264 TERENCE T. QUIRKE 
STRATIGRAPHY 
The geologic formations which compose the Killdeer Mountains 
are of Cenozoic age. The whole of Dunn County is underlain by 
_the Fort Union formation of the Eocene system; the upper 400 
feet of the strata forming the mountains is here referred to the 
White River formation of the Oligocene system. 
Fort Union formation.—In the region of the Killdeer Mountains 
the lower strata belong to the upper part of the Fort Union forma- 
tion. In general these beds differ from the lower members of the 
same formation by their darker color. As Leonard says, “The 
upper beds are composed of rather dark gray sandstone and shale, 
with many brown, ferruginous, sandy nodules and concretions.’”* 
The beds are of possible value by reason of lignite and beds of high- 
grade, white, plastic clay. Other characteristics of these members 
of the Fort Union formation are carbonaceous shales, nodular | 
ferruginous concretions, and many selenite crystals in beds of clay 
and shale. 
White River formation—In contrast to the dark color of the 
upper Fort Union formation the southern exposures of the Killdeer 
Mountain sides are as white as chalk. Throughout the entire 
thickness of 400 feet of the upper strata there is scarcely a trace 
of carbonaceous material, especially no lignite. The clays are 
calcareous and low in plasticity. Unlike the firm-grained, com- 
pact fire clays of the Fort Union formation, the clays are crumbly 
in texture and so loosely packed that some are porous. Limo- 
nitic coloring and concretions, common in the Fort Union 
beds, are rare in the upper strata. The small amount of iron 
present is in the ferrous form, tinging the rocks pale green. Cer- 
tain hard sandstones and some of the clays and coarse sands are 
dark green. There are several layers of limestone, whereas, save 
for a few thin beds, limestone has not been reported as a phase of 
the Fort Union formation in North Dakota. 
The outcrops on the mountain sides are few and nowhere con- 
tinuous from the bottom to the top. Fossils are either very rare 
tA. G. Leonard, Geol. Surv. North Dakota, 5th Bien. Rpt. (1908), p. 45. 
2 E. J. Babcock and C. H. Clapp, Geol. Surv. North Dakota, 4th Bien. Rpt. (1906), 
Pwge: 
