THICKNESS AND STKATIGRAPHIC RELATIONS 547 



beds by a few feet of white quartzitic sandstone, the basal member of the 

 formation. The characteristic massive limestone, with silicious network, 

 reappears in the Bridger range west of Deranch, at first thin, but gradu- 

 ally thickening to 40 feet on branches of Buffalo creek southeast of 

 Thermopolis, and to 50 feet or more in the upper canyon of Bighorn river. 



STRATIGRAPHIC RELATIONS 



The Bighorn formation lies unconformably between Middle Cambrian 

 and earlier Carboniferous beds and includes near its upper part an un- 

 conformity representing a long period of later Ordovician time. These 

 unconformities present no perceptible discordances in dips. The basal 

 contacts, especially at the base of the white sandstone, are well exposed, 

 but show mainly only a sharp change in materials. In a few cases there 

 are slight local irregularities apparently due to channeling. The subjacent 

 beds are Deadwood (Cambrian) limestones, but none of this material is 

 discernible in the Bighorn sandstone. Where the sandstone is absent and 

 the massive Bighorn limestone overlaps Deadwood limestone there appears 

 not to have been local uplift and truncation of the sandstone, but simply a 

 thinning out of the sandstone against the margin of a channel, or coast- 

 line, which is overlapped by the massive limestone. The contact between 

 the massive limestone and the overlying thin bedded limestone (Rich- 

 mond) unfortunately was not found sufficiently well exposed to afford 

 information as to the precise relations. Judging from their distribution, 

 the Eichmond beds lie in shallow basins on an eroded surface of the 

 massive limestone. The Bighorn-Madison contact is often exposed, but, 

 although the hiatus represents all of Silurian and Devonian time, neither 

 channeling nor fragmental products were observed, and usually it is not 

 possible to discern the plane of contact. In a few places there appears to 

 be a sudden change from one limestone to another, especially where the 

 basal Madison beds are darker gray and slightly sandy. 



The disappearance of the Bighorn formation to the southeast was ex- 

 amined with care, and, while there is some thinning of all the strata, the 

 principal diminution of thickness clearly is due to erosion from the top 

 down. Even in this part of the region neither discordance of dip nor 

 channeling of the surface of the Bighorn limestone was perceptible, and 

 there was no evidence of fragmental products in the overlying Madison 

 limestone. It was seen, however, that at first the top bed of the massive 

 Bighorn limestone was rapidly diminishing in thickness, and after it was 

 gone the next thick stratum became thinner and thinner, and finally 

 ended, so that Madison limestone came down onto Deadwood upper lime- 

 stones, as shown in plate 75. The basal Bighorn sandstone thinned 



