MANSFIELD: POST-PLEISTOCENE DRAINAGE. 19 
stream that carved Spiegel’s Gap originated in a similar manner, cut 
through the hard Minnekahta limestone and excavated the weaker 
strata beneath. At this time there could have been no master stream 
flowing along the west flank of the dome, else Sandy Creek and the 
Spiegel’s Gap creek would have joined it and would not have carved 
for themselves gateways outward to the Red valley. Field evidence 
suggests that this portion of Whitewood Creek has been formed by 
the coalescence of subsequent branches of several streams, through 
successive captures, and shows that the main stream has been diverted 
from a northerly course at the elbow three miles west of Crook Moun- 
tain. The present summit of the dome stands nearly five hundred 
feet above the level at which this capture occurred. At the time of 
the capture Crook Mountain must have already become a considerable 
eminence and superposition of Whitewood Creek upon it was there- 
fore impossible. The apparent deflection of Whitewood Creek by 
Crook Mountain seems to the writer to be best explained as a response 
to structure by the development of a subsequent stream along the 
strike of weak beds. Where the strata bow outward around the moun- 
tain, there the stream following the strike bows outward also. 
Conciusion. From a study of the field evidence and an investiga- 
tion of the literature, the following drainage history may be outlined 
for the region under discussion. By the increase of relative humidity 
which accompanied the close of the Tertiary period, the Pre-Oligocene 
Boulder valley was revived, and, during the prolonged erosion of 
early Pleistocene times, it was broadly ex 'avated. Later, by the 
return of more arid climatic conditions, it was aggraded to the level 
of the higher terrace. At that time its stream was probably joined 
by the predecessor of Bear Butte Creek and other tributaries and 
held its course east-northeast to the plains through the Spring Creek 
gap. At that time, too, Crook Mountain dome, already sufficiently 
uncovered to influence drainage, was sending radial streams in many 
directions. As a result of the northeast tilting, which followed the 
aggradation period, those streams which had northeasterly courses 
were given an advantage over their fellows and were allowed to cut 
more rapidly. Such a stream, working backward into Boulder Park, 
diverted the Bear Butte tributary. A similar stream, originally 
radial on Crook Mountain, found monoclinal shifting more easy than 
deep cutting. It accordingly shifted down the east flank of the dome 
and, by working rapidly headward through the soft Red Beds, diverted 
the main stream through Crook valley. The stream that made the 
next capture, two miles below Deadwood, was also favored by having 
