EVOLUTION OF MODERN SURFACE FEATURES 29 



trapped to form lakes, but the nature of their deposits indicates 

 that they possessed outlets through most of their existence. 



It is obviously impossible to mark out the courses that were pur- 

 sued by the Eocene streams. Even the courses of exterior drainage 

 much later in Tertiary time are problematical, and the early history 

 of the modern rivers of the Cordillera has been diversely interpreted, 

 as we shall see. 



Cenozoic Volcanic Activity. An additional item of the Eocene and 

 later Tertiary environments in the eastern part of the Cordillera 

 can be discussed appropriately at this point — the widespread and 

 persistent volcanism. 



Some trace of volcanic activity can be seen in almost all parts of 

 the region, but with variable manifestations — in places, isolated 

 volcanic cones, patches of lava, and shallow intrusions; in others, 

 extensive volcanic piles that stand as massive plateaus or moun- 

 tains. Time relations also vary. Some volcanic fields are so ancient 

 that only deeply eroded conduits and dike swarms are preserved; 

 others are recent enough that the forms of the cones, calderas, and 

 flows are still recognizable; still others present a record of inter- 

 mittent volcanic activity through much of Tertiary time, and 

 even later. 



Cause of the localization of the larger Tertiary volcanic fields is 

 not entirely certain. Many of the Tertiary intrusive bodies are 

 clearly aligned along zones of weakness in the country rock, related 

 to the Mesozoic orogenies; such alignments may also exist in the 

 conduits of the volcanic fields but have been hidden by the eruptives 

 that overspread the surface. It is perhaps significant that one of the 

 extensive volcanic fields, in the Absaroka Mountains and Yellow- 

 stone Park of northwestern Wyoming, lies at the east end of a 

 transverse zone of volcanic rocks of various ages that extends west- 

 ward across the Cordillera nearly to the Pacific Coast. Another 

 series of volcanic fields extends around the Colorado Plateau, from 

 the San Juan Mountains on the east through the Mogollon Plateau 

 and San Francisco peaks on the south to the High Plateaus of Utah 

 on the west, as though the plateau block were separated from its 

 neighbors by nearly continuous zones of weakness. 



Volcanism began in the eastern part of the Cordillera during the 

 later stages of Laramide orogeny (Fig. 5). Large volumes of andesitic 

 debris occur in the Upper Cretaceous and Paleocene deposits of 



