National Park ServiceU.S. Department of the Interior

Great Basin National Park Great Basin National Park Service

U.S. Department of the Interior

Great Basin National Park

Great Basin

The majority of the rocks in Great Basin Na-tional Park formed during the Cambrian, a timewhen western North America was located nearthe equator as part of a continent calledLaurentia. Thick layers of limestone, shale, andsandstone accumulated along a passive margin,where warm shallow seas lapped at the edge ofthe continent, similar to regions like islands ofthe modern Caribbean.

These rock units have survived numerousepisodes of mountain building (orogenies).These included the Antler Orogeny during theDevonian era, the Sonoma Orogeny during thePermian era, and the Sevier Orogeny during theCretaceous era (Figure 2). The Sevier Orogenypushed or “thrusted” thick layers of rock on topof one another that doubled the thickness of theearth’s crust. This caused the metamorphismthat turned the sandstone into quartzite, shaleinto slate, and limestone into marble. TheCretaceous is the same time when magma,molten rock formed within the earth’s interior,intruded or pushed its way into the overlyingrock layers. This magma cooled, forming thebodies of granite found throughout the park.

The modern basin and range topography isrelatively new, forming in the last 30 millionyears, during which time the crust of the GreatBasin has been subjected to extensive stretching

and thinning. The result of this stretching causedthe crust to break up into blocks. Some of theseblocks slid down past one another along faultsforming the valleys, and those blocks thatremained intact formed the mountains of today.

Geologic History

Great Basin National Park lies within the Basin and Range physiographic province.The entire region has experienced crustal thinning and deformation that producedextensive faulting. Along these roughly north-south-trending faults, mountainsuplifted and valleys down-dropped, producing the Basin and Range topography.

Geology

has been metamorphosed by heat andpressure. This extremely hard rock isresistant to erosive forces, explainingwhy the high peaks and cliffs in thepark are primarily quartzite.

Great Basin National Park covers a majority ofthe southern Snake Range, a mountain rangewith a maximum elevation of 13,063 feet (3,982m) at the summit of Wheeler Peak. The south-ern Snake Range is primarily composed ofquartzite, shale, and limestone intruded by smallbodies of granite. Quartzite is sandstone that

Limestone forms in warm water rich withcalcium carbonate and is easily eroded by waterinto exotic geologic features such as LexingtonArch and Lehman Caves. Shale forms with theaccumulation of silt and clay particles in rela-tively calm water. Granite, with its salt andpepper appearance, forms when magma intrudesthe earth’s crust and slowly cools. A variety ofsmall granite intrusions are located in JohnsonCirque, one of many glacial features in the upperelevations of the park.

Geologic Description

The Great Basin physiographic region ischaracterized by basin and range topography

As the earth’s crust stretched, faults formed alongwhich valleys down-dropped and mountains uplifted.

E X P E R I E N C E Y O U R A M E R I C A

The mountains in Great Basin National Parkhave been shaped by a variety of erosional anddepositional processes. The most influential ofthese was Pleistocene glaciation, that occurredduring several distinct “Ice Ages” over the last 1.5million years. The modern landscape of the parkprovides evidence for extensive glaciation from40,000 to 60,000 years ago and again from 14,000to 25,000 years ago.

As the glaciers grew in response to cooler sum-mers and snowier winters, they moveddownslope, eroding canyon walls and replacingv-shaped valleys (typical of stream erosion) withu-shaped valleys characteristic of glaciated areas.These glaciers carved out several cirques (featureswith steep ridges and shaped like a horseshoe).When the ice retreated, it left behind beautifulalpine lakes such as Johnson Lake, Baker Lake,and Teresa Lake.

Each glacial episode left behind moraines(deposits of debris shaped like ridges) thatindicate where the edge of the glacier was.Examples of moraines are the ridge of cobblesabove Brown Lake where the Bristlecone PineGrove is located, the pile of debris impounding

Stella Lake, and the prominent tree-covered ridgeseen looking towards Wheeler Peak from MatherOverlook. Using these features we can infer theextent of the glaciers during each glacial advance.

Rock glaciers are large masses of boulderscemented together by ice. Lehman rockglacier isa large tongue-shaped feature that you can eitherview by hiking up the Glacier Trailor the SummitTrail. Another smaller and less accessiblerockglacier can also be seen from the summit inthe cirque above Teresa Lake.

One remnant glacier is all that remains of thegiants that carved the alpine landscape thenmelted out around 10,000 years ago. It resides inLehman Cirque just above the Lehmanrockglacier. Earlier in the summer the glacier is abright clean white, but as the season progressesrocks and debris fall onto its surface, making itappear dark gray. This debris is dislodged fromthe steep walls of the cirque when water freezesand expands along cracks in the rock. You mayeven hear rocks falling from the steep walls as youhike up into the cirque.

Glaciation

Text and diagrams byJohn Van Hoesen,

UNLV, 2001

The formation of Lehman Caves began approxi-mately 550 million years ago during the Cam-brian when thick deposits of limestone, the rocktype where the caves are found, formed in awarm, shallow ocean. Limestone forms when theshells of dead sea creatures sink to the floor of theocean and accumulate. Over time, these layerssolidify in response to the weight of overlyingsediments and become limestone. The limestonein the park was subjected to a metamorphic event(increased heat and pressure that changes theminerals in a rock) turning some of the graylimestone into a whiter and harder rock calledmarble. The limestone was eventually brought tothe earth’s surface in response to numerousgeologic events. These same events were respon-sible for fracturing the limestone. During the

Pleistocene, water was more abundant through-out the southwestern United States. This causedmore water to seep into the ground and eventu-ally into the fractured bedrock. As the waterpassed through the soil and air it absorbedcarbon dioxide forming carbonic acid. Thisacidic groundwater is responsible for dissolvinglarge rooms and cavities in limestone and marble.The groundwater level slowly dropped leavingbehind empty cavities in the limestone, but theslow trickle of acidic water continued. Lime-stone and marble that was dissolved above thecavities remained in solution and eventuallyprecipitated on the ceilings, walls, and floors ofempty cavities. These processes created thebeautiful formations we see in Lehman Cavestoday. Such as stalactites, stalagmites, helictites,popcorn, and shields.

Lehman Caves Formation