66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
slopes of the earlier valley show that it must have reached a more 
advanced stage of development before revival than the later one 
has since attained. The slopes produced by the dissection and pene- 
tration of the terraces by the present streams are steeper than those 
above noted. They bear every evidence of youth and represent a still 
Fig. 4. Whitewood Creek looking southwest from a point half a mile below the 
west end of the gravel terrace. 
more recent stage in the history of Boulder Creek valley. These 
features are illustrated in Figure 2. 
In Whitewood Creek the lower terrace is not easily recognized but 
the upper terrace level may be seen extending southwest beyond 
Deadwood along the lower shoulders of the valley. Figure 3 repre- 
sents the view in that direction from a point on the east side of White- 
wood Creek just below the west end of the main gravel body. In it 
the terrace and the older valley sides, with the characteristic shoulder 
above, may be clearly seen. White Rock is a rather sharp-featured 
residual, but in the main the upper topography is subdued. Later 
incision has produced a steep-sided gorge, which is better shown in 
Figure 4, where the more gently sloping spurs of the older valley are 
truncated at about the terrace level. The latter may be traced down 
Whitewood Creek along the shoulders of the later gorge. On these 
shoulders lie the patch of gravel at locality 16 (Plate 1), already de- 
scribed, and a similar patch on the opposite side of the valley, mapped 
by H. G. Ferguson. The older Whitewood valley, in which the pres- 
ent gorge has been cut, was not so broadly opened as the ancient 
Boulder valley, and, since the rocks and structures in which both are 
cut are essentially the same, it may be inferred that less time was 
consumed in its excavation. 
