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).
[Illustration: 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_.
[Illustration: 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._]
[Illustration: Figure 9. _Cutaway view of a typical valley
glacier._]
[Illustration: 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._]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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._]