EVOLUTION OF MODERN SURFACE FEATURES 



11 



its record is most extensively preserved in the later Paleozoic, 

 Triassic, and Jurassic rocks. This is not the place to discuss the com- 

 plexities and variations of these primarily marine deposits, as we 

 are more concerned with the land areas. 



Miles 



Fig. 2. Map of western United States, showing generalized conditions 

 during middle part of the geosynclinal phase of the development of the 

 North American Cordillera (Pennsylvanian and Permian time). 1, 

 Oceanic area. 2, Eugeosynclinal area; stars indicate approximate posi- 

 tions of volcanic centers. 3, Miogeosynclinal area. 4, Foreland area, or 

 continental interior; partly land, but intermittently covered by ephemeral 

 seas. 5, Deeper sedimentary basins in continental interior. 6, Folds and 

 fold belts. 7, Direction of transport of sediments. 



Within the eugeosyncline, indications of any land areas are elu- 

 sive. Ephemeral islands are suggested by occasional gaps in the 

 sequence (such as absence of any Triassic rocks in parts of the Sierra 

 Nevada) and by local conglomerate layers. Many of the volcanic 

 units change markedly in thickness within short distances, as though 



