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



northwest of Buffalo contain large numbers of fossils representing the 

 Richmond fauna of the Upper Ordovician. The relations of this upper 

 series are not well exposed and apparently it is not continuous. In 

 some portions of the district the massive member of the Bighorn lime- 

 stone is succeeded by fine grained, light colored limestones containing 

 numerous corals, among which were recognized Halysites catenulatus and 

 other forms. 



LITTLEHORN LIMESTONE 



This formation comprises about 1,000 feet of beds, which constitute 

 the greater part of the high front range of the Bighorn mountains. The 

 rocks are mainly of light color and massively bedded. At the top there 

 is a series of pure limestones, 100 feet or more in thickness, which 

 weathers in typical castellated forms, as shown in plate 29. Many of 

 the lower beds contain some sand admixture, and they are of darker 

 color. Fossils of t} r pical Mississippian forms occur at several horizons. 



The formation is equivalent in the main to the Madison limestone of 

 Montana and the Pahasapa limestone of the Black hills ; but as the 

 stratigraphic limits of the formation are somewhat indefinite, a local 

 name is applied, derived from a great canyon on the east side of the 

 range, in which the series is finely displayed. It is possible though 

 not probable that in the basal portion of this formation the Silurian 

 and Devonian are represented, for there is no evidence of stratigraphic 

 break and no fossils have been discovered for the first 25 or 30 feet 

 above the Ordovician fossils in the Bighorn limestone. 



AMSDEN FORMATION 



The uppermost member of the Bighorn front range has been separated 

 as the Amsden formation, named from a branch of Tongue river west 

 of Dayton. It consists of the somewhat variable succession of red shales, 

 limestones, chert}'' and sandy members outcropping along the higher 

 outer slopes of the range. Its thickness is about 150 feet near the Mon- 

 tana line, but it increases gradually to the southward to a maximum 

 thickness of about 350 feet. Throughout its course the basal member 

 is a red, sandy shale or fine grained red sandstone, from 50 to 100 feet 

 thick — the amount gradually increasing to the southward — lying on the 

 massive bluish gray limestone of the Littlehorn formation with no ap- 

 parent unconformity. Next above is a somewhat variable series, includ- 

 ing very compact, pure white limestones, several thin beds of gray sand- 

 stone, occasional thin bodies of red shale, and in the greater part of the 

 region several thick beds of very cherty limestone. Some of the compact 



