GEOLOGIC BACKGROUND 9 



Rocky Mountain area which cut off the moist westerly winds and partly to the breaking down in 

 middle Miocene time of the continent Eria (in part the land bridge from Greenland), whereby the 

 colder Arctic currents came down along the Atlantic seaboard. More or less desert conditions 

 developed in the Cordilleran region and have prevailed there to the present time. The forests were 

 markedly reduced in areal extent, and this epoch as a whole, but particularly in the latter part, was 

 characterized by a decided increase in grass-covered plains, which produced great changes in food 

 conditions for the faunas. 



There was initiated a broad upwarp along the axis of the Rocky Mountains which continued 

 intermittently until its culmination in the Pleistocene. This resulted in the deposition of Miocene 

 sediments, not only over the Oligocene but beyond its boundaries as far south as Texas and nearly 

 across Nebraska and Kansas to the east. Deposition was not of course continuous over this whole 

 area at any one time. The lower Miocene formations (Gering, Rosebud, Monroe, and Harrison) 

 are generally coarser than the White River, as would be expected with the renewal of uplift to the 

 west. 



On the West Coast mountains and around Florissant Lake, Colorado, there prevailed a south 

 temperate climate, with a fair amount of moisture but not continuous humidity. Forests of willow, 

 poplar, oak, elm, and other trees grew in those areas. It was predominantly a hardwood flora, 

 and yet it had a southern aspect. 



The Great Basin came into existence in this epoch, and the Miocene sediments contained 

 therein are several thousands of feet thick. Farther north at this time the lava of the Columbia 

 Plateau was poured out over an area of more than 200,000 square miles. 



The Cascadian Revolution began in the middle Miocene and is still in progress at the present 

 time. The highlands of eastern Washington and Oregon were formed. Active volcanoes were 

 numerous over the Great Basin region, over the Colorado Plateau, and in Mexico. In fact, the 

 Miocene was an epoch of predominant vulcanism throughout the western part of North America. 

 The deposition during this epoch, especially in the northern areas, was characteristically of altered or 

 reworked volcanic ash, and in some places layers of pure ash are found. 



In the Oligocene there was a short period of faunal interchange between Europe and North 

 America which ended with the final phase of White River time. In the Deep River of the upper 

 Miocene there was another interchange by way of Siberia and Alaska, when the rhinoceroses and 

 elephants came from Asia and Africa. 



Oreodonts both small and large were abundant throughout this epoch, with a much more 

 marked diversity of form and with each branch more highly specialized than in the Oligocene. 



Between the Miocene and Pliocene again there is no distinct separation in the sedimentary 

 record, for the general trend of geologic events progressed into the latter epoch. The orogenic 

 movements continued until North America was more emergent than at any other epoch of the 

 Cenozoic, and this resulted in a more arid and increasingly much cooler climate. The whole series 

 of the Cordilleras was being elevated, and the contraction of streams and the summer droughts were 

 becoming more and more prevalent. The volcanic activity continued from the Rocky Mountains to 

 the Cascades and Sierras, but with diminishing vigor. 



Great numbers of land tortoises were present in the Pliocene, and the aspect of the land was for 

 the most part that of open, relatively arid country. It is significant that the rhinoceroses, browsing 

 horses, and browsing camels disappeared at this time, and that before the end of the middle 

 Pliocene the oreodonts had completely vanished. 



