GEOLOGY OF THE KILDEER MOUNTAINS 259 
may be seen: west of the gap uplands sloping to the west and 
streams flowing to the east, and east of the gap eastward flowing 
streams and remnants of an eastward sloping upland. The streams 
flowing northward likewise have cut into these long slopes, but since 
the heads of the streams have been working toward the south the 
action has been less effective, for direction of slope in this region 
makes a great difference in the rate of erosion. On steep, south- 
Fic. 3.—Indian Knob. This picture illustrates the withering effect of the sun’s 
strength on the south slopes; it shows also the three hard ledges. 
facing slopes in western North Dakota grass scarcely starts before 
the intensity of the sunshine and the shallowness of soil cause it to 
wither. On north-facing slopes the difference is remarkable, the 
lesser heat of the sun permitting a luxuriant growth of grass and 
several varieties of trees. Trees grow on the northward facing 
slopes to the top of the divides, but on the mountain top and on 
the southern slopes, outside of sheltered valleys, there are no trees. 
Indian Knob (Fig. 3), a conical hill on the southern border of the 
mountains, illustrates the effect of the sun. The south side is bare 
and precipitous; the north slope is clothed with brush to the very 
top. With little or no vegetation to hold the soil on the south 
