THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 31 
which are part of a flanking ridge dissected by cross-drainage. The flanking ridge appears 
also in the distance. In plate 8a the flanking ridge is broader; in plate 9a it is more 
nearly a terrace than a ridge. 
Similar relations between ridges and drainage lines are found in regions of steeply 
inclined strata, each ridge being determined by the outcrop of a resistant formation, 
or at least all of the preceding description might apply to the topography of such a 
region; but other characters remain to be mentioned, and these serve for discrimination. 
5 miles 
ee ee ee ee ee | 

Fic. 2.— Drainage map of Bolinas-Tomales Valley. Heavy broken lines show crests 
of bounding ridges. Light broken lines indicate limits of Rift topography. 
Where a steep-sided ridge is determined by the presence of a resistant formation, the 
determining rock follows and usually outcrops along its crest; but in the ridges under 
consideration there are few rock outcrops, and such as occur are not systematically 
related to the crest lines. The formation of the crest is not always the same thru the 
whole length of the ridge, and it is not always a rock of such character as to resist erosion. 
Between the ridges are linear valleys, and many of these are occupied by streams, but in 
a number of instances they are crost by the drainage. Often they include local depres- 
sions, with ponds or small swamps, this character being so pronounced that forty-seven 
such ponds were seen between Papermill Creek and Bolinas Lagoon, a distance of 11 
