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University of California . 



[Vol. 3. 



may have been coincident with them. The hypothesis of differ- 

 ential degradation is, therefore, adopted as the most satisfactory 

 one that can be suggested to account for these high tables. 

 Under this hypothesis the Summit Upland represents a quasi 

 mature geomorphy, the result of the reduction of a high 

 mountain range down to, and in places considerably below, the 

 upper surface of its core of granite. The present summits of 

 Mt. Whitney and Sheep Mountain were never less than 3000 feet 

 above base level. 



While the Summit Upland stood thus, in late Tertiary time, 

 a broad valley was evolved at the western base of that part of it 

 which forms the eastern rim of the Upper Kern Basin. This 

 takes now the form of a broad plateau at an altitude of about 

 11,500 feet above sea level. It appears to represent the base 

 level for the late stages of the cycle in which the maturity of the 

 Summit Upland was evolved. On morphological grounds it is 

 segregated from the Summit Upland as the Sub-summit Plateau, 

 but chronologically it is, in part, the correlative of the upland. 

 It must, therefore, be regarded, also, as the correlative of the late 

 Tertiary peneplain of the western flanks of the Sierra Nevada. 

 A remnant of a plateau at 11,200 feet, to the west of Mt. Van- 

 dever is correlated with the Sub-summit Plateau. The Summit 

 Upland and the Sub- summit Plateau thus appear to antedate the 

 uplift and faulting which dislocated the Sierra Nevada range 

 from the Great Basin. That uplift was not, however, a single 

 event. There were at least two distinct and important move- 

 ments widely separated in time. 



The first movement lifted this portion of the Sierra Nevada 

 about 2,500 feet, the difference in altitude between the Sub- 

 summit Plateau and the High Valley lands. This was undoubt- 

 edly the movement which inaugurated the westward tilting of 

 the peneplain of the western flanks of the range. It was also, 

 undoubtedly, but a part of the very much wider disturbance of 

 the Cordilleran region which inaugurated the Quaternary. By 

 this movement a wedge of the earth's crust 2,500 feet thick at 

 its butt, in the region of the Upper Kern, and tapering to nothing 

 toward the present Great Valley of California, which had hitherto 

 been below base level, was lifted into the zone of erosion; and 



