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University of California. 



[Vol. 2. 



incised and serrated forms. The differences .ire dependent in part 

 on differences in topographic age, and in part on differences in the 

 character of the rocks. 



Topographic age is estimated by the amount and character of 

 the contrast due to subaerial erosion, allowance being made in this 

 estimate for differences in the resistance of the rocks to, such 

 erosion. Where the rocks are hard, the contrasts will be greater, 

 the subaerial slopes less gentle and the topography more rugged, 

 for a given stage of development, than where the rocks are less 

 resistant. The influence of marine abrasion as a factor in deter- 

 mining topographic age depends upon circumstances. Given an 

 isolated elevated mass, standing on a coast line, or forming an 

 island, wave action, by decreasing the volume of the mass, aids 

 subaerial erosion in bringing the elevation nearer the condition of 

 ultimate base-level, and in this way tends to increase topographic 

 age. On the other hand, when a coast line is depressed, wave 

 action tends to obliterate the contrasts produced by previous 

 subaerial erosion, and to simplify the general topographic form. 

 Therefore, on re-elevation, the youth of the topography will have 

 been renewed, to a greater or less degree, so far as the surface has 

 been brought within reach of wave action. 



The forms of the Coast Range, in general, approach maturity, 

 and are characterized, for the most part, by narrow ridges and 

 V-shaped valleys. It is generally held by Pacific Coast geologists 

 that these forms belong mainly to the period of erosion following 

 the close of the Miocene, the land then standing at a greater eleva- 

 tion than now. 



Although the coastal forms are, on the whole, quite rugged, 

 there are many variations in the amount and character of the 

 dissection, due mainly to differences in the resistance of the rocks 

 from which the topography is carved. Some of the forms in the 

 less resistant rocks, especially where isolated, present, compara- 

 tively, very gentle slopes, and are not deeply dissected, although 

 the forms may still be called mature for that particular rock. 

 Along the immediate coast line the more rugged topographic 

 forms do not appear to have been greatly modified by wave action 

 in post-Pliocene times; the less resistant the rocks, the gentler the 



