88 
the peninsula of Lower California with its adjoining water bodies on 
both sides, 
The central or mountain belt is the dominating one of this strue- 
tural group and has exerted an important influence on each of the 
bordering belts. It is at the same time the barrier that separates them, 
determining their. main differences, and the link that relates them to 
each other. A complete understanding of the mountain belt can only 
be gained by a study which includes both of its bordering belts, and it 
is for this very reason that the plan of the excursion was so well adapted 
to the geologically inclined visitor—better, for example, than a much 
longer excursion that might have ended in the midst of the mountain 
belt, without reaching the border en the desert side. 
- The Coastal Belt—For some miles after leaving San Diego, the 
most striking geological feature is the broadly developed, gently sloping 
or practically flat terrace or ‘‘mesa’’ which extends up to the moun- 
tains. It represents a surface developed in earlier times when the 
coastal belt lay lower with respect to the sea level than it does at the 
present time. 
‘This terrace is commonly underlain by gravel with a rust-colored 
matrix, the original deposit laid down by the agents that shaped the 
old plain. This deposit conforms with the terrace surface and_ there- 
fore usually forms a horizontal or slightly inclined layer which in eliff 
or canyen sections is seen to contrast with the underlyine sandstones, 
shales and other rocks of the older formations whose strata usually He 
more markedly inclined to the horizontal, and sometimes dip at high 
angles. It is evident that these older rocks (Cretaceous and Tertiary, 
but whose time limits have, as far as I know, not been definitely ascer- 
tained) were disturbed by various earth movements—tilting, folding, 
faultine—before the terrace material was laid down, and the plain of 
which this terrace is the remnant was originally cut across the older 
strata by processes of abrasion. 
The ‘‘mesa’’ is separated from the ecean, and in many places from 
the valleys, by a number of narrower terraces, with intervening steeper 
slopes or cliffs. Along the immediate coast there is definite evidence 
that these terraces are raised ocean beaches; farther back, they are of 
stream origin. Just how large or how small areas should be appor- 
tioned between marine and fluvial abrasion was not determined, but, 
to a hasty view, the major portion of the broad terrace covering ap- 
peared to be the result, of stream deposition, and the valley extensions 
of the narrower terraces are evidently of river origin. 
Comparatively recent successive elevations of the coast have caused 
the streams to work to successively lower base-levels and the old plains 
} 
~ 
