Basin and Range Region:
Centered on the state of Nevada and extending from southern Oregon to western Texas, the Basin and Range Province is an immense region of alternating, north-south-trending, faulted mountains and flat valley floors. It has no counterpart elsewhere in the U.S. The province was created about 20 million years ago as the Earth's crust stretched, thinned, and then broke into some 400 mountain blocks that partly rotated from their originally horizontal positions. These mountains of late Precambrian and Paleozoic rock continue to erode and fill the intervening valleys with fresh sediment.

Continued Development:
The crust beneath the Basin and Range is still stretching today. Faults are active, mountains are pushing upwards, and basins are widening and filling with debris washed down from the high country. This landscape, which appears so everlasting, is actually in the midst of a geologic revolution, played out over millions of years. As the crust continues to stretch, the North American Plate will eventually be divided into two pieces, and a new ocean will form in between them. In other words, the state of California moves away from Colorado, (roughly "stable" North America) at approximately 1 centimeter per year.

Great Basin National Park encompasses most of the South Snake Range. The bulk of the rocks exposed in this range are formed of sediments like sand, mud and limey ooze (silt and clay particles mixed with calcium carbonate) that were laid down on the bottom of a shallow sea during the late Precambrian and Cambrian (around 560 million years ago). As layers accumulated upon layers, the sediments were turned into sedimentary rock. Sand lithified into sandstone, mud into shale, and limey ooze into limestone.

The rocks in the park were further changed during a mountain-building event that occurred around 200 million years ago during the Mesozoic Era. This event, the Sevier Orogeny, pushed layers of rock on top of each other, doubling the thickness of the crust. The layers at the bottom of the stack were metamorphosed slightly - sandstone changed gradually into quartzite, limestone to low-grade marble. Magma rose from deep within the Earth and pushed its way up into these layers. It did not come to the surface, however. Staying underground, it cooled to become granite. Where this hot magma was intruded, the surrounding rock was metamorphosed slightly more.

After all of this activity, the region still did not resemble the present landscape. The modern basins and ranges began to appear only within the last 30 million years or so, during the Cenozoic Era, when the Earth's crust in this area began to stretch in an east-west direction. Bedrock nearest the surface reacted to the crustal stretching by breaking into immense blocks several miles wide, tens of miles long, and thousands of feet thick. Many of these blocks fractured and the pieces tilted and spread out like a row of odd-sized books sliding out of place on a shelf. The remnants of these broken blocks lie beneath the sediment in the basins. Other blocks remained relatively intact and now form the mountain ranges. Because stretching is in an east-west direction, these ranges line up in a north-south direction. The South Snake Range was to see even more change. The younger unmetamorphosed layers of rock on top of the range slid off of the older metamorphosed rocks in a southeasterly direction, on a very low-angle fault line called a decollement. This event makes the South Snake Range a metamorphic core complex. The end of the Cenozoic Era witnessed more granitic intrusions into the park, as well as colder climates that further shaped the landscape.



-- Excerpts from:
USGS A Tapestry of Time and Terrain Website, 2002; and U.S. National Park Service, Great Basin National Park Website, 2002