234 The American Geologist. 
October. 1905 
erosion. As is shown by the map they cover all the lower 
country of the region and extend across the area at the 
head of Oak creek to the Kaya Paha (this latter was not 
positively determined to be Cretaceous). The formation is 
composed, principally, of dark gray shales containing brach- 
iopod shells and Baculites. A bluff of this formation on 
the south side of White river near the mouth of White 
Thunder creek is three hundred feet in hight; Baculites were 
obtained from its summit. The entire thickness of these 
shales is not exposed in the region examined, although their 
thickness there must exceed 600 feet. 
The shales are destitute of value, for they contain no 
stone, no building material, no water. The area they dom- 
inate is an alkali, salt grass, prairie-dog town country. 
The Oligocene formation forms the clay bad lands lying 
between the Cretaceous and the Loup Fork (Arikaree and 
Ogallala) formations, forming a rough broken strip which 
runs in a general east and west direction across the country. 
It once covered the whole region as is shown by detached 
patches beyond the confines of the area mapped, but has 
subsequently been removed. Its strata are usually hori- 
zontal. They are composed, for the most part, of light 
colored to pinkish soft shales with which are interstratified 
bands of harder material carrying some iron. Sandstone 
strata, however, are occasionally exposed. The formation 
beginning with a cobble-stone gravelly stratum, lies uncon- 
formablv on the Pierre shales. Its shales break down and 
so slack in water that they are readily carried off with it. 
When mixed with sand in the right proportion, the clays 
formed from the shales make a hard cement, which the 
Indians use in plastering. The cream colored to pinkish 
colored potato hills, cones, castles, terraces, and hog-backs, 
formed by the breaking down of these shales, make this 
strip picturesque. 
The formation has the appearance of having been be- 
gun by river action. Later, judging from the fineness of 
the material, the area must have become a deep lake and 
remained such for a long period. Finally, the lake was 
filled with sediment till the region reached the swamp stage; 
the deposition then ceased. 
