GEOLOGY OF THE KILDEER MOUNTAINS Dom 
of different lakes joined by a river or rivers, for the deposits are 
found only near the top of the divides. They were once probably 
continuous and widespread. Furthermore the major part of the 
Killdeer deposits are of fine materials derived from a basin rich in 
limestone. The fineness of the materials betokens long transpor- 
tation and thorough sorting, for the gravels are found only in the 
lower strata and only near the southern part of the deposits. The 
limestone layers are mechanical rather than chemical or organic 
deposits. The stream supplying the Oligocene lake must have been 
rather swift to carry so much calcareous and siliceous mud, and 
it must have come, without passing through any settling ponds, 
directly from the limestone highlands in the south. 
CONCLUSION 
The Killdeer Mountains are relict mountains on the divide 
between the Little Missouri and the Missouri rivers. They are 
deeply dissected by the tributaries of the Little Missouri River. 
Erosion has been hastened by the rejuvenation of the drainage due 
to a change in the course of the Little Missouri River. The change 
was caused by the advance of a pre-Wisconsin ice sheet, probably 
Kansan. The change increased greatly the gradient of the streams 
flowing from the mountains and gave rise to many cases of stream 
piracy; hence there was rejuvenation without uplift. 
The upper strata of the mountains are correlated with the White 
River formation. This extends considerably the known distribu- 
tion of the Oligocene series. Study of the formation leads to the 
conclusion that some of the lower beds were deposited by a river 
and that most of the rocks were laid down in a lake. The lake 
was fed by.a river flowing northward from the Black Hills and 
transporting large quantities of calcareous silt from South Dakota 
to western North Dakota. 
