20 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
The ridges exhibit for the most part a pronounced parallelism in a direction more or less 
oblique to the mean trend of the coast and of the Coast Ranges as a belt. The highest 
of these ridges rarely exceed 5,000 feet in altitude and their crests usually range between 
2,000 and 4,000 feet above sea-level. Rarely the tops of the ridges are more or less flat, 
presenting the character of a rolling upland, the rule being that the crests are deter- 
mined by the intersection of the valley slopes on either side. In the northern Coast 
Ranges, however, it is generally true that the ridge crests over wide areas reach about 
the same altitude and give the observer the impression of a dissected upland of fairly 
uniform and gentle slope. The valleys in which the streams flow are usually wide- 
bottomed in the softer formations and narrow in the harder rocks. In such portions 
of the region as have been geologically examined, it appears clear that the courses of 
these streams are closely, tho of course not wholly, controlled by the strike of the rocks 
or the strike of faults. The general scheme of drainage is that which might be termed 
subsequent, the streams having adjusted themselves to the structural lines, and having 
been greatly extended by headwater erosion along those lines at the expense of original 
consequent streams, traversing the region transversely to the trend of the structure to 
the sea on the one side, and to the Great Valley on the other. This interpretation is 
rendered more plausible by the fact that, in a general way, the broad structure of the 
Coast Ranges appears to be that of a geanticline, with various subordinate folds, the 
dissection of which by erosion reveals the Franciscan rocks in the central portion of the 
ranges, flanked on either side by rocks of later age. This interpretation appears to be 
quite acceptable for the Eel River and its various branches, which constitute the chief 
drainage of the northern end of the region. This drainage has all the characters of a 
subsequent system, and is in harmony with the mature aspect of the ridges and valley 
slopes. All the numerous tributaries of the river flow in longitudinal valleys, parallel 
or subparallel to one another, and connected by short transverse streams cutting thru 
the intervening ridges; and the course of the longitudinal valleys is that of the strike 
of the rocks, being, like the latter, oblique to general trend of the Coast Range belt. 
Thruout this region, within the hydrographic basin of the Eel River, there are below the 
crests of the ridges numerous instances of high valleys and broad, more or less obscure 
terraces, representing an inheritance from earlier stages of the geomorphic evolution of 
the region, when it stood at lower levels than at present. These have been described 
in a valuable paper by Diller.’ 
Between the headwaters of Eel River and the Bay of San Francisco the interpretation 
of the drainage as subsequent is not so certain, altho here the general geomorphic profile 
is even more mature than it is on the north, a fact referable to the softer character of 
certain geological formations which prevail. Here we have, as before, a system of stream 
valleys, notably Russian River Valley, Sonoma Valley, Napa Valley, and Berryessa, and 
Clear Lake Valleys, which are clearly evolved by stream erosion under the control of 
structure. The transverse connecting link from one longitudinal valley to another, which 
is so characteristic of subsequent drainage, is not apparent on the maps, but its absence 
may be more apparent than real. The lower stretch of Russian River, from Healds- 
burg to the sea, has the appearance of a transverse stream tapping a longitudinal valley 
of very mature character, and may be the remnant of an original consequent stream. 
This view, however, is open to the objection that the lower stretch of Russian River 
near the sea has a more youthful aspect than might reasonably be expected under the 
hypothesis. On account of the rather immature character of the transverse outlet of | 
Russian River, it has been suggested that it is of later date than Russian River and — 
represents a small stream which has cut its way back from the coast and captured the 
waters of the river, which formerly went to the Bay of San Francisco, the capture being 
1U.58. G. 58. Bulletin, 196. 


