Quaternary Volcanic Deposits and Ice Ages
Massive volcanic eruptions from the Yellowstone Volcano northwest of the area occurred 2.2 million, 1.3 million, and 630,000 years ago. Each catastrophic caldera-forming eruption was preceded by a long period of more conventional eruptions along even earlier volcanic episodes. One such event sent large amounts of Rhyolitic lava into the northern extent of Teewinot Lake. The resulting obsidian (volcanic glass) has been potassium-argon dated to 9 million years and was used by Native Americans starting thousands of years ago to make arrowheads, knives, and spear points. The lake was dry by the time a series of enormous pyroclastic flows from the Yellowstone area buried Jackson Hole under welded tuff. Older exposures of this tuff are exposed in the Bivouac Formation at Signal Mountain and Pleistocene-aged tuffs are found capping East and West Gros Venture Buttes (both the mountain and buttes are small fault blocks).
Climatic conditions in the area gradually changed through the Cenozoic as continental drift moved North America northwest from a sub-tropical to a temperate zone by the Pliocene epoch. The onset of a series of glaciations in the Pleistocene epoch saw the introduction of large glaciers in the Teton and surrounding ranges, which flowed all the way to Jackson Hole during at least three ice ages. Cascade, Garnet, Death and Granite Canyons were all carved by successive periods of glaciation.
The first and most severe of the known glacial advances in the area was caused by the Buffalo glaciation. In that event the individual alpine (mountain valley) glaciers from the Tetons' east side coalesced to form a 2,000 foot (610 m) thick apron of ice that overrode and abraded Signal Mountain and the other three buttes at the south end of Jackson Hole. Similar dramas were repeated on other ranges in the region, eventually forming part of the Canadian Ice Sheet, which at its maximum, extended into eastern Idaho. This continental-sized glacial system stripped all the soil and vegetation from countless valleys and many basins, leaving them a wasteland of bedrock strewn with boulders after the glaciers finally retreated. Parts of Jackson Hole that were not touched by the following milder glaciations still cannot support anything but the hardiest plants (smaller glaciers deposit glacial till and small rocks relatively near their source, while continental glaciers transport all but the largest fragments far away).
A less severe glaciation, known as Bull Lake, started sometime between 160 to 130 thousand years ago. Bull Lake helped repair some of the damage of the Buffalo event by forming smaller glaciers which deposited loose material over the bedrock. In that event, the large glacier which ran down Jackson Hole only extended just south of where Jackson, Wyoming now sits and melted about 100,000 years ago.
Then from 25,000 to 10,000 years ago the lower volume Wisconsin glaciation carved many of the glacial features seen today. Burned Ridge is made of the terminal moraine (rubble dump) of the largest of these glaciers to affect the area. Today this hummocky feature is covered with trees and other vegetation. Smaller moraines from a less severe part of the Pinedale were formed just below the base of each large valley in the Teton Range by alpine glaciers. Many of these piles of glacial rubble created depressions that in modern times are filled with a series of small lakes (Leigh, String, Jenny, Bradley, Taggart, and Phelps). Jackson Lake is the largest of these and was impounded by a recessional moraine left by the last major glacier in Jackson Hole. A collection of kettles (depressions left by melted stagnant ice blocks from a retreating glacier) south of the lake is called the Potholes. The basins that hold Two Ocean Lake and Emma Matilda Lake were created during the Bull Lake glaciation. Since then humans have built a dam over Jackson Lake's outlet to increase its size for recreational purposes.
All Pinedale glaciers probably melted away soon after the start of the Holocene epoch. The dozen small cirque glaciers seen today were formed during a subsequent neoglaciation 5000 years ago. Mount Moran has five such glaciers with Triple Glaciers on the north face, Skillet Glacier on the east face, and Falling Ice Glacier on the southeast face. All the glacial action has made the peaks of the Teton Range jagged from frost wedging. Other glaciers include Teton Glacier, below the east face of Grand Teton, Middle Teton Glacier, situated on the northeast slopes of Middle Teton, and the fast retreating Schoolroom Glacier, west of Grand Teton at Hurricane Pass.
Mass wasting events such as the 1925 Gros Ventre landslide continue to change the area. On June 22, 1925 an earthquake with an estimated magnitude of 4 weakened the side of a mountain located three miles (4.8 km) outside of the current park's southeastern border. The next day, 50 million cubic yards (38 million cubic meters) of water-saturated Pennsylvanian-aged Tensleep Sandstone slid 1.5 miles (2.4 km) from its source on Sheep Mountain and into the Gros Ventre River valley 2,100 feet (640 m) below, damming the river. Stressed by snow melt, the resulting 5 mile (8 km) long and 200 feet (60 m) deep lake breached the debris dam on May 18, 1927 and flooded the town of Kelly, Wyoming, killing six.
Read more about this topic: Geology Of The Grand Teton Area
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