THE NOETH PARK. H 



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upon the great uniformity throughout, in composition and physical habit, 

 and, in part, upon the palseontological evidence procured from the upper- 

 most beds, at several places, before they passed under the Tertiary deposits. 

 Organic remains from this horizon are abundant all over the Park, and 

 consist chiefly of the following forms: 



Baculites ovatus, 

 Inoceramus Barrdbini, 



which may occur in the lower sandstones of the Fox Hill, but which 

 characterize the Fort Pierre beds, even where the latter are made up of 

 nearly pure black clays. It should be stated that all through this massive 

 sandstone may be seen thin seams and partings of impure clays. The 

 entire thickness of the Colorado group, from the top of the Dakota 

 to the overlying Tertiary, has been roughly estimated at from 1,500 to 

 2,000 feet of strata. 



On the north side of the Park, lying directly upon the Archaean 

 spur of Bruin Peak, occurs an interesting development of the lower mem- 

 bers of the Colorado group. They are well shown along the Platte River, 

 which exposes a steep bank of marls and dark, earthy limestones, carry- 

 ing a small species of Ostrea. 



East of the Platte River, about 2 or 3 miles, and midway between the 

 river and the Laramie and North Park road, lies Sentinel Hill. It stands 

 out boldly, and somewhat isolated from the* main range, and, rising from 

 1,200 to 1,500 feet above the river-bottom, forms a prominent landmark, 

 plainly visible over the entire Park. This hill offers one of the most 

 instructive sections across the Mesozoic strata to be found in the region, as 

 all the strata are well represented from the base of the Red Beds well up 

 into the Colorado Cretaceous. On the north side of Sentinel Hill, the 

 bright-red sandstones of the Triassic occur, dipping into the hill. Over- 

 lying these are light-colored marls of Jurassic age, with the prominent 

 drab limestone, in turn overlaid by more marl beds, which pass up into 

 compact sandstone. The summit consists of Dakota Cretaceous, which 

 also forms the greater part of the southern and outer slope of the hill, 

 but near the base is overlaid by the bright beds of Colorado marls, which 



