EVOLUTION OF MODERN SURFACE FEATURES 19 



similarity to any modern features. The present Sierra Nevada, for 

 example, was produced much later by tilting and faulting of a block 

 within the orogenic belt; other parts were variously raised, broken 

 up, depressed, or buried. 



Sedimentation West of Nevadan Belt 



Beyond the strongly deformed and altered rocks of the Nevadan 

 orogenic belt, west of the Sierra Nevada and southwest of the 

 Klamath Mountains, are less altered Upper Jurassic and Lower 

 Cretaceous strata — the Knoxville formation and Shasta series — a 

 sequence of marine shales, sandstones, and minor conglomerates. 

 These are turned up against the Coast Ranges on the west side of 

 the Sacramento Valley, where they are as much as 8 miles thick. 



It has been thought that they were not laid down until after the 

 Nevadan orogeny, and that they accumulated in a new trough that 

 developed west of the Nevadan belt and east of the ancestral Coast 

 Ranges along the Pacific Ocean, with most of the sediments derived 

 from the latter (Anderson, 1938, pp. 25-29; Taliaferro, 1942, pp. 

 103-104). These views require reexamination, as recent paleontologic 

 work indicates that part of the deformed Jurassic rocks of the Sierra 

 Nevada on the east (Monte de Oro formation) are of the same age as 

 the Knoxville, and that a large part of the Franciscan group of the 

 Coast Ranges, once thought to underlie the Knoxville, is as young as 

 early Late Cretaceous (McKee et al., 1956, pp. 3; Irwin, 1957). The 

 dates of orogenic events and distribution of lands and seas during 

 Late Jurassic and Early Cretaceous time must therefore have been 

 quite different from those that have been inferred. 



It may be that deformation of the rocks during the Nevadan 

 orogeny diminished westward as well as eastward, so that the sites 

 of the Sacramento Valley and Coast Ranges were little disturbed 

 during the orogeny. The Franciscan, Shasta, and Knoxville strata 

 may have been deposited before, during, and after the climax of the 

 Nevadan orogeny, and have been laid down along the edge of the 

 continent and on the continental slope, in part beneath ocean water 

 of considerable depth. Further critical studies are needed to deter- 

 mine the source of this great body of sediments, but part of them, 

 perhaps the greater part, must have been derived from erosion of 

 rising lands in the orogenic belt to the east and northeast (Fig. 4). 



