin slabs. On the North Platte 

 river, south of Douglas, its upper portion is mottled dark gray, and at 

 the top there are 5 feet of more massive limestones of light color, which 

 is rather an unusual feature. Owing to overlap by Tertiary deposits, all 

 of the Chugwater formation does not appear again until Chugwater 

 creek is reached. Here its thickness is from 700 to 800 feet, measured 

 in beds dipping from 43 to 78 degrees. The rocks are the usual red 

 sandy shales, grading into soft, red sandstones. Near the middle of the 

 formation are two thin layers of limestones, which apparently are too 

 high to represent the Minnekahta limestone, a most unusual sequence 

 not elsewhere observed in the region to which this paper relates. 



In the extensive exposures on Horse creek there is a 20-foot ledge of 

 limestone 260 feet above the base of the formation which is strongly 

 suggestive of the Minnekahta. A bed of porous, sandy limestone 4 feet 

 thick lies 70 feet higher, as in the region northward and in the Bighorns, 

 and 100 feet higher is a 5-foot bed of massive white limestone. At the 

 top of the formation in this region there is a bed of pale reddish brown 

 massive sandstone, which continues far to the southward. Between the 

 base of the Chugwater formation and the Upper Carboniferous limestone, 

 on both Chugwater and Horse creeks, there are 65 feet of massive soft 

 sandstone of reddisn color, with thin beds of limestone, which may 

 possibly belong to the Chugwater formation, but, as explained above, 

 this series is believed to represent the Tensleep sandstone. 



SUNDANCE FORMATION 



» 



This formation extends along the slope of the Laramie range, present- 

 ing its usual characteristics to the northward, but thinning rapidly to 

 the southward. In extensive exposures at the east end of Casper moun- 

 tain it has a thickness of about 350 feet. At the base, overlying the 

 limestone which is supposed to represent the top of the Chugwater red 

 beds, there are 20 feet or more of white to red sandstones, then a few 

 feet of buff sandstones and shales with an 8-foot bed of gypsum, and 

 finally an upper series of dark shales with limestone layers and concre- 

 tions filled with characteristic fossils, including many Belemnites densus. 



Along North Platte river, in its big bend south of Douglas (figure 2, 

 plate 32) and to the west on Wagonhound creek, the succession is as 

 follows from the bottom up : 30 feet or more of massive gray sandstones ; 

 30 feet of pale greenish sandy shales ; 5 feet of soft greenish massive 

 sandstone ; 40 feet of bright reddish sandy beds ; 15 feet of massive buff 

 sandstone ; and at the top about 200 feet of green shales with a few thin, 

 beds of sandstone and limestone containing many upper Jurassic fossils. 



