EVOLUTION OF MODERN SURFACE FEATURES 



13 



this area had a history Hive that of the remainder of the stable con- 

 tinental nucleus, intermittently emergent or receiving the thin de- 

 posits of ephemeral seas. In later Paleozoic time, however, part of it 

 began to lose its previous stability. On the site of the Rocky Moun- 



130" 



120* 



100° 



.500 



1000 



Miles 



Fig. 3. Map of western United States, showing generalized conditions 

 late in the geosynclinal phase (Triassic and Early Jurassic time). 1, 

 Oceanic area. 2, Eugeosynclinal area; stars indicate approximate positions 

 of volcanic centers. 3, Miogeosynclinal area. 4, Sedimentary wedges that 

 spread from the geosynclinal area across the foreland: (a) dominantly 

 marine, (b) dominantly continental. 5, Land areas that probably did not 

 receive deposits. 



tains of Colorado and New Mexico several broad fold ridges were 

 raised; troughs subsided rapidly between them and received thick 

 accumulations of waste eroded from the uplifts (Fig. 2). These 

 "ancestral Rocky Mountains" are the ends of one of the w^estern 

 branches of the mountain belt that formed during Paleozoic time 



