200K ys TERENCE T. QUIRKE 
slopes erosion is much more rapid than it is on the north. Thus 
it is that streams which work their heads toward the north erode 
more rapidly than those which flow northward. 
Rejuvenation of the drainage.—The north and northwest drainage 
down the upland slopes interosculates with the heads of the east- 
ward flowing tributaries of Jim Creek, which have worked their 
way round North Mountain both to the north and to the south. 
They have cut off the heads of the upland slopes, leaving them 
separated from the mountain tops, stranded remnants of an older 
drainage system. This is indicated in Fig. 4. At present the 
chief agent of erosion is the Little Missouri River, whose tribu- 
taries have a maximum fall of 1,200 feet in 6 miles. The upland 
slopes show that there were once streams with a gradient much less 
steep. Previous to some pre-Wisconsin ice invasion, probably 
Kansan,‘ the Little Missouri River entered the Missouri through 
the valley of Tobacco Garden Creek? (Fig. 2). The ice sheet 
ponded the waters of the Little Missouri, Yellowstone, and Mis- — 
souri rivers, and caused them to overflow to the southeast. After 
the retreat of the ice sheet the Little Missouri River did not return | 
to its old course, but flowed eastward, north of the Killdeer Moun- 
tains, across the divide between its old course and the Missouri. 
This change rejuvenated the drainage of the Killdeer Mountains. 
The present surface of the valley now occupied by upper Cherry 
Creek and Tobacco Garden Creek is about 150 feet above the pres- 
ent level of the Little Missouri River channel, and that valley is 
30 miles farther away from the mountains than the Little Missouri. 
Thus, before the glacial period, the Killdeer Mountains were not 
being eroded so rapidly as now, because the gradient of their 
drainage system to the northwest was only about 1,050 feet in 36 
miles in place of 1,200 feet in.6 miles, 29 feet per mile in place of 
the present 200 feet per mile. The captured streams and numerous 
decapitated slopes about the Killdeer Mountains resulted from the 
Little Missouri River being turned by the glacial invasion from its 
t A. G. Leonard, Quar. Jour. Univ. North Dakota, VII (1917), 233. 
2 Frank A. Wilder, Geol. Surv. North Dakota, 3d Bien. Rpt. (1904), p. 119. Also 
U.S. Geol. Surv. Water Supply and Irrigation Paper 117 (1905), p. 43. A. G. Leonard, 
‘Pleistocene Drainage Changes in Western North Dakota,” Bull. Geol. Soc. America, 
XXVII (1916), 302. 
