With the uplift of the Teton Range and the formation of Jackson Hole
late in Cenozoic time the landscape gradually began to assume the
general outlines that we see today. Rain, wind, snow, and frost shaped
the first crude approximations of the present ridges and peaks. Streams
cut into the rising Teton fault block, eroding the ancestral canyons
deeper and deeper as the uplift continued. The most recent great chapter
in the story of the Teton landscape, however, remained to be written by
the glaciers of the _Ice Age_.
The reasons for the climatic changes that caused the Ice Age are still a
matter of much scientific debate. Various theories have been advanced
that attribute them to changes in solar radiation, changes in the
earth’s orbit and inclination to the sun, variations in the amount of
carbon dioxide in the atmosphere, shifts in the positions of the
continents or the poles, and to many other factors, but none has met
with universal acceptance. No doubt the explanation lies in some unusual
combination of circumstances, for widespread glaciation occurred only
twice before in the earth’s history—once in the late Precambrian and
once during the Permian. It is quite clear, however, that the glaciers
did not form in response to any local cause such as the uplift of the
Teton Range, for concurrent climatic changes and ice advance took place
throughout many parts of the world.
At least three times in the last 250,000 years glaciers from the
surrounding highlands invaded Jackson Hole. The oldest and most
widespread glaciation probably took place about 200,000 years ago; it
was called the _Buffalo Glaciation_ by Prof. Eliot Blackwelder in 1915
(see selected references). The age estimate is based on measurements of
the thickness of the decomposed layer on the surface of obsidian pebbles
in the glacial debris. Major sources of ice were the Beartooth Mountains
(fig. 1), the Absaroka Range, and the Wind River Range. The Gros Ventre
Mountains and Teton Range furnished lesser amounts of ice.
The ice from the Beartooth and Absaroka centers of ice accumulation
converged in the northeastern part of Grand Teton National Park and
flowed south along the face of the Teton Range in a giant stream that in
many places was 2,000 feet thick (fig. 57). All but the highest parts of
the Pinyon Peak and Mount Leidy Highlands were buried and scoured.
Signal Mountain, Blacktail Butte, and the Gros Ventres Buttes were
overridden and shaped by ice at this time. Another glacier, this one
from the Wind River Range, flowed northwest along the Continental
Divide, then down the Gros Ventre River Valley, and merged with the
southward-moving main ice stream west of Lower Slide Lake. Where Jackson
Hole narrows southward, the glacier became more and more confined, but
nevertheless flowed all the way through the Snake River Canyon and on
into Idaho.
[Illustration: Figure 58. _Glacial deposits, outwash, and loess
exposed along Boyle Ditch in Jackson Hole National Elk Refuge.
Indicated are middle Pliocene Teewinot formation (A), oldest till
(B), Bull Lake outwash gravel (C), and post-Bull Lake loess (D),
which here contains snail shells dated by Carbon-14 as 15,000 years
old. Height of cliff is about 30 feet._]
The volume of this great ice mass was probably considerably more than
1,000 cubic miles. When it melted, nearly all the previously accumulated
soil in Jackson Hole was washed away and a pavement of quartzite
boulders mantled much of the glaciated surface. In areas not
subsequently glaciated, the lack of soil and abundance of quartzite
boulders drastically influenced the topography, later drainage,
distribution of all types of vegetation, especially conifers and grass,
and the pattern of human settlement and industry.
[Illustration: Figure 59. _View west from the Snake River overlook
showing at upper right the Burned Ridge moraine (with trees) merging
southward with the highest (oldest) Pinedale outwash plain. The next
lower surface is composed of outwash from the Jackson Lake moraine
which lies to the right, out of the picture. At the bottom is
Deadman’s Bar, a gravel deposit at the present river level. Photo by
H. D. Pownall._]
The second glaciation, named _Bull Lake_, was less than half as
extensive as the first. A large tongue of ice from the Absaroka center
of accumulation flowed down the Buffalo River Valley and joined ice from
the Tetons on the floor of Jackson Hole. An enormous outwash fan of
quartzite boulders extended from near Blacktail Butte southward
throughout most of southern Jackson Hole. Glaciers in the Gros Ventre
Mountains did not advance beyond the east margin of the valley floor.
Carbon-14 ages and data from weathered obsidian pebbles suggest that
this glaciation took place between 35,000 and 80,000 years ago.
Bull Lake moraines and outwash deposits are overlain directly in the
southern part of Jackson Hole by fine silt, rather than by deposits of
the third glaciation (fig. 58). This silt, of windblown origin, is
called _loess_ and contains fossil shells dated by Carbon-14 as between
13,000 and 19,000 years old. Wherever the loess occurs, it is marked by
abundant modern coyote dens and badger burrows.
[Illustration: Figure 60. _Air oblique view west toward the Teton
Range, showing effects of Pinedale Glaciation on the landscape. Mt.
Moran is at top left; the mountain front is broken by U-shaped
valleys from which ice emerged into the area now occupied by Jackson
Lake. The timbered area bordering Jackson Lake is the Jackson Lake
moraine. One of the braided outlet channels breaching the Jackson
Lake moraine can be seen crossing the outwash plain at the left
center. Lakes at lower right occupy “potholes” near where the
9,000-year-old snail shells occur. Snake River is in foreground.
Photo by R. L. Casebeer._]
The third and last glaciation, named the _Pinedale_, was even less
extensive than the others. Nevertheless it was of great importance for
it added the final touches to the present landscape. The jagged
intricately ice-carved peaks (fig. 4) and the glittering lakes and broad
gravelly plains are vivid reminders of this recent chapter in geologic
history.
Pinedale glaciers advanced down Cascade, Garnet, Avalanche, and Death
Canyons and spilled out onto the floor of Jackson Hole, where they built
the outermost loops of the conspicuous terminal moraines that now
encircle Jenny, Bradley, Taggart, and Phelps Lakes (fig. 13). Ice
streams from Glacier Gulch and Open Canyon also left prominent moraines
on the valley floor, but these do not contain lakes. Ice from Leigh
Canyon and all of the eastward-draining valleys to the north combined to
form a large glacier in roughly the present position of Jackson Lake.
This ice entirely surrounded Signal Mountain, leaving only the upper few
hundred feet projecting as an island or _nunatak_.
[Illustration: Figure 61. _The Pinedale Glaciers in the central part
of Jackson Hole as they might have appeared at the time the Jackson
Lake moraine was built. Solid color areas are lakes; dark irregular
pattern shows areas of moraine deposited during the maximum advance
of the Pinedale Glaciers. Pattern of open circles shows older
Pinedale outwash plains; pattern of fine dots shows outwash plains
built at the time the glaciers were in the positions shown in the
drawing. Coarser dots near the margins of the glaciers represent
concentrations of rock debris in the ice._]
The southernmost major advance of Pinedale ice from Jackson Lake is
marked by a series of densely timbered moraines that cross the Snake
River Valley. This series is collectively named the _Burned Ridge
moraine_ (fig. 61). Extending southward for 10 miles from this moraine
is a remarkably flat surfaced gravelly outwash deposit. It was spread by
streams that poured from the glacier at the time the moraine was being
built (fig. 59). East of the Snake River, the main highway from a point
just north of Blacktail Butte to the Snake River overlook is built on
this flat untimbered surface. We assume that the outwash is younger than
15,000 years because it apparently overlies loess of that age.
The glacier withdrew rapidly northward from the Burned Ridge moraine,
leaving behind many large irregular masses of stagnant, debris-covered
ice. The sites of these became kettles, locally known as “The Potholes”
(fig. 12). The main glacier retreated to a position marked by the loop
of moraines just south of Jackson Lake (fig. 60). Figure 61 is a sketch
map showing how the glaciers in this part of Jackson Hole might have
appeared at the time the Jackson Lake moraine was built.
Abundant snail shells have been found in lake sediments in the bottoms
of the kettles north of the Burned Ridge moraine (fig. 60) as well as on
low ridges between them. Carbon-14 age determinations indicate that the
snails lived about 9,000 years ago, either in a lake already present
before the Pinedale ice advanced and formed the Burned Ridge moraine or
in ponds that filled kettles left as the ice melted behind this moraine.
In either case, the shells indicate that the Pinedale glaciers probably
existed on the floor of Jackson Hole as recently as 9,000 years ago, at
a time when Indians were already living in the area. We can easily
imagine the fascination with which these primitive peoples may have
watched as year after year the glaciers wasted away, slowly retreating
back into the canyons, then withdrawing into the sheltered recesses of
the high mountains, eventually to dwindle and disappear.
Many bits of evidence, both from North America and Europe, indicate that
there was a period called the _climatic optimum_ about 6,000 years ago
when the climate was significantly warmer and drier than at present. We
suspect, though there is as yet no direct proof, that the Pinedale
glaciers wasted away entirely during this interval.
The modern pattern of vegetation in Jackson Hole is strongly influenced
by the distribution of Pinedale glacial moraines and outwash deposits.
Almost without exception the moraines are heavily forested, whereas the
nearby outwash deposits are covered only by a sparse growth of
sagebrush. This is probably because the moraines contain large amounts
of clay and silt produced by the grinding action of the glaciers.
Material of this type retains water much better and, because of the
greater variety of chemical elements, is more fertile than the porous
quartzite gravel and sand on the outwash plains.