256 TERENCE T. QUIRKE 
PHYSIOGRAPHY 
Character and growth of the valleys —The areal distribution of 
valleys in the Killdeer Mountains is relatively large. The main 
valleys are wide-mouthed, steep-sided, and nearly flat-bottomed. 
Numerous short, steep-sided gulches cut into the level upland. 
The slopes of the main valley floors are gradual because they are 
in uniformly soft clay and sand formations; but the heads and sides 
of the valleys are steep because they are bounded by the resistant 
interbedded limestone and sandstone in the upper part of the 
plateaus. The gulches and valley heads are still in infancy, whereas 
the lower valleys are farther along in their work of lowering the 
valley floors to the level of the plain. The valley floors are strewn | 
with masses of resistant rock from the upper slopes. In some places 
the beds of the upper slopes have slipped down from their original 
position in such a way that they retain their horizontal position, 
although about 100 feet below the level in which they were in place. 
In some places streams have cut their way in behind the slump, 
thus increasing the slump by continually undercutting the already 
sliding cliff face. In some cases the slumps were started by the 
formation of caves. Several of these caves have the position of 
fissures, but their size and their irregular shape and the high lime 
content of the rock show that they were enlarged, at least, by solu- 
tion. Nearly parallel to the south-facing cliff of South Mountain, 
near Oakdale, there is a cave opening known as Medicine Hole, 
which gives access to a crack at least 90 feet deep and a score or 
more feet long. This opening cuts through the limestone forma- 
tion which caps that part of the mesa, and the bottom of the aper- 
ture is in the weak sand formation. Less than too feet south of the 
fissure is the cliff face of the upper hard ledge. As soon as this 
ledge is undermined a little more the separated mass will slump. 
Spurs.—The advance of the valleys has left little but the spurs 
of what was once a much larger mountain. The spurs are long, 
skeletal, and straggling. Perhaps the most striking spur is the 
southeastern end of the South Mountain, just above Oakdale, but 
the southwestern end of the mountain is also notable (Fig. 1). It 
is almost a semicircle, half a mile in periphery, with precipitous 
inner walls. Below the cliffs lie benches and hollows due to land- 
