shallow gutters down each side, owes its existence to major 

 geological faults that define its east and west borders. Outside 

 these, the whole land surface has risen to form the North and 

 South Montane Blocks, and the Cascade-Sierra Nevada ranges 

 The tlrcat Basin has for long, and still is. being squeezed be 

 tween these, and this has caused a great deal of subsidiary 

 faulting in the basin itself, along with consequent mountain 

 building, so that today the whole province is covered with 

 parallel strings of mountain ranges These are all about fifty to 

 seventy-five miles long and six to fifteen wide, but rise only 

 some three to five thousand feet ofT the plateau Apart from the 

 Snake River, which really only passes through the area, there is 

 no drainage system leading out of it. and precious little leading 

 into it. What rain falls on it cascades down these little moun- 

 tains into the depressions between and either sinks below ground 

 or evaporates. Were there enough rainfall, this water would 

 find its way downhill over the surface until it formed a river 

 system going to the sea. As there is not enough and has not been 

 for a very great time, other mechanisms have had long enough 

 to become effective. 



The infrequent rains are often torrential, and. falling upon 

 such bare ground, they wash enormous quantities of material 

 down the mountains to form great fans and outwashes in the 

 valleys. Further, the great differences between day and night 

 temperatures, especially during the winters, crack up the rock 

 surfaces and cause screes of exceptional dimensions. All of these 

 structures may meet across valleys and create dams and barriers. 

 so that the level land between the mountain ranges is formed 

 into strings of bowl- or basin-shaped depressions. These become 

 little catchment basins for whatever surface moisture there is. 

 and those lined with fine sediments and clays may retain water 

 in the form of shallow lakes. A large percentage of the whole 

 province is covered with these structures, and a great many 

 of them contain either permanent or intermittent lakes or dried- 

 out lake floors. Two of these are of enormous size but of most 

 irregular outline and are filled with "islands." These are the 

 ancient or "fossil" lakes today called Lahontan and Bonneville. 



The depth and size and even the very existence of these lakes 



and incipient lakes are entirely dependent upon the amount of 

 rain that falls This varies from year to year and in various 

 longer cycles: it has also varied very greatly during the geo- 

 logically recent ice advances and retreats, notably during the 

 latter, when great volumes of ice on the surrounding montane 

 glaciers melted. 



LAHONTAN AND BONNEVILLE 



The Great Salt Lake of Utah is the largest natural inland body of 

 water in the United States west of the Mississippi, but it is a 

 paltry remnant of a much greater body of water that not too 

 long ago rivaled the Great Lakes of the east. So big was it. and 

 its sister Lake Lahontan to the west, that both figure prominently 

 on the accompanying map. These lakes are really only areas 

 that are lower than the surrounding territory and have no out- 

 lets. They are formed of checkerboards of the smaller bowls and 

 basins and pans described above. Both are alleged to have been 

 created in the first place by the melting of snow and ice on the 

 surrounding mountains. Bonneville was once 350 miles long 

 and 150 miles wide and some 1000 feet deep. Lahontan was 

 about 250 by 100 but was only 500 feet deep. Today the former 

 has shrunk to the Great Salt and Provo lakes; the latter, which 

 was always of more irregular outline and contained many more 

 islands, has left numerous remnants, notably lakes Honey. 

 Pyramid. Winnemucca. North Carson, and Walker. 



Both lakes Bonneville and Lahontan were initially fresh, and 

 they went to depths of 910 and 320 feet respectively. They then 

 went through a dry period when they became saline, but this was 

 followed by a wetter period when they rose to 1000 and 530 feet 

 in depth. At this time Bonneville found an outlet over Red Rock 

 Pass to the north, and some 375 feet of its water drained off in 

 a very few years into the Snake River. Then the snows stopped 

 melting and/or the rains came less torrentially on the mountains, 

 and the two lakes started evaporating, thus becoming increasingly 

 saline. The exact heights to which the waters rose can be de- 

 termined from altitude measurements of the noticeable wave- 



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