130 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
that are elsewhere completely hidden under alluvium (Figure 8). It is 
very clear that the southern valley is in process of being lengthened by 
headward growth or retrogressive erosion at the expense of the northern 
valley. Southward from the divide, there are terraces on the side of 
the deeper valley, attesting the once greater height of its floor. It was 
not possible for us to determine surely any local or general cause for 
the change that is here so actively in progress. It may, perhaps, be in- 
dependent of the uplift in response to which the larger streams have 
cut down their canyons ; nevertheless, we seem to have here as before a 
Figure 8. 
Migrating divide at “Cedar Ridge”’ under Echo cliffs. The geological section here seen 
includes weak Permian strata under the Shinarump sandstone in the foreground; weak 
Triassic strata in the monoclinal valley at mid-distance, and the heavy Triassic sand- 
stones of Echo cliffs in the background. The valley with shaded floor on right (south) 
is encroaching upon higher-floored valley on left (north). Constructed from field notes. 
revival of activities that should have long since become quiescent if the 
erosion of the region had resulted from a single uplift. 
It may be that a similar migration is in progress at the divide be- 
tween the northward and southward drainage of the monoclinal House- 
rock valley, but as this divide lay several miles north of our road we 
could not examine it. 
Migrating Divides in Arid Regions. — It should be recalled in this con- 
nection that the migration of divides in general depends only indirectly 
on the action of streams ; it is directly the work of weathering, creep- 
