UPPER CRETACEOUS. 673 



Along the foot of the Rocky Mountains and the Laramie Front Range the formation usually 

 presents three limestone layers, a lower massive bed and two upper layers, separated by limy 

 shales, the uppermost overlain by impure limestones, which weather to a bright-yeUow color and 

 always contain flat masses of limestone consisting of colonies of Ostrea congesta. The formation 

 thins to the north in Colorado and Wyoming, becoming about 400 feet thick northwest of Chey- 

 enne, and about 200 feet on the slopes of the Black Hills. The bright-yellow color of 

 the weathered beds is a conspicuous feature. The point farthest west at which the formation 

 was noted is southwest of Casper. It is not characterized along the slopes of the Bighorn Moun- 

 tains, although doubtless it is there represented by some gray shales, not distinguishable from 

 those of the adjoining formations, for there is no suggestion of hiatus in the sedimentary series. 



Pierre shale. — The great shale series of the Pierre formation occupies a vast area in the 

 central Great Plains and was probably originally of even greater extent, for it appears to have 

 been removed by erosion in the mountain uplifts, in eastern Nebraska, and in southern and east- 

 ern Kansas. No special investigation has been made of the Pierre stratigraphy, and, although 

 the beds seem to be uniform in composition, probably a careful study of the distribution of its 

 numerous fossils would show widespread stages. One of these is the upper horizon of concre- 

 tions with Lucina occidentalis, giving rise to "tepee buttes," which appears to extend from 

 Arkansas Valley through Colorado to and aU around the Black HiUs. In places • along the 

 western margin of the area great variations in thickness are presented, the shales becoming 

 thicker and local sandstone beds being included. West of Denver the formation appears to have 

 a thickness of over 7,700 feet, and of considerably over 3,000 feet at Florence and near Boulder. 



Fox Hills formation. — The Fox Hills formation appears to be present everywhere between 

 the Pierre and the Laramie, merging into both formations and constituting beds of passage 

 between them. In some districts the Fox Hills beds begin abruptly with a sudden change 

 from the dark shales of the Pierre to sandstones or sandy shales of the Fox Hills containing 

 some distinctive species. It is probable that this change does not take place at the same horizon 

 throughout, and the Fox Hills fauna appears in connection with the sandy sediments. 



Usually the Fox HUls deposits are less than 300 feet thick, but in the Denver region, where 

 they comprise a thick mass of sandy clays in their lower portion, they attain a thickness of a 

 thousand feet. The top member in this region is a persistent characteristic sandstone, 50 feet 

 thick, which appears to be the same as the Trinidad sandstone- in the Spanish Peaks district 

 in south Colorado. The top of the Fox Hills formation is not always clearly definable and in 

 most cases can only be placed arbitrarily at the horizon where the first deposits of evident 

 fresh-water origin appear. 



The divisions thus distinguished are commonly classified as the Dakota sand- 

 stone, the Colorado group (Benton and Niobrara formations), and the Montana 

 group (Pierre and Fox Hills of the eastern plains) . The term Dakota was proposed 

 by King*"^" and given the present definition by Eldridge,^"^ who at the same time 

 proposed the term Montana. The strata, faunas, and distribution of the Colorado 

 have been described in detail by Stanton."^ 



K 12. NOBTHEASTERN tTTAH, NOBTHW^ STEBN COLOBADO, AND SOTTTHWESTEBN 



WYOMING. 



The Cretaceous strata of the eastern Uinta Range were described by Powell ^^^ 

 under the names Henrys Fork, Sulphur Creek, Salt Wells, and Point of Rocks 

 groups. The latest work in this region has been by Gale,^^^ who has described 

 the Rangely oil district, which lies in Colorado just north of the fortieth parallel. 

 In a correlation table Gale shows that the Cretaceous of western Colorado is now 



48011°— 12 43 



