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GUIDEBOOK OF THE WESTERN Ul^TED STATES. 



with great outpourings of black lava, wliich spread out sheet on sheet, 

 buried the old land surface, and partly filled the vallej with molten 

 rock^ which solidified and has remained to this day undisturbed 

 except for the gorges that the streams have cut in it. In some places 

 old mountains project through the petrified lava flood as islands pro- 

 ject above the sm-face of the sea, and old ridges stick out into it as 

 capes and promontories. 



The description of the Snake River plain below given^ is taken 

 from a report written in 1901 by I, C. Russell. 



^ Soutliem Idalio is a region composed of 

 geologically old rockSj ^vhicli formed an 

 ancient land surface having a nigged 

 relief. In the depressions of this sur- 

 face, during latei' geologic timej exten- 

 sive lake and stream deposits and vast 

 lava flows were spread out. The older 

 rocks, sharply separated from the younger 

 by a long time inter\'al, during which 

 extensive movements in the earth's crust 

 and deep erosion took placOj ai'e mainly 

 granite, rhyolite, quartzite, and lime- 

 stone. The younger of these is probably 

 the li mestone which is thought to b e of Car- 

 boniferous age. These rocks were variously 

 folded, faulted, and upheaved into prom- 

 inent mountains, and deeply dissected by 

 a large river, with many tributaries, 

 which was long lived. The valley of the 

 main stream^ the ancient representative 

 of Snake River, became broad and had 

 many important tributaiy valleys open- 

 Lag from it and extendiiig far into the 

 bordering mountains. The sharp-ctested 



volcanoes and became deeply filled. 

 These sediments, which have a known 

 depth of over 1,000 feet, are now well ex- 

 posed, particularly in southwestern Idaho, 

 In places they contain impressions of 

 leaves of trees which grew on the borders 

 of the old lake, the shells of fresh-water 

 mollusks, the bones of land mammals, and 

 other remains. The fossils record a Ter- 

 tiarj^ (Miocene) age. 



Before Lake Pavette came to an end 

 the vast lava flows which now form such a 

 conspicuous feature of the Snake River 

 basin began to be outpoured. In fact, 

 the lava and the sediments of Lake Pay- 

 ette and of a later lakfe in the same basin 

 were contemporaneous, the lava and lake 

 sediments being interbedded. Some of 

 the lava flows entered the lake, and the 

 j occurrence of thick beds of, volcanic 

 fragments (lapilli) and of scorlaceous, 

 glaasj^ la\'a, with a torn and slaglike 

 structiu'e, at the base of thick sheets ol 

 usually compact basalt records the enci^y 



mountain spurs between the lateral val- of the steam explosions that followed. 

 leys are in some instances prolonged far Highly liquid lava continued to be poured 



into the main dep 



After the topography had passed matu- 

 rity — that is, after the streams had exca- 

 vated deep valleys, leaving pharp-crested 

 or serrated divides between them — ^the 

 main stream was obstructed, possibly 

 by lava flows, but more probably by an 

 upward movement of the rocks athwart its 

 cour^^e, in the r^ion now included in 

 western Idaho and eastern Oregon, and a 

 lake was formed which occupied a large 



included in the 



part of the country 

 Snake River plains. This water body, 

 named by Lindgren Lake Payette, re- 

 eeived the. sediment brought in by trib- 

 utary streams and the dust blown out by 



out at^various intervals from a large num- 

 ber of volcanic vents and spread out in 

 the pre\'iously formed basin, making, in 

 truth, lakes of molten rocka. Besides 

 these two processes of upbuilding — that 

 is, sedimentation in lakes and the out- 

 pouring of lava which spread widely 

 there was a third, the washing of debris 

 from the uplands and its deposition in 

 alluvial cover and widely extended sheets 

 of sand, gravel, and silt in the valleys. 

 In addition, there are widespread eolian 

 [wind] deposits. The volcanic eruption 

 continued after the lakes were either 

 filled or drained, bo that by far the larger 

 portion of the Snake River plains is 



