93 
not have obtained during the development of the advanced mature or 
peneplain surface of the summit region. If that were in process of 
formation during the Cretaceous and early Tertiary, then the present 
relations must have been established by differential movement of the 
mountain belt with respect to the desert and coastal belts since that 
time. The sequence of major movements and resulting conditions and 
activities have not been definitely determined. The evidence at hand, 
however, would seem to indicate that the latest movements, which re- 
sulted in physiographic relations of the elevated and depressed belts, 
essentially as at present, took place in rather late geological times—not 
earlier than late Tertiary, possibly even in early Quaternary. To its 
comparative recency of development, and to the arid climate are due the 
steepness and rugeedness of the mountain front that must be descended 
to reach the Imperial valley. 
The Desert Belt.—We ran but a short distance out on to the valley 
floor, but far enough to see some of the important features of its western 
edge. The streams, for all their shortness of water, bring much detritus 
down into the valley, including very coarse material which is mostly 
deposited not far from the edge of the mountains. One might have ex- 
peeted that all the older formations of the valley would be buried from 
sight by the more recent detritus. But evidently the western edge of 
the valley itself is now in proeess of dissection and erosion, and shows 
exposures of older formations in the sides of the present stream trenches. 
And it is not only the streams which have their sources in the mountains 
that have cut such trenches. There are many small branching gulehes 
formed by the temporary streams that result from the oecéasional desert 
rains. They work back into the old valley floor by headwater erosion 
from the trenches of the mountain-born creeks. 
So by the side of the road we found exposed stratified sediments— 
shales, sandstones, etcetera—tilted up at a considerable angle. They 
could be traced up to and into a high ridge that extends into the desert 
to the north of the road (Carrizo Mountain). These rocks have been 
referred to the upper Miocene by Arnold. They evidently had been 
formed and then disturbed, tilted, folded, and faulted before the pres- 
ent general relations of mountain and desert were established. 
Kew has pointed out that the fossils found in these beds are strik- 
ingly different in species and character of development from those found 
in beds of presumably the same age found in the coastal province. Only 
8.5 per cent of the species are common to the two regions. The differ. 
ences are of the same general nature as now found between the fauna 
of the shores of the Pacifie Ocean and of the Gulf of California and in- 
dicate that at that time, as at present, the mountain belt existed as a 
