410 N. H. DARTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



earlier Carboniferous times. The red shales in the lower portion of 

 this formation, including its basal red quartzite, may represent the Upper 

 red shale member underlying the Minnelusa formation of the Black 

 Hills region. 



OPECHE FORMA TION 



This member, 60 feet in thickness, is seen west of Guernsey, lying on 

 the massive, white sandstone at the top of the Hartville formation, a 

 member which probably represents the Tensleep sandstone. 



MINNEKAHTA LIMESTONE 



A small but characteristic exposure of this limestone was observed 

 with the underlying Opeche red beds north of North Platte river, 15 

 miles northwest of Guernsey. 



SPEARFISH FORMATION 



The Spearfish red beds are exposed at the above mentioned locality 

 15 miles northwest of Guernsey, with an estimated thickness of about 

 450 feet. 



SUNDANCE FORMATION 



This formation appears in the banks of North Platte river northwest 

 of Guernsey and for some distance northward. The lower 145 feet are 

 sandstones with a few thin beds of shales, over which there are 60 feet 

 of interbedded, slabby sandstones and clays, all containing characteristic 

 marine Jurassic fossils. The top of the formation appears in a canyon 

 crossing the anticline on Old Woman creek. 



MORRISON FORMA TION 



The characteristic greenish and purplish, massive clays of this forma- 

 tion appear in the localities above mentioned, west of Guernsey and on 

 Old Woman creek. 



LAKOTA-DAKOTA 



The sandstones of these formations appear in the basin northwest of 

 Guernsey, and they constitute the higher part of the anticlinal range on 

 the east side of Old Woman creek. The thickness is from 150 to 350 

 feet, comprising hard, massive, coarse grained sandstones, which to the 

 northward are separated by gray and purple clays and thin bedded sand- 

 stones believed to represent the Fuson formation of the Black hills. 



In the sandstones there are thin conglomeratic streaks, general cross- 

 bedding, and the colors are predominately buff to dirty gray. h\ the 



