EVOLUTION OF MODERN SURFACE FEATURES 



17 



Sierra Nevada, and perhaps also the plutonic phase, was accom- 

 pHshed during a relatively brief interval near the end of Jurassic 

 time, but before its close (Taliaferro, 1942, pp. 102-105). It has been 

 inferred, as well, that much of the deformation in the eugeosynclinal 



I00» 



3 VpA /;// 



500 



1000 



Miles 



Fig. 4. Map of western United States, showing generalized conditions 

 during early part of orogenic phase (Late Jurassic and Early to Middle 

 Cretaceous time). 1, Oceanic areas. 2, Clastic deposits laid down along 

 edges of orogenic belts; arrows indicate direction of transport of sedi- 

 ments. 3, Marine deposits of foreland area. 4, Folds produced by Nevadan 

 orogeny, late in Jurassic and early in Cretaceous time. 5, Folds produced 

 by orogeny later in Cretaceous time. 6, Miogeosynclinal area. 7, Volcanic 

 centers. 



belt to the north and south, where age relations are less clear, took 

 place during the same epoch. 



Various considerations suggest that events were more complex 

 and prolonged than thus implied, even in the Sierra Nevada region. 

 The eugeosynclinal rocks were first folded, steeply upturned, and 



