PLIOCENE 519 



northwestern Texas into Oklahoma. Another small Pliocene lake 

 occurred in southern Idaho, but its place in the series is not yet 

 known. 



The volcanic activity in the Rocky Mountain and Pacific coast 

 regions, which had begun in the Cretaceous, continued through 

 the Pliocene. The great outflow of rhyolite which built up the 

 Yellowstone Park plateau is referred to the Pliocene. Near the 

 end of the epoch, or perhaps after its close, occurred the enor- 

 mous fissure eruptions, which flooded northern California and 

 Nevada, southern Idaho, eastern Oregon and Washington, with 

 thick sheets of basalt, obliterating the valleys and revolutionizing 

 the system of drainage. 



A problematical formation is the Lafayette, whose geological 

 position and mode of origin are still debated. The Lafayette is 

 a belt of sands and gravels which runs through Maryland, Virginia, 

 the Carolinas, and the Gulf States, around the southern end of the 

 Appalachians, up to southern Illinois, whence it turns southwest- 

 ward to Texas.' As in the typical exposures the Lafayette rests 

 unconformably upon the Miocene and is unconformably overlaid 

 by the Pleistocene, many authorities refer it to the Pliocene and 

 regard it as a marine formation, while others believe it to be Pleis- 

 tocene and to have been largely formed by waters derived from 

 the melting of the first ice-sheet. The almost complete absence 

 of fossils is a great obstacle to the settling of these questions. 



At or near the close of the Pliocene, extensive upheavals took 

 place in several different parts of the continent, especially on the 

 Pacific slope. The rise of the Rocky Mountains continued, rais- 

 ing the western part of the Miocene lake beds 3000 feet higher 

 than the eastern. The height of the Sierra was greatly increased 

 by the rise of the mountains along the eastern fault plane and the 

 tilting of the whole block westward. The new valleys cut through 

 the late basalt sheets of the Sierras are much deeper than the 

 older valleys excavated in Cretaceous and Tertiary times, which 

 is due to the greater height of the mountains and consequent 

 greater fall of the streams. The fault blocks which form the 

 Basin Ranges were still further displaced, increasing their height. 



