158 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ville time this part of the mountain base was well protected by the grow- 
ing delta of Weber river. 
The Erosion of the Spur-Facets. —It is evident from what has been 
said in the section on the mountain face that the retreat of a mountain 
front from its initial fault scarp will be greater on the stream lines than 
on the interstream surfaces, and again greater at the apex of a facet than 
at its base. The concentration of drainage—even if it be only wet- 
weather drainage — along the stream line of the ravines, and the increase 
in the volume of the streams from head to mouth has given them strength 
enough to remove the waste that weathers and creeps down from the 
ravine walls. There is, however, at present no such concentration of 
removing agencies along the foot of the mountains ; and as the duration 
of the Bonneville waters at their various levels has been but a small frac- 
tion of the whole life of the mountains, it may be said that there has pre- 
vailingly been no active agent available for the removal of waste from 
the mountain base between the mouths of the streams. 
If the fault plane were vertical, a large amount of rock waste would 
have fallen from it, and in the absence of any effective removing agency 
along the mountain base, some of the waste should accumulate there as a 
talus, Figure 13, whose foot should advance in front of the fault line. 
The conspicuous absence of such talus makes it probable that the fault 
plane was by no means vertical, and suggests that the slope of the spur 
facets may not be greatly unlike the slope of the faults. In the Spanish 
Wahsatch, the small facets slope at an angle of 38° or 40° ; in the Provo 
Wahsatch, the slope is from 32° to 38°. Certain ranges in northern 
Nevada had similarly steep basal slopes. 
Other Parts of Block Mountains. — In the early stages of faulting, the 
back slope of a tilted mountain block should exhibit its prefaulting 
form little changed, except that all the slopes and streams which had 
been steepened by the tilting would show signs of more active erosion 
than the other parts. The lower part of the back slope would be 
buried under accumulating waste. 
In a later stage of faulting, it might be impossible to recognize any 
survivors of the prefaulting forms, unless near the back base where the 
small depth to which erosion could penetrate would delay change. The 
back base line would expectably be much more sinuous than the front 
base line, for at the back of the range the gravels and sands of the 
intermont depression would mount obliquely upon a surface in which 
the inequalities of prefaulting time had been somewhat exaggerated by 
the revived erosion of the early stages of tilting. 
