Effects on Jackson Hole
Rock debris is carried toward the end of the glacier or along the margins where it is released as the ice melts. The semicircular ridge of rock fragments that marks the downhill margin of the glacier is called a terminal moraine; that along the sides is a lateral moraine (figs. [9] and [10]). These are formed by the slow accumulation of material in the same manner as that at the end of a conveyor belt. They are not built by material pushed up ahead of the ice as if by a bulldozer. Large boulders carried by ice are called erratics; many of these are scattered on the floor of Jackson Hole and on the flanks of the surrounding mountains ([fig. 11]).
Figure 7. Rock surface polished and grooved by ice on the floor of Glacier Gulch.
Great volumes of water pour from melting ice near the lower end of a glacier. These streams, heavily laden with rock flour produced by the grinding action of the glacier and with debris liberated from the melting ice, cut channels through the terminal moraine and spread a broad apron of gravel, sand, and silt down-valley from the glacier terminus (end). Material deposited by streams issuing from a glacier is called outwash; the sheet of outwash in front of the glacier is called an outwash plain. If the terminus is retreating, masses of old stagnant ice commonly are buried beneath the outwash; when these melt, the space they once occupied becomes a deep circular or irregular depression called a kettle ([fig. 12]); many of these now contain small lakes or swamps.
As a glacier retreats, it may build a series of terminal moraines, marking pauses in the recession of the ice front. Streams issuing from the ice behind each new terminal moraine are incised more and more deeply into the older moraines and their outwash plains. Thus, new and younger layers of bouldery debris are spread at successively lower and lower levels. These surfaces are called outwash terraces.
Figure 8. East face of the Teton Range showing some of the glacial features, air oblique view. Cascade Canyon, the U-shaped valley, was cut by ice. The glacier flowed toward the flats, occupied the area of Jenny Lake (foreground), and left an encircling ring of morainal debris, now covered with trees. The flat bare outwash plain in foreground was deposited by meltwater from the glacier. The lake occupies a depression that was left when the ice melted away. National Park Service photo by Bryan Harry.
Figure 9. Cutaway view of a typical valley glacier.
Figure 10. Recently-built terminal moraine of the Schoolroom Glacier, a small ice mass near the head of the south fork of Cascade Canyon. The present glacier lies to the right just out of the field of view. The moraine marks a former position of the ice terminus; the lake (frozen over in this picture) occupies the depression left when the glacier wasted back from the moraine to its present position. Crest of the moraine stands about 50 feet above the lake level. Many of the lakes along the foot of the Teton Range occupy similar depressions behind older moraines. Photo by Philip Hyde.
Figure 11. Large ice-transported boulder of coarse-grained pegmatite and granite resting on Cretaceous shale near Mosquito Creek, on the southwest margin of Jackson Hole. Many boulders at this locality are composed of pegmatite rock characteristic of the middle part of the Teton Range. This occurrence demonstrates that boulders 40 feet in diameter were carried southward 25 miles and left along the west edge of the ice stream, 1,500 feet above the base of the glacier on the floor of Jackson Hole.
Just as the jagged ridges, U-shaped valleys, and ice-polished rocks of the Teton Range attest the importance of glaciers in carving the mountain landscape, the flat gravel outwash plains and hummocky moraines on the floor of Jackson Hole demonstrate their efficiency in transporting debris from the mountains and shaping the scenery of the valley.
Glaciers sculptured all sides of Jackson Hole and filled it with ice to an elevation between 1,000 and 2,000 feet above the present valley floor. The visitor who looks eastward from the south entrance to the park can see clearly glacial scour lines that superficially resemble a series of terraces on the bare lower slopes of the Gros Ventre Mountains. Southward-moving ice cut these features in hard rocks. Elsewhere around the margins of Jackson Hole, especially where the rocks are soft, evidence that the landscape was shaped by ice has been partly or completely obliterated by later events. Rising 1,000 feet above the floor of Jackson Hole are several steepsided buttes (figs. [13] and [55]) described previously, that represent “islands” of hard rock overridden and abraded by the ice. After the ice melted, these buttes were surrounded and partly buried by outwash debris.
Figure 12. “The Potholes,” knob and kettle topography caused by melting of stagnant ice partly buried by outwash gravel. Air oblique view north from over Burned Ridge moraine (see [fig. 61] for orientation). Photo by W. B. Hall and J. M. Hill.
The story of the glaciers and their place in the geologic history of the Teton region is discussed in more detail later in this booklet.
Figure 13. Radar image of part of Tetons and Jackson Hole. Distance shown between left and right margins is 35 miles. Lakes from left to right: Phelps, Taggart, Bradley, Jenny, Leigh, Jackson. Blacktail Butte is at lower left. Channel of Snake River and outwash terraces are at lower left. Burned Ridge and Jackson Lake moraines are in center. Lava flows at upper right engulf north end of Tetons. Striated surfaces at lower right are glacial scour lines made by ice moving south from Yellowstone National Park. Image courtesy of National Aeronautics and Space Administration.