68 DESCRIPTIVE GEOLOGY. 



feet more. For the entire thickness of the deposit, the beds would indicate 

 from 1,200 to 1,500 feet. 



As ah-eady mentioned, the beds are found lying unconformably upon 

 the older uplifted strata and overlapping the area of the Miocene basin. 

 South of the Union Pacific Railroad, they occur abutting against Mesozoic 

 formations; just north of Granite Canon, they lie next the Archaean 

 mass; and a short distance beyond, at the mouth of Crow Creek Canon, are 

 found essentially horizontal against nearly vertical Palaeozoic limestones. 

 From Crow Creek northward, as indicated upon the geological map, they 

 may be seen resting directly upon every formation, from the Archaean to 

 the Fox Hill Cretaceous. • 



In general, the lithological aspect of the Niobrara Tertiary strata 

 closely resembles that of the Miocene basin, as already described ; but 

 from the much greater area over which they occur, and from the many 

 widely separated exposures, affords peculiarities not observed in the limited 

 outcrop of the latter. The most prominent feature in the Pliocene, in dis- 

 tinction from the underlying formation, is the prevailing arenaceous nature 

 of the beds, and the greater persistency in character, through a wider verti- 

 cal range.. Marls, clays, coarse and fine sandstones, conglomerates, with 

 some nearly pure limestones, make up the strata. Of these, fine, marly 

 sandstones are the predominant beds, while light-colored limestones are 

 exceptional, and would appear to occupy only small areas, probably lentic- 

 ular masses, filling shallow basins and depressions. 



Over the greater part of these Tertiary plains, the upper layers are 

 made up of coarse, hard sandstones, which near the mountains may be 

 called a conglomerate, firmly held together by a fine cementing material, 

 forming a sort of concrete. They consist of fine and coarse pebbles, crys- 

 tals of quartz, and feldspar ; chiefly material from the Archaean granites and 

 gneisses, varying in size from a pea up to a foot in diameter. The upper 

 stratum withstands atmospheric agencies remarkably well, and serves to 

 protect the more easily eroded beds beneath. The character of these over- 

 lying conglomerates is well shown along the gently-sloping bench upon 

 which the railroad reaches the mountains, and along the bluffs upon both 

 sides of South Crow Creek, where it not infrequently overhangs, eaves- 



