JURASSIC. 567 



feet, as measured near the base of Blue Mountain. * * * The lower strata of these varie- 

 gated beds are of marine origin, as shown by the fossils they contain; the upper part, by analogy 

 with other and better-known sections, is supposed to be composed largely of fresh-water deposits. 



The variegated and shaly beds are hmited at the base by a second group of sandstones 

 which, like the Dakota, usually forms a steep rocky ridge. * * * It is usually very white 

 and massive and cross-bedded to an extreme degree. 



The variegated beds are of Jurassic age and probably correspond in their upper or sup- 

 posedly fresh-water part to the Morrison formation east of the Rocky Mountains. They have 

 also been variously named "Flaming Gorge," McEhno, and in part Gunnison in the Uiata 

 Mountain and western Rocky Mountain regions. * * * The cross-bedded white sandstone 

 is also of Jurassic age, as is shown by fossils obtained from this field. It is with httle doubt 

 the equivalent to the "White CMff" sandstone mapped by Powell on the north side of the 

 Uinta Mountains, and also seems to be with almost equal certainty equivalent to the La Plata 

 sandstone of the southwestern Colorado sections. 



Gale gives a list of fossils which are regarded as characteristically Jurassic. 



K-L 13. WYOMING AND SOUTH DAKOTA. 



The Jurassic is represented in the Bighorn Mountains and throughout central 

 Wyoming by the Sundance formation and possibly by the Morrison formation, 

 although the latter may prove to be of Cretaceous age. (See Chapter XIV, 

 pp. 606-608.) Darton'^'^ states : 



Typical marine Jurassic deposits, with an abundant fauna, extend continuously around 

 the Bighorn uplift, and they are so similar to the deposits in the Black Hills that the same 

 name is applicable. The thickness averages about 300 feet and the succession comprises a 

 sandy series below and a considerable thickness of greenish fossihferous shales above. At or 

 near the base there is usually a hard fossihferous hmestone layer having a thickness of from 

 3 to 5 feet, increasing locally to 25 feet. Next above are soft sandy beds often containing large 

 numbers of GrypJisea calceola var. nebrascensis. The greenish shales above contain thin layers 

 of highly fossihferous limestones and a few thin sandy layers. 



In another paper "^^ Darton describes the Sundance formation as follows : 



The Sundance formation occupies a large area in central Wyoming, but it is absent in the 

 Laramie Basin south of Rock River. In Wind River Basin it is mostly covered by Tertiary, 

 but it outcrops extensively near Fort Washakie, Lander, and Dallas. It appears along both 

 flanks of Owl Creek Mountains, on the north slope of Rattlesnake Mountains, in southeastern 

 Natrona County, in northeastern Carbon County, in the flexures south of Douglas and east and 

 northeast of Medicine Bow, and along the east side of Laramie Mountains from Iron Mountain 

 station to Crow Creek. Although there is a long time interval between the Sundance and 

 Chugwater formations, marked erosional unconformity is rare and in places it is difficult to 

 draw the line between them. This is probably because the first sediments of the upper forma- 

 tion were derived from the one below. The upper hmits are similarly iU defined. There is no 

 discordance in dips of underlying or overlying formations. 



In the Owl Creek Mountain region the Sundance formation consists of about 200 feet of 

 soft gray sandstones and green shales with conspicuous hard layers. * * * The local stra- 

 tigraphy varies somewhat. At the base are sandstones, the middle and upper beds are mainly 

 shales, and there is more or less sandstone for about 40 feet at the top. The shales are greenish 

 or dark gray and contain limestone in concretions and thin layers, often highly fossihferous. 



