32 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
miles. (See plates 9B, 10, 43, 54a.) The valleys range in width from 20 or 30 feet to 
about 500 feet, the majority falling between 100 and 200 feet; and each of them is 
approximately uniform in width, unless occupied by a stream. In a typical cross- 
profile, the side of the valley is somewhat definitely distinguished from the bottom by a 
change of slope (see fig. 5), the distinction appearing at one or both sides. 
In view of these characters, and especially of the abundance of ponds, it is evident 
that these little valleys are not products of stream erosion; and that in so far as they 
are occupied by streams the streams are adventitious. Their true explanation is sug- 
gested by their relation to certain of the earthquake phenomena of April, 1906. As will 
5 miles 
——————E SS eS eee Be eee 

Fic. 3.— Hypothetic drainage map of Bolinas-Tomales Valley, if developed without 
influence of Rift displacement. Compare fig. 2. 
presently be described in detail, the trace of the earthquake fault thru the greater part 
of its course in the larger valley follows the edge of one or another of these small valleys; 
and in places where the fault movement included vertical dislocation, such dislocation 
nearly always tended to increase the depth of the valley. (See plate 108 and fig. 6.) 
Of the numerous minor or secondary cracks developed by the earthquake in the immedi- 
ate vicinity of the main fault, a considerable proportion occurred at the edges of the little 
valleys, following more or less closely the line along which the bottom meets the side; 
and with these cracks also there was usually a little vertical dislocation, the ground 
