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University of California Publications in Geology [Vol. 10 



These are the oil-bearing sandstones of the productive Grass Creek 

 field, which lies about sixteen miles to the west-southwest. 



Cody Shale. 7 ' — The Cody shale in this region consists of soft blue 

 black shales, dark sandy shales, and thin brown and gray sandstones. 

 In the lower part the soft black shales carrying numerous limonitic 

 and iron-rich calcareous concretions predominate. Toward the top 

 the shales become more sandy and slightly lighter in color. Thin, 

 more or less lenticular sandstones occur throughout the formation. 

 No attempt was made to separate the formation into the Basin and 

 Pierre shales. The thickness as calculated from dip readings and 

 width of outcrop is 1800 feet. 



This formation is exposed on the flanks of both the Sunshine and 

 Gooseberry Creek anticlines, and where unprotected by the Wasatch 

 formation or the Quaternary gravel it occupies valleys characterized 

 by bad-land topography. 



Mesaverde Formation. — The Mesaverde formation conformably 

 overlies the Cody shale and within the limits of the area mapped 

 consists of about 900 feet of tan and buff sandstones, buff sandy shales, 

 gray sandstones, carbonaceous shales and coal beds. The coal beds 

 are associated with a thick massive white to gray sandstone not far 

 above the base of the formation. Carbonized wood fragments and 

 plant remains are numerous. No detailed study of this formation 

 was made in the field. 



The Mesaverde formation is exposed on the east, west, and north 

 sides of the area mapped. Its outcrop is usually marked by steep 

 cliffs several hundred feet high where it rises above the soft Cody 

 shale. It is locally known as the "rim-rock" and is an effective 

 barrier to transportation except where cut through by streams. 



TEETIAEY SYSTEM 



Wasatch Formation. — The Wasatch formation unconformably 

 overlies all of the older formations exposed in the area. It consists 

 of conglomerates, conglomeratic sandstones, sandstones, tuffs, and clays 

 of continental origin. The lower part is almost wholly composed of 

 conglomerates and rather hard massive sandstones with thin layers of 

 white tuff often containing small rather angular pebbles. The pebbles 

 of the conglomerates and the grains of the sand are subangular to 

 fairly well rounded. The upper part is made up chiefly of white to 



3 This formation includes the Basin shale and Pierre shale of Hintze 's classi- 

 fication. Loc. ext., pp. 76, 77. 



