Lawson. 



Coast of Northern California. 



251 



In general the stream topography is sufficient!}' advanced to 

 bring into prominence the great control which geological structure 

 has exerted upon the evolution of the drainage. A glance at the 

 drainage map shows a very remarkable parallelism of all the main 

 valleys of the western slope of the Coast Ranges. Not only are 

 they parallel with one another, but their general direction is coinci- 

 dent with a remarkably straight stretch of shore line which extends 

 from Point Arena to the head of Tomales Ha)-, and is continued as 

 a strongly marked topographic feature through Ballenas Bay across 

 the entrance of the Golden Gate. This remarkable line is the trace 

 of a great fault which was established in pre-Miocene time, and was 

 again, in post-Miocene time, functional as a plane of dislocation. 

 This line is coincident with the general strike of the (earlier?) Meso- 

 zoic rocks which make up the greater part of the portion of the Coast 

 Ranges here considered. Beyond Point Arena the shore bends 

 away from this line and assumes a more northerly trend; and most 

 of the rivers of the region emerge, therefore, on the coast obliquely. 

 This great system of parallel valleys and dividing ridges is clearly 

 an illustration of fully developed subsequent drainage on an exten- 

 sive scale. Historically these subsequent streams are of two kinds. 

 In the more mature portions of the original low, baseleveled tract 

 which formed the surface of the block whose sculpture we are now 

 discussing, the streams may be presumed to have wandered indiffer- 

 ently across the plain with a general direction normal to the coast. 

 The tendency of the geological structure to control their courses 

 prior to the plain condition may, as Davis suggests, * be assumed to 

 have lost its effectiveness in the later stages of the old geomorphic 

 cycle. In the early stages of the new cycle inaugurated by the 

 uplift these water courses persisted as antecedent streams, and to 

 them were added new consequent streams. As the relief became 

 more marked, the influence of the underlying structure made itself 

 felt. The dominant lines of weakness in the structure of the block 

 began to appear, and along these lines subsequent streams were 

 rapidly pushed. As the degradation proceeded, the}- became the 

 main drainage lines. The antecedent and consequent streams 

 dwindled, and many suffered atrophy. A few persisted as short 

 * Journal of Geology, Vol. II. No. 1, p. 80. 



