Ransome.1 



The Great Valley. 



413 



dissected Pleistocene plain. Near the falls these bluffs are very 

 low, and swing in close to the river; but further west they attain an 

 elevation of probably 100 feet or more, and are separated from the 

 river by broad stretches of level alluvium. Upon surmounting the 

 crest of the bluff, a similar level plain is revealed stretching back to 

 the low rounded foot-hills. Such a terrace can hardly indicate any- 

 thing else than a rise of this portion of the Great Valley's edge 

 relative to the local baselevel of the Merced River, which is deter- 

 mined by the San Joaquin. Thus the lowest flanks of the range 

 show evidence, not of a dragging down or subsidence, but of a 

 gradual elevation up to very recent times. The result has been to 

 confine more and more the latest deposits in the Great Valley to 

 narrowing strips of flood-plain bordering the rivers, and to increase 

 the area of the Sierra Nevada foot-hills at the expense of the level 

 plain. 



It seems fitting at this point to consider the application of the 

 isostatic hypothesis- to the Great Valley, made by Becker.* He 

 considers that the elevation of the Sierra Nevada was not effected 

 by a more or less simple tilting of a great crustal block, as is gen- 

 erally supposed, but by the cumulative result of a series of innu- 

 merable small northwest and southeast faults of Pliocene age, hav- 

 ing their upthrows on the eastern side. The effective force for 

 this faulting and elevation he supposes to have been derived from 

 the isostatic subsidence of the sediment-laden valley floor, which 

 produced not only a viscous flow of material underneath the rising 

 Sierra mass, but supplied also the tangential force by which that 

 elevation was mainly effected. 



This hypothesis is plainly open to any objection that has already 

 been brought against isostatic subsidence in the valley, when no 

 particular cognizance was taken of the further mechanical conse- 

 quences of such adjustment. But Becker's hypothesis, by following 

 out these consequences, and attributing to the energy of the sub- 

 siding mass a certain mechanical result in the elevation of the 

 adjacent mountain range, has made it possible to apply to it a rough 

 quantitative test. 



The accompanying diagram is intended to represent a transverse 



* Loc. cit. , p. 73. 



