VALLEY OF THE NORTH PLATTE. 153 



growing in little ridges along these cracks gives to the surface the appear- 

 ance of a planted field. These sandstones form the southern member of a 

 broad, shallow, synclinal fold, which occupies the valley north of Sage 

 Creek, the northern member of which, as already described, is very steep. 

 On the opposite side of the Platte River, east and southeast from the mouth 

 of Sage Creek, the Cretaceous beds, dipping from 1° to 3° to the northeast, 

 rise in long level benches of friable coarse gray sandstone, stretching com- 

 pletely across the valley to Ellc Mountain, where they again dip westerly, 

 and may be traced to the beds, already described, on Pass Creek. South- 

 ward the limit of Cretaceous strata is less definitely determined, as, in a 

 nearly horizontal position, they lie concealed beneath Tertiary deposits 

 of similar lithological habit, which have been referred to the North Park 

 beds. These Tertiary beds extend down the valley of the Platte as far as 

 Cottonwood Creek, and rest in plateau-like ridges against the crystalline 

 gneisses of the Medicine Bow Range. Near the mountains, in a number of 

 localities, they may be seen lying unconformably upon the upturned edges 

 of the Cretaceous. 



As thus described, by the aid of the geological map it will be seen 

 that the Fox Hill Cretaceous forms a remarkable synclinal basin, irregular 

 in shape, but extending in an east and west line for 32 miles, and measuring 

 across its broadest expanse about 16 miles in width. The persistency of the 

 bluff-like formation of the Fox Hill beds over so wide an area is a marked 

 feature in the geological structure of the region. 



Perhaps equally noteworthy is the occurrence of the upper members 

 of Colorado Cretaceous clays, which crop out along the base of the bluffs, 

 as seen on the Medicine Bow River, at Elk Mountain, along the Sage 

 Creek Bluffs, and at the Mount Steele Ridge, in all of which localities the 

 beds of the lower series pass up gradually into the upper sandstones, both 

 divisions being perfectly conformable. 



In general, these Fox Hill beds are not rich in organic remains ; but 

 sufiicient palseontological evidence was obtained at various points, in rude 

 casts of Inoceramus and BacuUtes, and some characteristic plant-remains, to 

 define their horizon as belonging to this group. 



A number of sections were made through portions of the Fox Hill 



