GEOLOGY OF THE KILDEER MOUNTAINS 257 
slips. The map shows that almost nothing of the mountains 
remains but spurs. If the contour intervals were smaller they 
would show that the slopes are benched on account of the varying 
hardness of the rock layers. ‘There are three main benches, which 
are caused by hard layers referred to in the rock section as the 
lower, middle, and upper hard ledges, respectively. All the benches 
show as outcrops in the picture of Indian Knob. 
Work of the wind.—At the base of thick resistant layers the wind 
has undercut the cliff as much as six feet. Undermining of the 
Fic. 1.—Southwest spur of South Mountain. It is almost a semicircle, half a 
mile in periphery, with precipitous inner walls, and landslide topography below. 
resistant layers causes the overhanging mass to be held in place 
by the shearing strength of the rock. If there were caves along 
the zone of stress, rupture would be likely to follow the plane of 
weakness. ‘The layers of interstratified sand and limestone show 
the wind’s work. The wind has scooped out the sandstone for 
eight or nine inches, leaving layers of the limestone projecting to 
this extent. In a less arid and less windy climate these conditions 
might be reversed. | 
Work of running water.—Charlie Bob Creek on the northwest, 
and Jim Creek on the northeast, both tributaries of the Little 
Missouri, are the chief agents of erosion. The tributaries of the 
Missouri River, particularly Knife River and its tributary, Spring 
