GEOLOGY OF THE KILDEER MOUNTAINS 263 
which flows east follows probably the valley of a smaller tributary 
of the Missouri. Jim Creek, a tributary of the Little Missouri 
River, captured the head waters of Spring Creek, a tributary of 
the Missouri. Then the captured additions of Jim Creek pushed . 
back into the gap between the two mountains until they cut off 
the head waters of Charlie Bob Creek, a fellow-tributary of the 
Little Missouri River. Fig. 4 shows the change in the divide 
between North and South mountains, and the change in the 
Fic. 6.—View from the northeast end of South Mountain, looking toward North 
Mountain. A vertical line through the center of the picture indicates the locus of 
the old divide. Compare with Fig. 4, and note the decapitated slopes. 
course of the creek which flows from the north valley of South 
Mountain. 
Origin of the mountain features——The character of the topog- 
raphy proves that the Killdeer Mountains are relict mountains 
greatly dissected by the ordinary agents of erosion. The rock 
strata are flat-lying and of unequal resistance. There are three 
strong layers which have given rise to the plateau tops and to two 
benches, one preserved only in the peaks, and one below the main 
table-land (Fig. 6). The decapitated slopes and the cases of stream 
piracy were caused by rejuvenation of the drainage due to a post- 
Kansan change in the course of the Little Missouri River. 
