92 
cyele occupied at least all of the Cretaceous and possibly the early 
Tertiary, though its limits are yet to be determined. 
During the time of the production of this old surface the region 
of the present mountain belt must have had a much lower position with 
respect to sea level than it has at present. For marine sediments formed 
from its waste are found near San Diego, and such a topography could 
not be formed at a high elevation so near a, base-level of efficient erosion. 
The subsequent uplift of the region to its present altitude was not 
a continuous process. There is evidence within the mountain belt and 
along its edges, that it took place during several suecessive periods with 
longer or shorter periods of quiet between. Early in this history of 
uplift a beginning was made in the dissection and destruction of the 
old surface form and later movements have only tended to accelerate 
the destructive process. Under recent conditions of elevation and base 
level the streams are cutting deep canyons along the border of the 
mountain belt and these are extending their influence back towards or 
into the summit region. With the long history of attack, it is remark- 
able how the influence of the older surface persists in the present 
topography. 
The Eastern Transition Zone.—At the east edge of the mountain 
helt, the most striking feature is the steep descent to the broad Imperial 
valley. From a large area of rather mild topography, we have an 
abrupt transition into high, precipitous and rugged slopes which in a 
short distance lower the surface several thousand feet, and then again 
an abrupt passage into the low relief—to a large extent an almost feat- 
ureless floor—of the valley of the Salton Sea. The view from the upper 
edge of the mountains is very extensive and is most impressive. The 
contrasts are marked. 
The mountain front is notched by canyons, and these are being 
extended back into the summit region. The progress of their attack 
and the stage of their development are among the more interesting 
eeological features of the eastern part of the mountain belt. ‘They are 
far less developed than the canyons of the western part, and have done 
much less damage to the earlier erosion forms. Without more definite 
data this might be supposed as possibly in part due to a later develop- 
ment of the eastern than of the western mountain slope, or to an initial 
west slope of the plateau surface, or a western tilt developed during 
uplift, that would determine longer and more effective drainage lines 
towards the west. It is probably dependent in part on the difference in 
rainfall on the east and west slopes. 
It is clear that the present relations of the mountain belt and the 
Imperial valley, likewise of the mountain and the coastal belts could 
