Chapter III: Front Matter (3)

QUATERNARY
Recent
Modern 0-200± Sand, gravel, and Floor of Jackson
stream, silt along present Hole and in canyons
landslide, streams; jumbled and on
glacial and broken rock in mountainsides
talus deposits landslides and on throughout the
talus slopes; region.
debris around
existing glaciers.
Pleistocene
Glacial 0-200± Gravel, sand, silt, Floor of Jackson
deposits and and glacial debris. Hole.
loess
Unnamed upper 0-500 Shale, brown-gray, Gros Ventre River
lake sequence sandstone, and Valley.
conglomerate.
Unnamed lower 0-200 Shale, siltstone, National Elk Refuge.
lake sequence and sandstone,
gray, green, and
red.
? Pleistocene or Pliocene
Bivouac 0-1,000 Conglomerate, with Signal Mountain and
Formation purplish-gray West Gros Ventre
welded tuff in Butte.
upper part.
TERTIARY
Pliocene
Teewinot 0-6,000 Limestone, tuff, National Elk
Formation and claystone, Refuge, Blacktail
white, soft. Butte, and eastern
margin of Antelope
Flat.
Camp Davis 0-5,500 Conglomerate, red Southernmost tip of
Formation and gray, with Jackson Hole.
white tuff,
diatomite, and red
and white claystone.
Miocene
Colter 0-7,000 Volcanic Pilgrim and Ditch
Formation conglomerate, tuff, Creeks, and north
and sandstone, end of Teton Range.
white to
green-brown, with
locally-derived
basalt and andesite
rock fragments.
Oligocene
Wiggins 0-3,000 Volcanic Eastern margin of
Formation conglomerate, gray Jackson Hole.
to brown, with
white tuff layers.
Eocene
Unnamed upper 0-1,000 Tuff, conglomerate, Eastern margin of
and middle sandstone, and Jackson Hole.
Eocene claystone, green,
sequence underlain by
variegated
claystone and
quartzite pebble
conglomerate.
Wind River 2,000-3,000 Claystone and Eastern margin of
and Indian sandstone, Jackson Hole.
Meadows variegated, and
Formations locally-derived
conglomerate;
persistent coal and
gray shale zone in
middle.
Paleocene
Unnamed 1,000-2,000 Sandstone and Eastern margin of
greenish-gray claystone, Jackson Hole.
and brown greenish-gray and
sandstone and brown,
claystone intertonguing at
sequence base with quartzite
pebble conglomerate.
Pinyon 500-5,000 Conglomerate, Eastern part of
Conglomerate brown, chiefly of Jackson Hole, Mt.
rounded quartzite; Leidy and Pinyon
coal and claystone Peak Highlands, and
locally at base. north end of Teton
Range.

During the early part of the Tertiary Period, mountain building and basin subsidence were the dominant types of crustal movement. Seas retreated southward down the Mississippi Valley and never again invaded the Teton area. Environments on the recently uplifted land were diverse and favorable for the development of new forms of plants and animals.

Rise and burial of mountains

The enormous section of Tertiary sedimentary rocks in the Jackson Hole area (table 5) is one of the most impressive in North America. If the maximum thicknesses of all formations were added, they would total more than 6 miles, but nowhere did this amount of rock accumulate in a single unbroken sequence. No other region in the United States contains a thicker or more complete nonmarine Tertiary record; many areas have little or none. The accumulation in Jackson Hole reflects active uplifts of nearby mountains that supplied abundant rock debris, concurrent sinking of nearby basins in which the sediments could be preserved, and proximity to the great Yellowstone-Absaroka volcanic area, one of the most active continental volcanic fields in the United States. The volume and composition of the Tertiary strata are, therefore, clear evidence of crustal and subcrustal instability.

The many thick layers of conglomerate are evidence of rapid erosion of nearby highlands. The Pinyon Conglomerate (fig. 45), for example, contains zones as much as 2,500 feet thick of remarkably well-rounded pebbles, cobbles, and boulders, chiefly of quartzite identical with that in the underlying Harebell Formation and derived from the same source, the Targhee uplift. Like the Harebell the matrix contains small amounts of gold and mercury. Rock fragments increase in size northwestward toward the source area (fig. 46) and most show percussion scars, evidence of ferocious pounding that occurred during transport by powerful, swift rivers and steep gradients.

Conglomerates such as the Pinyon are not the only clue to the time of mountain building. Another type of evidence—faults—is demonstrated in figure 16. The youngest rocks cut by a fault are always older than the fault. Many faults and the rocks on each side are covered by still younger unbroken sediments. These must, therefore, have been deposited after fault movement ceased. By dating both the faulted and the overlying unbroken sediments, the time of fault movement can be bracketed.

Observations of this type in western Wyoming indicate that the Laramide Revolution reached a climax during earliest Eocene time, 50 to 55 million years ago. Mountain-producing upwarps formed during this episode were commonly bounded on one side by either reverse or thrust faults (fig. 16B and 16C) and intervening blocks were downfolded into large, very deep basins. The amount of movement of the mountain blocks over the basins ranged from tens of miles in the Snake River, Salt River, Wyoming, and Hoback Ranges directly south of the Tetons to less than 5 miles on the east margin of Jackson Hole (the west flank of the Washakie Range shown in figure 1). The ancestral Teton-Gros Ventre uplift continued to rise but remained one of the less conspicuous mountain ranges in the region (fig. 47).

The Buck Mountain fault, the great reverse fault which lies just west of the highest Teton peaks (see geologic map and cross section), was formed either at this time or during a later episode of movement that also involved the southwest margin of the Gros Ventre Mountains. The Buck Mountain fault is of special importance because it raised a segment of Precambrian rocks several thousand feet. Later, when the entire range as we now know it was uplifted by movement along the Teton fault, the hard basement rocks in this previously upfaulted segment continued to stand much higher than those in adjacent parts of the range. All of the major peaks in the Tetons are carved from this doubly uplifted block.

The brightly colored sandstone, mudstone, and claystone in the Indian Meadows and Wind River Formations (lower Eocene) in the eastern part of Jackson Hole were derived from variegated Triassic, Jurassic, and Lower Cretaceous rocks exposed on the adjacent mountain flanks. Fossils in these Eocene Formations show that it took less than 10 million years for the uplifts to be deeply eroded and partially buried in their own debris.

The Laramide Revolution in the area of Grand Teton National Park ended during Eocene time between 45 and 50 million years ago, and as the mountains and basins became stabilized a new element was added. Volcanoes broke through to the surface in many parts of the Yellowstone-Absaroka area and the constantly increasing volume of their eruptive debris was a major factor in the speed of filling of basins and burial of mountains throughout Wyoming. This entire process only took about 20 million years, and along the east margin of Jackson Hole it was largely completed during Oligocene time (fig. 48). However, east and northeast of Jackson Lake a Miocene downwarp subsequently formed and in it accumulated at least 7,000 feet of locally derived sediments of volcanic origin.

The First Big Lake

_Teewinot Lake_ (fig. 49), the first big freshwater lake in Jackson Hole, was formed during Pliocene time, about 10 million years ago, and in it the Teewinot Formation was deposited. These lake strata consist of more than 5,000 feet of white limestone, thin-bedded claystone, and _tuff_ (solidified ash made up of tiny fragments of volcanic rock and splinters of volcanic glass). The claystones contain fossil snails, clams, beaver bones and teeth, aquatic mice, suckers, and other fossils that indicate deposition in a shallow freshwater lake environment. These beds underlie Jackson Lake Lodge, the National Elk Refuge, part of Blacktail Butte, and are conspicuously exposed in white outcrops that look like snowbanks on the upper slopes along the east margin of the park across the valley from the Grand Teton.

Teewinot Lake was formed on a down-faulted block and was dammed behind (north of) a fault that trends east across the floor of Jackson Hole at the south boundary of the park. Lakes are among the most short-lived of earth features because the forces of nature soon conspire to fill them up or empty them. This lake existed for perhaps 5 million years during middle Pliocene time; it was shallow, and remained so despite the pouring in of a mile-thick layer of sediment. This indicates that downdropping of the lake floor just about kept pace with deposition.

_Uintatherium_ 6-horned, saber-toothed plant eater
_Stylinodon_ gnawing-toothed mammal
_Palaeosyops_ early titanothere
_Helaletes_ primitive tapir
_Sciuravus_ squirrel-like rodent
_Smilodectes_ lemurlike monkey
_Trogosus_ gnawing-toothed mammal
_Hyrachyus_ fleet-footed rhinoceros
_Ischyrotomus_ marmotlike rodent
_Homacodon_ even-toed hoofed animal
_Orohippus_ ancestral horse
_Patriofelis_ large flesh eater
_Mesonyx_ hyenalike mammal
_Helohyus_ even-toed hoofed mammal
_Metacheiromys_ armadillolike edentate
_Machaeroides_ saber-toothed mammal
_Hyopsodus_ clawed, plant-eating mammal
_Saniwa_ monitorlike lizard
_Crocodilus_ crocodile
_Echmatemys_ turtle

Other lakes formed in response to similar crustal movements in nearby places. One such lake, _Grand Valley Lake_ (fig. 49), formed about 25 miles southwest of Teewinot Lake; both contained sediments with nearly the same thickness, composition, appearance, age, and fossils. Although these two lakes are on opposite sides of the Snake River Range, the ancestral Snake River apparently flowed through a canyon previously cut across the range and provided a direct connection between them.

Development of mammals

The Cenozoic Era is known as the “Age of Mammals.” Small mammals had already existed, though quite inconspicuously, in Wyoming for about 90 million years before Paleocene time. Then about 65 million years ago their proliferation began as a result of the extinction of dinosaurs, obliteration of seaways that were barriers to distribution, and the development of new and varied types of environment. These new environments included savannah plains, low hills and high mountains, freshwater lakes and swamps, and extensive river systems. The mammals increased in size and, for the first time, became abundant in numbers of both species and individuals. The development and widespread distribution of grasses and other forage on which many of the animals depended were highly significant. Successful adaptation of _herbivores_ (vegetation-eating animals) led, in turn, to increased varieties and numbers of predatory _carnivores_ (meat-eating animals).

During early Eocene time, coal swamps formed in eastern Jackson Hole and persisted for thousands of years, as is shown by 60 feet of coal in a single bed at one locality. Continuing on into middle Eocene time, the climate was subtropical and humid, and the terrain was near sea level. Tropical breadfruits, figs, and magnolias flourished along with a more temperate flora of redwood, hickory, maple, and oak. Horses the size of a dog and many other small mammals were abundant. Primates, thriving in an ideal forest habitat, were numerous. Streams contained gar fish and crocodiles (fig. 50).

_Trigonias_ early rhinoceros
_Perchoerus_ early peccary
_Mesohippus_ 3-toed horse
_Aepinacodon_ remote relative of hippopotamus
_Archaeotherium_ giant piglike mammal
_Protoceras_ bizarre horned ruminant
_Hesperocyon_ ancestral dog
_Hyracodon_ small fleet-footed rhinoceros
_Poëbrotherium_ ancestral camel
_Hypisodus_ very small chevrotainlike ruminant
_Ictops_ small insect-eating mammal
_Brontotherium_ titanothere
_Protapirus_ ancestral tapir
_Glyptosaurus_ extinct lizard
_Hoplophoneus_ saber-toothed cat
_Subhyracodon_ early rhinoceros
_Merycoidodon_ sheeplike grazing mammal
_Hyaenodon_ archaic hyenalike mammal
_Hypertragulus_ chevrotainlike ruminant

Early in the Oligocene Epoch, between 30 and 35 million years ago, the climate in Jackson Hole became cooler and drier, and the subtropical plants gave way to the warm temperate flora of oak, beech, maple, alder, and ash. The general land surface rose higher above sea level, perhaps by accumulation of several thousand feet of Oligocene volcanic rocks (fig. 52) rather than by continental uplift. _Titanotheres_ (large four-legged mammals with the general size and shape of a rhinoceros) flourished in great numbers for a few million years and then abruptly vanished. Horses by now were about the size of a very small modern colt. Rabbits, rodents, carnivores, tiny camels, and other mammals were abundant in Jackson Hole, and the fauna, surprisingly, was essentially the same as that 500 miles to the east, at a much lower elevation, on the plains of Nebraska and South Dakota (fig. 51).

The Miocene Epoch (15 to 25 million years ago) was the time of such intense volcanic activity in the Teton region that animals must have found survival very difficult. A few skeletons and fragmentary parts of camels about the size of a small horse and other piglike animals called _oreodonts_ comprise our only record of mammals; nothing is known of the plants. Farther east the climate fluctuated from subtropical to warm temperate, gradually becoming cooler toward the end of the epoch.

Fossils in the Pliocene lake deposits (8 to 10 million years old; see description of Teewinot Formation) include shallow-water types of snails, clams, diatoms, and ostracodes, as well as beavers, mice, suckers, and frogs. Pollen in these beds show that adjacent upland areas supported fir, spruce, pine, juniper, sage, and other trees and shrubs common to the area today. Therefore, the climate must have been much cooler than in Miocene time. No large mammals of Pliocene age have been found in Jackson Hole. The record of life during Quaternary time is discussed later.

Volcanoes

Volcanoes are one of the most interesting parts of the geologic story of the Teton region. Although ash from distant volcanoes had settled in northwestern Wyoming at least as far back in time as Jurassic, the first nearby active volcanoes (since the Precambrian) erupted in the Yellowstone-Absaroka region during the early Eocene, about 50 million years ago. From then on, the volcanic area grew in size and the violence of eruptions and volume of debris increased until Pliocene time. This debris had a profound influence on the color and composition of the sediments and on the environment and types of plants and animals.

The color of the volcanic rocks and the sediments derived from them varies significantly from one epoch to another. For example, the middle Eocene rocks are white to light-green, red, and purple, upper Eocene are dark-green, Oligocene are light-gray, white, and brown, Miocene are dark-green, brown, and gray, and Pliocene are white to red-brown.

As mentioned earlier, it is probable that the vast outpouring of volcanic rocks during late Tertiary time in the Teton region and to the north and northeast is directly related to the subsidence of Jackson Hole and the rise of the Tetons.

The spectacular banded cliffs of the Wiggins Formation on both sides of Togwotee Pass (fig. 52) and farther north in the Absaroka Range are remnants of Oligocene volcanic conglomerate and tuff that once spread as a blanket several thousand feet thick across eastern Jackson Hole and partially or completely buried the nearby older folded mountain ranges.

About 25 million years ago, with the start of the Miocene Epoch, volcanic vents opened up within, and along the borders of, Grand Teton National Park. Major centers of eruption were at the north end of the Teton Range, east of Jackson Lake, and south of Spread Creek. They emitted a prodigious amount of volcanic ash and fragments of congealed lava. For example, adjacent to one vent a mile in diameter, about 4 miles north-northeast of Jackson Lake Lodge, is a continuous section, 7,000 feet thick, of waterlaid strata derived in large part from this volcanic source. These sedimentary rocks comprise the Colter Formation which is darker colored and contains more iron and magnesium than the Wiggins Formation. The site of deposition at this locality was a north-trending trough that represented an early stage in the downwarping of Jackson Hole.

Pliocene volcanoes erupted in southern and central Yellowstone Park. The volcanoes emitted viscous, frothy, pinkish-gray and brown lava called _rhyolite_. This is an extrusive igneous rock that has the same composition as granite, but is much finer grained. In several places, lava apparently flowed into the north end of Teewinot Lake, chilled suddenly, and solidified into a black volcanic glass called _obsidian_. Because it chips easily into thin flakes having a smooth surface, obsidian was prized by the Indians, who used it for spear and arrow points (fig. 54). Some of this obsidian has a potassium-argon date of 9 million years.

After Teewinot Lake was filled with sediment, the floor of Jackson Hole became a flat boulder-covered surface. Nearby vents erupted heavy fiery clouds of gaseous molten rock that rolled across this plain and then congealed into hard layers with the general appearance of lava flows. Under a microscope, however, the rock is seen to be made up of compressed fragments of glass that matted down and solidified when the clouds stopped moving. This kind of rock is called a _welded tuff_. One of these forms the conspicuous ledge in the Bivouac Formation on the north and east sides of Signal Mountain (fig. 55), and is especially important because it has a potassium-argon date of 2.5 million years. More of this _welded tuff_ flowed southward from Yellowstone National Park, engulfed the north end of the Teton Range (fig. 53), and continued southward along the west side of the mountains for 35 miles and along the east side for 25 miles.

THE LAST HUNDREDTHS ABSOLUTE TIME IMPORTANT EVENTS
OF AN INCH OF THE (Years ago)
YARDSTICK
0 0 Last glaciation followed by
faulting
¹/₁₀₀₀ 50000 Second glaciation
²/₁₀₀₀ 100000 ?—First glaciation
⁶/₁₀₀₀ 700000 ?—Second Quaternary lake
⁸/₁₀₀₀ 1 million ?—Tilting and faulting of
southern part of Jackson Hole
¹¹/₁₀₀₀ 1.3 million ?—First Quaternary lake
¹²/₁₀₀₀ 1.5 million } Complex series of volcanic
eruptions in southern Jackson
Hole
¹⁵/₁₀₀₀ 1.9 million }
¹⁶/₁₀₀₀ 2 million ?—Development of Hoback
normal fault
²/₁₀₀ 2.5 million Eruption of welded tuff in
Bivouac Formation
²⁴/₁₀₀₀ 3 million

QUATERNARY—TIME OF ICE, MORE LAKES, AND CONTINUED CRUSTAL DISTURBANCE

The Quaternary Period is represented by less than 15-thousandths of the last inch on our yardstick of time (fig. 56) and the entire Ice Age takes up less than 2-thousandths of an inch (less than the thickness of this page). Nevertheless, the spectacular effects of various forces of nature on the Teton landscape during this short interval of time are of such significance that they warrant a separate discussion. The role of glaciers in carving the rugged Teton peaks and shaping the adjacent valleys was mentioned in the first part of this booklet, but is discussed in more detail here. The magnitude and complexity of crustal movements increased during the final 2 million years of time—so much so that the beginning of Quaternary time has not yet been identified with any single event. Figure 56 shows the major events described below.

Hoback normal fault

The _Hoback normal fault_, 30 miles long, with a mile or more displacement, developed in the southernmost part of Jackson Hole about 2 million years ago. This fault is on the east side of the valley. Thus, the valley block was downdropped between this fault and the Teton fault that borders the west side.

Volcanic activity

During or shortly after major movement on the Hoback fault, and perhaps related to it, there was a complex series of volcanic eruptions west and north of the town of Jackson, along the south boundary of the park. In rapid succession, lavas of many types, with a combined thickness of more than 1,000 feet, were extruded and volcanic plugs intruded into the near-surface sedimentary rocks. These volcanic rocks can be seen on the East and West Gros Ventre Buttes.

There are no active volcanoes in the Teton region today and no postglacial lava flows or cinder cones. Five miles north of Grand Teton National Park are boiling springs (Flagg Ranch hot springs) that are associated with the youngest (late Quaternary) lavas in southern Yellowstone Park. Elsewhere in Jackson Hole are a number of lukewarm springs but their relation to volcanic rocks has not been determined.

What happened to the vast thicknesses of volcanic debris? We know they existed because sections of them have been measured on the eroded edges of uptilted folds and fault blocks. Many cubic miles of these rocks are now buried beneath the floor of Jackson Hole, but a much greater volume was carried completely out of the region by water, ice, and wind during the final chapter of geologic history.

Preglacial lakes

Remnants of two sets of lake deposits in Jackson Hole record preglacial events in Quaternary time. Downdropping of southern Jackson Hole along the Hoback and Teton faults blocked the southwestward drainage of the Snake River, and a new lake formed overlapping and extending south of the site of the long-vanished Teewinot Lake. Incorporated in the lake sediments are fragments of lava like that in nearby Quaternary flows. From this we know that the lake formed after at least some of the lava was emplaced. Apparently subsidence was more rapid than filling, for a time, at least, because this new lake was deep. Fossil snails preserved in olive-drab to gray fine-grained claystone overlying lava flows at the north end of East Gros Ventre Butte are the kind now living at depths of 120 to 300 feet in Lake Tahoe, California-Nevada. Near the margins of the lake, pink and green claystone and soft sandstone were deposited. The duration of this lake is not known but it lasted long enough for 200 feet of beds to accumulate. Subsequent faulting and warping destroyed the lake, left tilted remnants of the beds perched 1,000 feet up on the east side of Jackson Hole, and permitted the Snake River to reestablish its course across the mountains to the southwest.

Later downdropping of Jackson Hole impounded a second preglacial lake. Little is known about its extent because nearly everywhere the soft brown and gray shale, claystone, and sandstone deposited in it were scooped out and washed away during subsequent glaciations. A few remnants of the lake deposits are preserved in protected places, however; two are within the Gros Ventre River Valley—one downstream from Lower Slide Lake about a mile east of the park and the other 4 miles farther east. The latter remnant is nearly 500 feet thick and the upper half is largely very fine grained shale and claystone. This fine texture suggests that the lake existed for a good many thousand years, for such deposits commonly accumulate more slowly than coarser grained debris.

The Ice Age

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.

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.

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.

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_.

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.

Modern glaciers

About a dozen small rapidly dwindling glaciers exist today in shaded reentrants high in the Teton Range. They are probably vestiges of ice masses built up since the climatic optimum, during the so-called “_Little Ice Age_.” These glaciers, while insignificant compared to those still present in many other mountain ranges, are fascinating working models of the great ice streams that shaped the Tetons during Pleistocene time.

The Teton Glacier (fig. 6) is one of the best known. It is an ice body about 3,500 feet long and 1,100 feet wide that lies at the head of Glacier Gulch, shaded by the encircling ridges of the Grand Teton, Mount Owen, and Mount Teewinot. Ice in the central part is moving at a rate of more than 30 feet a year.

THE PRESENT AND THE FUTURE

The geologic story of the Teton country from the time the earth was new to the present day has been summarized. What can we learn from it? We become aware that events recorded in the rocks are not a chaotic jumble of random accidents but came in an orderly, logical succession. We see the majestic parade of life evolving from simple to complex types, overcoming all natural disasters, and adapting to ever-changing environments. We can only speculate as to the motivating force that launched this fascinating geologic and biologic venture and what the ultimate goal may be. New facts and new ideas are added to the story each year, but many unknown chapters remain to be studied; these offer an irresistible, continuing challenge to inquisitive minds, strong bodies, and restless, adventurous spirits.

Most geologic processes that developed the Teton landscape have been beneficial to man; a few have interfered with his activities, cost him money, time, effort, and on occasion, his life. Postglacial faulting and tilting along the southern margin of Grand Teton National Park diverted drainage systems (such as Flat Creek, southwest of the Flat Creek fault on the south edge of the geologic map), raised hills, dropped valleys, and made steep slopes unstable. Flood-control engineers wage a never-ending struggle to keep the Snake River from shifting to the west side of Jackson Hole as the valley tilts westward in response to movement along the Teton fault. Each highway into Jackson Hole has been blocked by a landslide at one time or another and maintenance of roads across slide areas requires much ingenuity. We see one slide (the Gros Ventre) that blocked a river; larger slides have occurred in the past, and more can be expected. Abundant fresh fault scarps are a constant reminder that public buildings, campgrounds, dams, and roads need to be designed to withstand the effects of earthquakes. Some of these problems have geologic solutions; others can be avoided or minimized as further study increases our understanding of this region.

Man appeared during the last one-fiftieth of an inch on our yardstick of time gone by. In this short span he has had more impact on the earth and its inhabitants than any other form of life. Will he use wisely the lessons of the past as a guide while he writes his record on the yardstick of the future?