DEAD WOOD FORMATION AND BIGHORN LIMESTONE 395 



DEAD WOOD FORMATION 



As the lowest series in the Bighorn uplift so closely resembles the 

 Deadwood formation of the northern Black hills, this name has been 

 applied to it. Its thickness is greater, averaging from 900 to 1,000 feeti 

 but the rocks comprise similar sandstones, shales, limestones, and con- 

 glomerates. At the base there are usually at least 10 to 30 feet of brown 

 sandstone lying on the granite, and this member often is conglomeratic 

 near the contact. Next above are gray and greenish gray shales, usually 

 with thin sandstone or sandy shale intercalations, which ordinarily have 

 a thickness of 250 to 300 feet. They are succeeded by a bed of coarse 

 gray sandstone, averaging only from 25 to 40 feet in thickness, but 

 apparently of wide extent. This sandstone is overlain by several hun- 

 dred feet of shales and thin bedded sandstones containing considerable 

 glauconite, at most localities, and a few thin layers of limestone. This 

 series merges upward into an alternation of impure limestone and a 

 very characteristic conglomerate of flat limestone pebbles, which are 

 mostly green with glauconite on the surface, but vary from gray to pale 

 pink inside. This limy series has a thickness of about 200 feet, and is 

 generally succeeded abruptly by a bed of white sandstone in places 25 

 feet thick, which is arbitrarily regarded as the top of the Deadwood for- 

 mation. There are very few local variations in the character of the 

 formation, the principal change being in the thickness of the basal sand- 

 stones, which in some areas attain a thickness of 300 feet, as shown in 

 plate 28. Fossils occur at various horizons, mostly in thin limestone 

 layers somewhat above the middle of the formation, in the medial 

 sandstone, and in the basal sandstone. The prominent forms are Decel- 

 lomus politics, and Ptychoparia oweni, which occurs mainly in the basal 

 sandstone. 



BIGHORN LIMESTONE 



This member is one of the most prominent of the sedimentary series 

 in the Bighorn mountains, rising in high cliffs along the inner face of 

 the limestone front range (see plate 28). Its principal mass consists of 

 a hard, massive, impure limestone of light gray or faint buff color, with 

 reticulating network of silica, which on weathering gives it a very coarse, 

 irregularly pitted or honeycombed surface. The thickness ranges from 

 200 to 300 feet in greater part, and the bedding planes are mostly from 

 20 to 30 feet apart. It usually yields very few fossils, but those which 

 have been found are of late Ordovician age. In its upper portion the rock 

 becomes less massive and the top member included in the formation 

 consists of some thin bedded, impure limestones, which at one locality 



LIII— Bur,i,. Geoi,. Soc. Am.. Vol. 15. 1903 



