324 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
Above these shales the Niobrara formation has at its base 20 to 30 
feet of limestone, compact, brittle, and fossiliferous. The remaining 
300 feet vary from calcareous shales to shaly limestones; they are 
fossiliferous and bituminous, and certain beds, from a few inches to 
a few feet in thickness, are almost solid masses of Ostrea shells. 
The topmost beds are of a gritty, shaly limestone having a buff color. 
Overlying these and prominent from their contrast in color are the 
dark shales at the base of the Pierre. They are-spar ingly fossiliferous, 
but contain much finely disseminated carbonaceous matter. The 
Pierre has a total thickness in this vicinity of probably 7,000 feet. 
The dark color belongs more particularly to the lower part, the 
remainder being generally lighter and having, after weathering, a 
characteristically greenish-drab tint. 
While, as a whole, a remarkably uniform mass of shale, the Pierre 
has occasional beds which are more or less sandy, the constitution of 
these varying from clay shale to pure sand. Only One such sand- 
stone attracts attention by its outcrop. This is a gritty bed lying 
about 2,000 feet above the base. Three miles north of Boulder it is 
about 100 feet thick, but it thickens rapidly toward the north. From 
about 3 miles north of Boulder its outcrop is almost continuous for 
many miles to the north. There has been considerable speculation 
concerning the relation of this sandstone to the oil, but, as will be seen 
below, the stratum has no special significance. The texture of the 
Pierre will be mentioned more particularly in connection with the 
subjects of drilling and the occurrence of the oil. 
Structure. — All the strata are steeply upturned against the moun- 
tains. The Red Beds immediately west of the oil fields dip toward 
the plains at an angle of about 55°. Higher strata outcropping 
farther to the east have successively greater dips until the Niobrara 
is reached, which is about vertical. Eastward from this line there is 
a rapidly diminishing dip, and within a very few miles the position 
may be horizontal or even show local westward dips. 
Minor folds almost certainly exist. In the absence of ledge-making 
strata they can not be determined by outcrops and have no influence 
on the topography. Excavations have, however, revealed occasional 
dips which are plainly not a part of the general eastward inclination. 
Also the data from one small group of wells, but 2^ miles from the 
foothills, strongly indicate local folding with dips as high as 15° or 
20°. Here again the data are so limited that the direction of the 
folding is uncertain. If parallel to the mountains, it may be regarded 
as attendant upon mountain making; if the minor folds trend east 
and west, they are in harmony with the Boulder arch so far as that 
existed. a Other indications of a compressive force acting north and 
south are locally observable in the sinuous strike of the Niobrara 
limestone. 
"Eldridge, G. H., Mori. U. S. Geol. Survey Vol. XXVII, p. 110. 
