32 REPORT UNITED STATES GEOLOGICAL SURVEY. 



clearly shown as it is in the eastern portion, but yet the lowest portion 

 of this group is everywhere of such a character lithologically as to be 

 not clearly separable from the upper part of the Colorado Group. 



In a general way, the lower division of the Fox Hills Group, which 

 is about 800 feet in thickness, may be defined as a series of thin-bedded 

 sandstones and sandy shales, which are often so easily disintegrated as 

 to become covered with debris resulting from their own disintegration. 

 In some places, therefore, the strata of this lower division seldom pre- 

 sent escarpment faces; but in other places, especially in the western 

 portion of the district, it contains one or two massive strata of firm sand- 

 stone, near the top of the division, that form prominent features in all 

 the exposures there of this portion of the group. The principal one of 

 these massive strata is from 30 to 50 feet thick, and is especially con- 

 spicuous in escarpments, of which it forms the cap. Where this great 

 massive stratum exists, those below it gradually pass into the soft strata 

 of the Colorado Group, and the strata above it are also soft, and are in 

 turn capped by another massive stratum ; and upon these comes the 

 remainder of the upper division. This description applies particularly 

 to the strata in the vicinity of Raven Park. 



Upon the lower division rest about 1,000 feet more or less of regu- 

 larly-bedded ordinary sandstones which constitute the upper division, 

 and which form hogbacks wherever the strata are considerably flexed, 

 and present bold escarpments under other conditions of flexion and 

 erosion. These last-named characteristics of the group are particularly 

 observable in and around Agency Park and the eastern portion of Axial 

 Basin. 



Several carbonaceous horizons were observed in different parts of the 

 district among the strata of this group, including both the upper and 

 lower divisions, but comparatively little coal was discovered. Near the 

 base of the group, however, a bed of coal, probably of workable thick- 

 ness and quality, is to be found in some places. 



The strata of this group occupy quite a large part of the surface of 

 the district, and the upper strata enter into the composition of a large 

 part of the various hogbacks and escarpments within its limits. They 

 are exposed in the north and west borders of Agency Park, in all the 

 borders of Raven Park, and in both the north and south as well as the 

 east borders of the eastern portion of Axial Basin. They occupy that 

 portion of the surface between Yampa River and Williams Fork which 

 lies adjacent to the latter stream, constitute a large part of Pifion Ridge, 

 and are upturned to view in the Midland Flexure from one end of the 

 district to the other. 



THE POST-CRETACEOUS PERIOD. 



Resting conformably upon the Fox Hills Group there is another series 

 ,of strata which differs materially from any of the preceding groups in 

 the character of its invertebrate fossils, although the sedimentation 

 which produced it appears to have been continuous and unbroken from 

 those groups into and throughout the series under discussion. The 

 thickness of this series of strata is greater than that of any of the Cre- 

 taceous groups proper that preceded it, reaching a maximum thickness 

 iu the district here reported upon of at least 3,500 feet. A few hundred 

 feet, constituting the upper portion of this series in the valley of Bitter 

 Creek, Wyoming, has been, by Professor Powell * and myself, separated 



* See Geology of the Uinta Mountains, and Article XXV, Vol. Ill, Bull. U. S. Geol. 

 Surv. Ter. 



