338 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
miles distant from the fault, with 2 mountain crests intervening. In this interval the 
intensity had diminished from X to less than VII, but at Lakeport and Upper Lake it 
rose to IX. The topographical and geological maps of the Clear Lake district, published 
by Becker,’ show that Lakeport and Kelseyville are on an alluvial plain, the under- 
lying deposits of which are of Quaternary age; and the same conditions prevail at Upper 
Lake. Between this area of alluvium and Bartlett Springs, on rocky ground, 10 miles 
to the east of Upper Lake, the intensity dropt to VI. At Lower Lake, situated on 
Tejon sandstone, the intensity had similarly dropt to VI; and these intensities are about 
the normal for the distances at which Bartlett Springs and Kelseyville lie from the fault 
along the coast. At Highland Springs, the intensity was between VII and VI, which 
is also about the normal for its distance from the fault. It thus appears that the high 
apparent intensity was confined to the alluvial or recent lake deposits of the area about 
Lakeport. These facts indicate that the high apparent intensity for this area was prob- 
ably not due to a local earthquake, coincident or nearly so with the main shock, but 
that the destructive action of the latter was locally augmented by conditions inherent 
in the underlying incoherent deposits. For if there had been a local dislocation, its 
effects would undoubtedly have made themselves manifest over a wider area than that 
occupied by these deposits. The character of the shock, as described by those who 
experienced it in the vicinity of Clear Lake, agrees, moreover, with that of the shock 
emanating from the fault at the coast. Becker’s geological map of the district shows 
no faults traversing it. 
In general, then, while from the nature of the case it is not possible to deny positively 
that a local earthquake may have occurred on the morning of April 18, 1906, at the same 
time as the main shock, no evidence appears to sustain that view. On the other hand 
the evidence here as elsewhere supports the belief that the apparent intensity is a func- 
tion of the underlying formations to the extent manifested in this district. 
Coming now to Santa Rosa Valley, we encounter an interesting case of high intensity, 
associated with an alluvial valley-bottom. The valley may be described as an oval- 
shaped area, extending for 24 miles from Healdsburg to the vicinity of Penn’s Grove, 
with a maximum width of 8 miles on a line lying between Santa Rosa and Sebastopol. 
The general trend of the central axis of the valley is about N. 30° W. It is thus not 
far from parallel with the general trend of the fault along the coast. Over a considerable 
expanse the valley-floor is perfectly even, and appears level to the eye. At its widest 
portion, however, it has a slope from an elevation of 170 feet above sea-level in the eastern 
part of the city of Santa Rosa, to about 50 feet above sea-level, a descent of 15 feet to the 
mile. In this section there are no terraces, but a perfectly even profile. To the north of 
Santa Rosa, however, the floor of the valley is less even, and it is stept in a few broad 
terraces, the lowest of which is the present flood plain of the Russian River. 
The geomorphogeny of the valley is not altogether simple; the primary fact in its 
development, however, is that it has been carved by stream erosion to its full width out 
of a great syncline of Merced (late Pliocene) strata.? The upturned edges of these Merced 
strata, planed down to an even but now somewhat dissected surface, constitute the floor 
of the upper terrace lying to the north of Mark West Creek at an altitude of about 200 
feet above the flood plain of the Russian River. On a somewhat lower terrace is the town 
of Windsor. South of Mark West Creek, the valley is in general deeply alluviated and the 
wells a little to the east of the city of Santa Rosa (150 feet deep) show that the alluvium 
is saturated with ground-water to within a short distance of the surface. The distribu- 
tion of this ground-water thruout the valley is, however, not well known, no systematic 
investigation ever having been made. On the western side of the valley from Sebastopol 

* U.S. Geological Survey, Monograph XIII. 
> Cf. Osmont, Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3. 
