Chapter II: Front Matter (2)

The Teton landscape is a battleground, the scene of a continuing unresolved struggle between the forces that deform the earth’s crust and raise the mountains and the slow processes of erosion that strive to level the uplands, fill the hollows, and reduce the landscape to an ultimate featureless plain. The remainder of this booklet is devoted to tracing the seesaw conflict between these inexorable antagonists through more than 2.5 billion years as they shaped the present landscape—and the battle still goes on.

Evidence of the struggle is all around us. Even though to some observers it may detract from the restfulness of the scene, perhaps it conveys to all of us a new appreciation of the tremendous dynamic forces responsible for the magnificence of the Teton Range.

The battle is indicated by the small faults that displace both the land surface and young deposits at the east base of Mount Teewinot, Rockchuck Peak (fig. 15), and other places along the foot of the Tetons.

Jackson Hole continues to drop and tilt. The gravel-covered surfaces that originally sloped southward are now tilted westward toward the mountains. The Snake River, although the major stream, is not in the lowest part of Jackson Hole; Fish Creek, a lesser tributary near the town of Wilson, is 15 feet lower. For 10 miles this creek flows southward parallel to the Snake River but with a gentler gradient, thus permitting the two streams to join near the south end of Jackson Hole. As tilting continues, the Snake River west of Jackson tries to move westward but is prevented from doing so by long flood-control levees built south of the park.

Recent faults also break the valley floor between the Gros Ventre River and the town of Jackson.

The ever-changing piles of rock debris that mantle the slopes adjacent to the higher peaks, the creeping advance of rock glaciers, the devastating snow avalanches, and the thundering rockfalls are specific reminders that the land surface is restless. Jackson Hole contains more landslides and rock mudflows than almost any other part of the Rocky Mountain region. They constantly plague road builders (fig. 17) and add to the cost of other types of construction.

All of these examples of the relentless battle between constructive and destructive processes modifying the Teton landscape are but minor skirmishes. The bending and breaking of rocks at the surface are small reflections of enormous stresses and strains deep within the earth where the major conflict is being waged. It is revealed every now and then by a convulsion such as the 1959 earthquake in and west of Yellowstone Park. Events of this type release much more energy than all the nuclear devices thus far exploded by man.

ENORMOUS TIME AND DYNAMIC EARTH

Framework of time

One of geology’s greatest philosophical contributions has been the demonstration of the enormity of geologic time. Astronomers deal with distances so great that they are almost beyond understanding; nuclear physicists study objects so small that we can hardly imagine them. Similarly, the geologist is concerned with spans of time so immense that they are scarcely comprehensible. Geology is a science of time as well as rocks, and in our geologic story of the Teton region we must refer frequently to the geologic time scale, the yardstick by which we measure the vast reaches of time in earth history.

Rocks and relative age

Very early in the science of geology it was recognized that in many places one can tell the comparative ages of rocks by their relations to one another. For example, most _sedimentary rocks_ are consolidated accumulations of large or small rock fragments and were deposited as nearly horizontal layers of gravel, sand, or mud. In an undisturbed sequence of sedimentary rocks, the layer on the bottom was deposited first and the layer on top was deposited last. All of these must, of course, be younger than any previously formed rock fragments incorporated in them.

_Igneous rocks_ are those formed by solidification of molten material, either as lava flows on the earth’s surface (_extrusive igneous rocks_) or at depth within the earth (_intrusive igneous rocks_). The relative ages of extrusive igneous rocks can often be determined in much the same way as those of sedimentary strata. A lava flow is younger than the rocks on which it rests, but older than those that rest on top of it.

An intrusive igneous rock must be younger than the rocks that enclosed it at the time it solidified. It may contain pieces of the enclosing rocks that broke off the walls and fell into the liquid. Pebbles of the igneous rock that are incorporated in nearby sedimentary layers indicate that the sediments must be somewhat younger.

All of these criteria tell us only that one rock is older or younger than another. They tell us little about the absolute age of the rocks or about how much older one is than the other.

Fossils and geologic time

Fossils provide important clues to the ages of the rocks in which they are found. The slow evolution of living things through geologic time can be traced by a systematic study of fossils. The fossils are then used to determine the relative ages of the rocks that contain them and to establish a geologic time scale that can be applied to fossil-bearing rocks throughout the world. Figure 18 shows the major subdivisions of the last 600 million years of geologic time and some forms of life that dominated the scene during each of these intervals. Strata containing closely related fossils are grouped into _systems_; the time interval during which the strata comprising a particular system were deposited is termed a _period_. The periods are subdivisions of larger time units called _eras_ and some are split into smaller time units called _epochs_. Strata deposited during an epoch comprise a _series_. Series are in turn subdivided into rock units called _groups_ and formations. Expressed in tabular form these divisions are:

Subdivisions of Time-rock units Rock units
geologic time

Era
Period System
Epoch Series
Group
Formation

The time scale based on the study of fossil-bearing sedimentary rocks is called the stratigraphic time scale; it is given in table 1. The subdivisions are arranged in the same order in which they were deposited, with the oldest at the bottom and the youngest at the top. All rocks older than Cambrian (the first period in the Paleozoic Era) are classed as Precambrian. These rocks are so old that fossils are rare and therefore cannot be conveniently used as a basis for subdivision.

The stratigraphic time scale is extremely useful, but it has serious drawbacks. It can be applied only to fossil-bearing strata or to rocks whose ages are determined by their relation to those containing fossils. It cannot be used directly for rocks that lack fossils, such as igneous rocks, or metamorphic rocks in which fossils have been destroyed by heat or pressure. It is used to establish the relative ages of sedimentary strata throughout the world, but it gives no information as to how long ago a particular layer was deposited or how many years a given period or era lasted.

Radioactive clocks

The measurement of geologic time in terms of years was not possible until the discovery of natural radioactivity. It was found that certain atoms of a few elements spontaneously throw off particles from their nuclei and break down to form atoms of other elements. These decay processes take place at constant rates, unaffected by heat, pressure, or chemical conditions. If we know the rate at which a particular radioactive element decays, the length of time that has passed since a mineral crystal containing the elements formed can be calculated by comparing the amount of the radioactive element remaining in the crystal with the amount of disintegration products present.

MILLIONS OF
YEARS AGO

0 Man
0-60 CENOZOIC QUATERNARY and TERTIARY Mammals
60-130 MESOZOIC CRETACEOUS
130-180 JURASSIC Dinosaurs
180-220 TRIASSIC
220-260 PALEOZOIC PERMIAN Reptiles
260-350 PENNSYLVANIAN, Amphibians
MISSISSIPPIAN
350-400 DEVONIAN Fishes
400-440 SILURIAN Sea scorpions
440-500 ORDOVICIAN Nautiloids
500-530 CAMBRIAN Trilobites
530- PRECAMBRIAN Soft-bodied
creatures

Three principal radioactive clocks now in use are based on the decay of uranium to lead, rubidium to strontium, and potassium to argon. They are effective in dating minerals millions or billions of years old. Another clock, based on the decay of one type of carbon (_Carbon-14_) to nitrogen, dates organic material, but only if it is less than about 40,000 years old.

The uranium, rubidium, and potassium clocks are especially useful in dating igneous rocks. By determining the absolute ages of igneous rocks whose stratigraphic relations to fossil-bearing strata are known, it is possible to estimate the number of years represented by the various subdivisions of the stratigraphic time scale.

The yardstick of geologic time

Recent estimates suggest that the earth was formed at least 4.5 billion years ago. To visualize the length of geologic time and the relations between the stratigraphic and absolute time scales, let us imagine a yardstick as representing the length of time from the origin of the earth to the present (fig. 19). On one side of the yardstick we plot time in years; on the other, we plot the divisions of the stratigraphic time scale according to the most reliable absolute age determinations.

Era System or period Series or epoch

Cenozoic Quaternary Recent
Pleistocene
Tertiary Pliocene
Miocene
Oligocene
Eocene
Paleocene
Mesozoic Cretaceous
Jurassic
Triassic
Paleozoic Permian
Pennsylvanian
Mississippian
Devonian
Silurian[1]
Ordovician
Cambrian
— Precambrian

[1]_The Silurian is the only major subdivision of the stratigraphic time
scale not represented in Grand Teton National Park._

We are immediately struck by the fact that all of the subdivisions of the stratigraphic time scale since the beginning of the Paleozoic are compressed into the last 5 inches of our yardstick! All of the other 31 inches represent Precambrian time. We also see that subdivisions of the stratigraphic time scale do not represent equal numbers of years. We use smaller and smaller subdivisions as we approach the present. (Notice the subdivisions of the Tertiary and Quaternary in table 1 that are too small to show even in the enlarged part of figure 19). This is because the record of earth history is more vague and incomplete the farther back in time we go. In effect, we are very nearsighted in our view of time. This “geological myopia” becomes increasingly evident throughout the remainder of this booklet.

ABSOLUTE TIME (Years ago) INCHES STRATIGRAPHIC TIME SCALE

First man → 0 CENOZOIC
1 MESOZOIC
First dinosaurs → 2 PALEOZOIC
3
500 million 4
First abundant fossils → 5 PRECAMBRIAN
6
7
1 billion 8
9
10
11
12
13
14
Oldest known fossils → 15
2 billion 16
17
18
19
20
21
22
23
3 billion 24
25
26
27
Oldest dated rocks → 28
29
30
31
4 billion 32
33
34
35
Minimum age of the earth → 36

ENLARGEMENT OF THE LAST SIX INCHES
ABSOLUTE TIME INCHES STRATIGRAPHIC TIME SCALE
(Years Ago)

0 0 CENOZOIC QUATERNARY
TERTIARY
MESOZOIC CRETACEOUS
1 JURASSIC
TRIASSIC
2 PALEOZOIC PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN
3 DEVONIAN
SILURIAN
ORDOVICIAN
500 million 4 CAMBRIAN
5 PRECAMBRIAN
6

PRECAMBRIAN ROCKS—THE CORE OF THE TETONS

The visitor who looks at the high, rugged peaks of the Teton Range is seeing rocks that record about seven-eighths of all geologic time. These Precambrian rocks are part of the very foundation of the continent and are therefore commonly referred to by geologists as basement rocks. In attempting to decipher their origin and history we peer backward through the dim mists of time, piecing together scattered clues to events that occurred billions of years ago, perhaps during the very birth of the North American Continent. To cite an oft-quoted example, it is as though we were attempting to read the history of an ancient and long-forgotten civilization from the scattered unnumbered pages of a torn manuscript, written in a language that we only partially understand.

Ancient gneisses and schists

The oldest Precambrian rocks in the Teton Range are layered gneisses and schists exposed over wide areas in the northern and southern parts of the range and as scattered isolated masses in the younger granite that forms the high peaks in the central parts. The layered gneisses may be seen easily along the trails in the lower parts of Indian Paintbrush and Death Canyons, and near Static Peak.

The _layered gneisses_ are composed principally of quartz, feldspar, _biotite_ (black mica), and _hornblende_ (a very dark-green or black mineral commonly forming rodlike crystals). Distinct layers, a few inches to several feet thick, contain different proportions of these minerals and account for the banded appearance. Layers composed almost entirely of quartz and feldspar are light-gray or white, whereas darker gray layers contain higher proportions of biotite and hornblende.

Some layers are dark-green to black _amphibolite_, composed principally of hornblende but with a little feldspar and quartz. In many places the gneisses include layers of _schist_, a flaky rock, much of which is mica. At several places on the east slopes of Mount Moran thin layers of impure gray marble are found interleaved with the gneisses. West of Static Peak along the Alaska Basin Trail a heavy dark rock with large amounts of _magnetite_ (strongly magnetic black iron oxide) occurs as layers in the gneiss.

In some places the gneiss contains dark-reddish crystals of garnet as much as an inch in diameter. Commonly the garnet crystals are surrounded by white “halos” which lack biotite or hornblende, probably because the constituents necessary to form these minerals were absorbed by the garnet crystals. In Death Canyon and on the slopes of Static Peak some layers of gray gneiss contain egg-shaped masses of magnetite as much as one-half inch in diameter (fig. 20). These masses are likewise surrounded by elliptical white halos and have the startling appearance of small eyes peering from the rock. Appropriately, this rock has been called the “bright-eyed” gneiss by Prof. Charles C. Bradley in his published study (Wyoming Geological Association, 1956) of this area.

What were the ancient rocks from which the gneisses of the Teton Range were formed? Most of the evidence has been obliterated but a few remaining clues enable us to draw some general conclusions. The banded appearance of many of the gneisses suggests that they were formed from sedimentary and volcanic rocks that accumulated on the sea floor near a chain of volcanic islands—perhaps somewhat similar to the modern Aleutians or the islands of Indonesia. When these deposits were buried deep in the earth’s crust the chemical composition of some layers may have undergone radical changes. Other layers, however, still have compositions resembling those of younger rocks elsewhere whose origins are better known. For example, the layers of impure marble were probably once beds of sandy limestone, and the lighter colored gneiss may have been muddy sandstone, possibly containing volcanic ash. Some dark amphibolite layers could represent altered lava flows or beds of volcanic ash; others may have resulted from the addition of silica to muddy magnesium-rich limestone during metamorphism. The magnetite-rich gneiss probably was originally a sedimentary iron ore.

Minerals that were most easily altered at depth reacted with one another to form new minerals more “at home” under the high temperature and pressure in this environment just as the ingredients in a cake react when heated in an oven. Rocks formed by such processes are called _metamorphic rocks_; careful studies of the minerals that they contain suggest that the layered gneisses developed at temperatures as high as 1000°F at depths of 5 to 10 miles. Under these conditions the rocks must have behaved somewhat like soft taffy as is shown by layers that have been folded nearly double without being broken (fig. 21). Folds such as these range from fractions of an inch to thousands of feet across and are found in gneisses throughout the Teton Range. In a few places folds are superimposed in such a way as to indicate that the rocks were involved in several episodes of deformation in response to different sets of stress during metamorphism.

When did these gneisses form? Age determinations of minerals containing radioactive elements show that granite which was intruded into them after they were metamorphosed and folded is more than 2.5 billion years old. They must, therefore, be older than that. Thus, they probably are at least a billion years older than rocks containing the first faint traces of life on earth and 2 billion years older than the oldest rocks containing abundant fossils. How much older is not known, but the gneisses are certainly among the oldest rocks in North America and record some of the earliest events in the building of this continent.

Figure 21. _Folds in layered gneisses._

Irregular bodies of granite gneiss are interleaved with the layered gneisses in the northern part of the Teton Range. The _granite gneiss_ is relatively coarse grained, streaky gray or pink, and composed principally of quartz, feldspar, biotite, and hornblende. It differs from enclosing layered gneisses in its coarser texture, lack of layering, and more uniform appearance. The dark minerals (biotite and hornblende) are concentrated in thin discontinuous wisps that give the rock its streaky appearance.

The largest body of granite gneiss is exposed in a belt 1 to 2 miles wide and 10 miles long extending northeastward from near the head of Moran Canyon, across the upper part of Moose Basin, and into the lower reaches of Webb Canyon. This gneiss may have been formed from granite that invaded the ancient sedimentary and volcanic rocks before they were metamorphosed, or it may have been formed during metamorphism from some of the sediments and volcanics themselves.

At several places in Snowshoe, Waterfalls, and Colter Canyons the layered gneisses contain discontinuous masses a few tens or hundreds of feet in diameter of heavy dark-green or black _serpentine_. This rock is frequently called “_soapstone_” because the surface feels smooth and soapy to the touch. Indians carved bowls (fig. 22) from similar material obtained from the west side of the Tetons and from the Gros Ventre Mountains to the southeast. Pebbles of serpentine along streams draining the west side of the Tetons have been cut and polished for jewelry and sold as “_Teton jade_”; it is much softer and less lustrous than real jade. The serpentine was formed by metamorphism of dark-colored igneous rocks lacking quartz and feldspar.

Granite and pegmatite

Contrary to popular belief, _granite_ (crystalline igneous rock composed principally of quartz and feldspar) forms only a part of the Teton Range. The Grand Teton (fig. 6) and most surrounding subsidiary peaks are sculptured from an irregular mass of granite exposed continuously along the backbone of the range from Buck Mountain northward toward upper Leigh Canyon. The rock is commonly fine grained, white or light-gray, and is largely composed of crystals of gray quartz and white feldspar about the size and texture of the grains in very coarse lump sugar. Flakes of black or dark-brown mica (biotite) and silvery white mica (_muscovite_) about the size of grains of pepper are scattered through the rock.

From the floor of Jackson Hole the granite cliffs and buttresses of the high peaks appear nearly white in contrast to the more somber grays and browns of surrounding gneisses and schists. These dark rocks are laced by a network of irregular light-colored granite dikes ranging in thickness from fractions of an inch to tens of feet (fig. 23).

The largest masses of granite contain abundant unoriented angular blocks and slabs of the older gneisses. These inclusions range from a few inches in diameter (fig. 24) to slabs hundreds of feet thick and thousands of feet long.

Dikes or irregular intrusions of pegmatite are found in almost every exposure of granite. _Pegmatite_ contains the same minerals as granite but the individual mineral crystals are several inches or even as much as a foot in diameter.

Some pegmatites contain silvery plates or tabular crystals of muscovite mica as much as 6 inches across that can be split into transparent sheets with a pocket knife. Others have dark-brown biotite mica in crystals about the size and shape of the blade of a table knife.

A few pegmatites contain scattered red-brown crystals of garnet ranging in size from that of a BB shot to a small marble; a few in Garnet Canyon and Glacier Gulch are larger than baseballs (fig. 25). The garnets are fractured and many are partly altered to _chlorite_ (a dull-green micaceous mineral) so they are of no value as gems.

Figure 23. _Dikes of granite and pegmatite._

Pegmatite _dikes_ (tabular bodies of rock that, while still molten, were forced along fractures in older rocks) commonly cut across granite dikes, but in many places the reverse is true. Some dikes are composed of layers of pegmatite alternating with layers of granite (fig. 26), showing that the pegmatite and granite are nearly contemporaneous. Prof. Bruno Giletti and his coworkers at Brown University, using the rubidium-strontium radioactive clock, determined that the granite and pegmatite in the Teton Range are about 2.5 billion years old.

Black dikes

Even the most casual visitor to the Teton Range notices the remarkable black band that extends down the east face of Mount Moran (figs. 27 and 28) from the summit and disappears into the trees north of Leigh Lake. This is the outcropping edge of a steeply inclined dike composed of _diabase_, a nearly black igneous rock very similar to basalt. Thinner diabase dikes are visible on the east face of Middle Teton, on the south side of the Grand Teton, and in several other places in the range (see geologic map inside back cover).

The diabase is a heavy dark-greenish-gray to black rock that turns rust brown on faces that have been exposed to the weather. It is studded with small lath-shaped crystals of feldspar that are greenish gray in the fresh rock and milk white on weathered surfaces.

The black dikes formed from molten rock that welled up into nearly vertical fissures in the older Precambrian rocks. Toward the edges of the dikes the feldspar laths in the diabase become smaller and smaller (fig. 29), indicating that the wall rocks were relatively cool when the _magma_ or melted rock was intruded. Rapid chilling at the edges prevented growth of large crystals. In many places hot solutions from the dike permeated the wall rocks, staining them rosy red.

The black dike on Mount Moran is about 150 feet thick near the summit of the peak. This dike has been traced westward for more than 7 miles. Where it passes out of the park south of Green Lakes Mountain it is 100 feet thick. The amount of molten material needed to form the exposed segment of this single dike could fill Jenny Lake three times over. The other dikes are thinner and not as long: the dike on Middle Teton is 20 to 40 feet thick, and the dike on Grand Teton is 40 to 60 feet thick.

The black dikes must be the youngest of the Precambrian units because they cut across all other Precambrian rocks. The dikes must have been intruded before the beginning of Cambrian deposition inasmuch as they do not cut the oldest Cambrian beds. Gneiss adjacent to the dike on Mount Moran contains biotite that was heated and altered about 1.3 billion years ago according to Professor Giletti. The alteration is believed to have occurred when the dike was emplaced; therefore this and similar dikes elsewhere in the range are probably about 1.3 billion years old.

Quartzite

At about the same time as the dikes were being intruded in the Tetons, many thousands of feet of sedimentary rocks, chiefly sandstone, were deposited in western Montana, 200 miles northwest of Grand Teton National Park. The sandstone was later recrystallized and recemented and became a very dense hard rock called _quartzite_. Similar quartzite, possibly part of the same deposit, was laid down west of the north end of the Teton Range, within the area now called the Snake River downwarp (fig. 1).

The visitor who hikes or camps anywhere on the floor of Jackson Hole becomes painfully aware of the thousands upon thousands of remarkably rounded hard quartzite boulders. He wonders where they came from because nowhere in the adjacent mountains is this rock type exposed. The answer is that the quartzites were derived from a long-vanished uplift (figs. 42 and 46), carried eastward by powerful rivers past the north end of the Teton Range, and then were deposited in a vast sheet of gravel that covered much of Jackson Hole 60 to 80 million years ago. Since then, these virtually indestructible boulders have been re-worked many times by streams and ice, yet still retain the characteristics of the original ancient sediments.

A backward glance

So far we have seen that the Precambrian basement exposed in the Teton Range contains a complex array of rocks of diverse origins and various ages. Before passing on to the younger rocks, reference to our yardstick may help to place the Precambrian events in their proper perspective.

In all of Precambrian time, which encompasses more than 85 percent of the history of the earth (31 of the 36 inches of our yardstick), only two events are dated in the Teton Range: the intrusion of granite and pegmatite about 2.5 billion years ago, and the emplacement of the black dikes about 1.3 billion years ago. These dates are indicated by heavy arrows on the time scale (fig. 30). The ancient gneisses and schists were formed sometime before 2.5 billion years ago, and probably are no older than 3.5 billion years, the age of the oldest rocks dated anywhere in the world.

The close of the Precambrian—end of the beginning

More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocks—a longer period of time than has elapsed since the beginning of the Paleozic Era. During this enormous interval the Precambrian rocks were uplifted, exposed to erosion, and gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly floundered and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years. It is to the sediments deposited in these seas that we turn to read the next chapter in the geologic story of the Teton Range.

ABSOLUTE TIME (Years ago) INCHES

Beginning of the Paleozoic. First abundant fossils → 4
1 billion 8
Maximum age of black dikes → 10
Oldest known fossils 15
2 billion 16
Old granite and pegmatite 20
3 billion 24
Gneisses and schists formed sometime in this interval 20-27
Oldest dated rocks → 28
4 billion → 32
Minimum age of the earth 36

THE PALEOZOIC ERA—TIME OF LONG-VANISHED SEAS AND THE DEVELOPMENT OF LIFE

The Paleozoic sequence

North, west, and south of the highest Teton peaks the soaring spires and knife-edge ridges of Precambrian rock give way to rounded spurs and lower flat-topped summits, whose slopes are palisaded by continuous gray cliffs that resemble the battlements of some ancient and long-abandoned fortress (fig. 31). As mentioned previously, the cliffs are the projecting edges of layers of sedimentary rocks of Paleozoic age that accumulated in or along the margins of shallow seas. At one time the layers formed a thick unbroken, nearly horizontal blanket across the Precambrian basement rocks, but subsequent uplift of the eastern edge of the Teton fault block tilted them westward. They were then stripped from the highest peaks.

The Paleozoic and younger sedimentary rocks in the Teton region are subdivided into _formations_, each of which is named. A formation is composed of rock layers which, because of their similar physical characteristics, can be distinguished from overlying and underlying layers. They must be thick enough to be shown on a geologic map. Table 2 lists the various Paleozoic formations present in and adjacent to Grand Teton National Park and gives their thicknesses and characteristics. These sedimentary rocks are of special interest, for they not only record an important chapter of geologic history but elsewhere in the region they contain petroleum and other mineral deposits.

The Paleozoic rocks can be viewed close at hand from the top of the Teton Village tram (fig. 32) on the south boundary of the park. A less accessible but equally spectacular exposure of Paleozoic rocks is in Alaska Basin, along the west margin of the park, where they are stacked like even layers in a gigantic cake (fig. 33).

Alaska Basin—site of an outstanding rock and fossil record

Strata in Alaska Basin record with unusual clarity the opening chapters in the chronicle of seas that flowed and ebbed across the future site of the Teton Range during most of the Paleozoic Era. In the various rock layers are inscribed stories of the slow advance and retreat of ancient shorelines, of the storm waves breaking on long-vanished beaches, and of the slow and intricate evolution of the myriads of sea creatures that inhabited these restless waters.

Careful study of the fossils allows us to determine the age of each formation (table 3). Even more revealing, the fossils themselves are tangible evidence of the orderly parade of life that crossed the Teton landscape during more than 250 million years. Here is a record of Nature’s experiments with life, the triumphs, failures, the bizarre, the beautiful.

Age Formation Thickness Description Where exposed
(feet)

Permian Phosphoria 150-250 Dolomite, gray, North and west
Formation cherty, sandy, flanks of Teton
black shale and Range, north
phosphate beds; flank of Gros
marine. Ventre Mountains,
southern Jackson
Hole.
Pennsylvanian Tensleep 600-1,500 Tensleep Sandstone, North and west
and Amsden light-gray, hard, flanks of Teton
Formations underlain by Range, north
Amsden Formation, flank of Gros
a domolite and red Ventre Mountains,
shale with a basal southern Jackson
red sandstone; Hole.
marine.
Mississippian Madison 1,000-1,200 Limestone, North and west
Limestone blue-gray, hard, flanks of Teton
fossiliferous; Range, north
thin red shale in flank of Gros
places near top; Ventre Mountains,
marine. southern Jackson
Hole.
Devonian Darby 200-500 Dolomite, dark-gray North and west
Formation to brown, fetid, flanks of Teton
hard, and brown, Range, north
black, and yellow flank of Gros
shale; marine. Ventre Mountains,
southern Jackson
Hole.
Ordovician Bighorn 300-500 Dolomite, North and west
Dolomite light-gray, flanks of Teton
siliceous, very Range, north and
hard; white dense west flanks of
very fine-grained Gros Ventre
dolomite at top; Mountains,
marine. southern Jackson
Hole.
Cambrian Gallatin 180-300 Limestone, blue North and west
Limestone gray, hard, flanks of Teton
thin-bedded; Range and Gros
marine. Ventre Mountains.
Gros Ventre 600-800 Shale, green, North and west
Formation flaky, with Death flanks of Teton
Canyon Limestone Range and Gros
Member composed of Ventre Mountains.
about 300 feet of
hard cliff-forming
limestone in
middle; marine.
Flathead 175-200 Sandstone, North and west
Sandstone reddish-brown, flanks of Teton
very hard, Range and Gros
brittle; partly Ventre Mountains.
marine.

The regularity and parallel relations of the layers in well-exposed sections such as the one in Alaska Basin suggest that all these rocks were deposited in a single uninterrupted sequence. However, the fossils and regional distribution of the rock units show that this is not really the case. The incomplete nature of this record becomes apparent if we plot the ages of the various formations on the absolute geologic time scale (fig. 34). The length of time from the beginning of the Cambrian Period to the end of the Mississippian Period is about 285 million years. The strata in Alaska Basin are a record of approximately 120 million years. More than half of the pages in the geologic story are missing even though, compared with most other areas, the book as a whole is remarkably complete! During these unrecorded intervals of time either no sediments were deposited in the area of the Teton Range or, if deposited, they were removed by erosion.

Madison Limestone
Darby Formation
Bighorn Dolomite
Gallatin Limestone

Advance and retreat of Cambrian seas: an example

The first invasion and retreat of the Paleozoic sea are sketched on figure 35. Early in Cambrian time a shallow seaway, called the _Cordilleran trough_, extended from southern California northeastward across Nevada into Utah and Idaho (fig. 35A). The vast gently rolling plain on Precambrian rocks to the east was drained by sluggish westward-flowing rivers that carried sand and mud into the sea. Slow subsidence of the land caused the sea to spread gradually eastward. Sand accumulated along the beaches just as it does today. As the sea moved still farther east, mud was deposited on the now-submerged beach sand. In the Teton area, the oldest sand deposit is called the Flathead Sandstone (fig. 36).

The mud laid down on top of the Flathead Sandstone as the shoreline advanced eastward across the Teton area is now called the Wolsey Shale Member of the Gros Ventre Formation. Some shale shows patterns of cracks that formed when the accumulating mud was briefly exposed to the air along tidal flats. Small phosphatic-shelled animals called _brachiopods_ inhabited these lonely tidal flats (fig. 37A and 37B) but as far as is known, nothing lived on land. Many shale beds are marked with faint trails and borings of wormlike creatures, and a few contain the remains of tiny very intricately developed creatures with head, eyes, segmented body, and tail. These are known as trilobites (fig. 37C and 37D). Descendants of these lived in various seas that crossed the site of the dormant Teton Range for the next 250 million years.

Mount Meek
Madison Limestone
Bighorn Dolomite
Death Canyon Limestone Member
Flathead Sandstone
Precambrian Rock

As the shoreline moved eastward, the Death Canyon Limestone Member of the Gros Ventre Formation (fig. 33) was deposited in clear water farther from shore. Following this the sea retreated to the west for a short time. In the shallow muddy water resulting from this retreat the Park Shale Member of the Gros Ventre Formation was deposited. In places underwater “meadows” of algae flourished on the sea bottom and built extensive reefs (fig. 38A). From time to time shoal areas were hit by violent storm waves that tore loose platy fragments of recently solidified limestone and swept them into nearby channels where they were buried and cemented into thin beds of jumbled fragments (fig. 38B) called _“edgewise” conglomerate_. These are widespread in the shale and in overlying and underlying limestones.

AGE (Numbers FORMATION (Thickness) ROCKS AND FOSSILS
show age in
millions of
years)

(310)
MISSISSIPPIAN MADISON LIMESTONE Uniform thin beds of
(Total about 1,100 blue-gray limestone and
feet, but only lower sparse very thin layers of
300 feet preserved in shale. Brachiopods, corals,
this section) and other fossils abundant.
(345)
LATE AND DARBY FORMATION (About Thin beds of gray and buff
MIDDLE DEVONIAN 350 feet) dolomite interbedded with
layers of gray, yellow, and
black shale. A few fossil
brachiopods, corals, and
bryozoans.
(390)
(425)
LATE AND BIGHORN DOLOMITE (About Thick to very thin beds of
MIDDLE 450 feet; Leigh blue-gray or brown dolomite,
ORDOVICIAN Dolomite Member about white on weathered surfaces.
40 feet thick at top) A few broken fossil
brachiopods, bryozoans, and
horn corals. Thin beds of
white fine-grained dolomite
at top are the Leigh Member.
(440)
(500)
LATE CAMBRIAN GALLATIN LIMESTONE (180 Blue-gray limestone mottled
feet) with irregular rusty or
yellow patches. Trilobites
and brachiopods.
(530)
MIDDLE CAMBRIAN GROS VENTRE FORMATION
PARK SHALE MEMBER Gray-green shale containing
(220 feet) beds of platy limestone
conglomerate. Trilobites,
brachiopods, and fossil algal
heads.
DEATH CANYON Two thick beds of
LIMESTONE MEMBER dark-blue-gray limestone
(285 feet) separated by 15 to 20 feet of
shale that locally contains
abundant fossil brachiopods
and trilobites.
WOLSEY SHALE MEMBER Soft greenish-gray shale
(100 feet) containing beds of purple and
green sandstone near base. A
few fossil brachiopods.
FLATHEAD LIMESTONE (175 Brown, maroon, and white
feet) sandstone, locally containing
many rounded pebbles of
quartz and feldspar. Some
beds of green shale at top.
(570)
PRECAMBRIAN Granite, gneiss, and
pegmatite.

STRATIGRAPHIC SCALE ABSOLUTE ENLARGED PIECE OF
TIME (Years YARDSTICK SHOWN ON
ago) FIGURE 19

2
PALEZOIC PENNSYLVANIAN ?
300 million
MISSISSIPPIAN MADISON
DEVONIAN DARBY
3
400 million
SILURIAN
ORDOVICIAN BIGHORN
500 million 4
CAMBRIAN GALLATIN
GROS VENTRE
FLATHEAD
600 million
PRECAMBRIAN 5

Figure 35. _The first invasions of the Paleozoic sea._

Once again the shoreline crept eastward, the seas cleared, and the Gallatin Limestone was deposited. The Gallatin, like the Death Canyon Limestone Member, was laid down for the most part in quiet, clear water, probably at depths of 100 to 200 feet. However, a few beds of “edgewise” conglomerate indicate the occurrence of sporadic storms. At this time, the sea covered all of Idaho and Montana and most of Wyoming (fig. 35B) and extended eastward across the Dakotas to connect with shallow seas that covered the eastern United States. Soon after this maximum stage was reached slow uplift caused the sea to retreat gradually westward. The site of the Teton Range emerged above the waves, where, as far as is now known, it may have been exposed to erosion for nearly 70 million years (fig. 35C).

The above historical summary of geologic events in Cambrian time is recorded in the Cambrian formations. This is an example of the reconstructions, based on the sedimentary rock record, that have been made of the Paleozoic systems in this area.

Figure 37. _Cambrian fossils in Grand Teton National Park._

A-B. _Phosphatic-shelled brachiopods, the oldest fossils found in
the park. Actual width of specimens is about ¼ inch._

C-D. _Trilobites. Width of C is ¼ inch, D is ½ inch. National Park
Service photos by W. E. Dilley and R. A. Mebane._

Younger Paleozoic formations

Formations of the remaining Paleozoic systems are likewise of interest because of the ways in which they differ from those already described.

Figure 38. _Distinctive features of Cambrian rocks._

The Bighorn Dolomite of Ordovician age forms ragged hard massive light-gray to white cliffs 100 to 200 feet high (figs. 32 and 33). _Dolomite_ is a calcium-magnesium carbonate, but the original sediment probably was a calcium carbonate mud that was altered by magnesium-rich sea water shortly after deposition. Corals and other marine animals were abundant in the clear warm seas at this time.

Dolomite in the Darby Formation of Devonian age differs greatly from the Bighorn Dolomite; that in the Darby is dark-brown to almost black, has an oily smell, and contains layers of black, pink, and yellow mudstone and thin sandstone. The sea bottom during deposition of these rocks was foul and frequently the water was turbid. Abundant fossil fragments indicate fishes were common for the first time. Exposures of the Darby Formation are recognizable by their distinctive dull-yellow thin-layered slopes between the prominent gray massive cliffs of formations below and above.

The Madison Limestone of Mississippian age is 1,000 feet thick and is exposed in spectacular vertical cliffs along canyons in the north, west, and south parts of the Tetons. It is noted for the abundant remains of beautifully preserved marine organisms (fig. 39). The fossils and the relatively pure blue-gray limestone in which they are embedded indicate deposition in warm tranquil seas. The beautiful Ice Cave on the west side of the Tetons and all other major caves in the region were dissolved out of this rock by underground water.

The Pennsylvanian System is represented by the Amsden Formation and the Tensleep Sandstone. Cliffs of the Tensleep Sandstone can be seen along the Gros Ventre River at the east edge of the park. The Amsden, below the Tensleep, consists of red and green shale, sandstone, and thin limestone. The shale is especially weak and slippery when exposed to weathering and saturated with water. These are the strata that make up the glide plane of the Lower Gros Ventre Slide (fig. 5) east of the park.

The Phosphoria Formation and its equivalents of Permian age are unlike any other Paleozoic rocks because of their extraordinary content of uncommon elements. The formation consists of sandy dolomite, widespread black phosphate beds and black shale that is unusually rich not only in phosphorus, but also in vanadium, uranium, chromium, zinc, selenium, molybdenum, cobalt, and silver. The formation is mined extensively in nearby parts of Idaho and in Wyoming for phosphatic fertilizer, for the chemical element phosphorus, and for some of the metals that can be derived from the rocks as byproducts. These elements and compounds are not everywhere concentrated enough to be of economic interest, but their dollar value is, in a regional sense, comparable to that of some of the world’s greatest mineral deposits.

Figure 39. _A glimpse of the sea floor during deposition of the
Madison Limestone 330 million years ago, showing the remains of
brachiopods, corals, and other forms of life that inhabited the
shallow warm water._

THE MESOZOIC—ERA OF TRANSITION

The Mesozoic Era in the Teton region was a time of alternating marine, transitional, and continental environments. Moreover, the highly diversified forms of life, ranging from marine mollusks to tremendous, land-living dinosaurs, confirm and reinforce the story of the rocks. Living things, too, were in transition, for as environment changed, many forms moved from the sea to land in order to survive. It was the time when some of the most spectacularly colored rock strata of the region were deposited.

Colorful first Mesozoic strata

Bright-red soft Triassic rocks more than 1,000 feet thick, known as the Chugwater Formation, comprise most of the basal part of the Mesozoic sequence (table 4). They form colorful hills east and south of the park. The red color is caused by a minor amount of iron oxide. Mud cracks and the presence of fossil reptiles and amphibians indicate deposition in a tidal flat environment, with the sea lying several miles southwest of Jackson Hole. A few beds of white _gypsum_ (calcium sulfate) are present; they were apparently deposited during evaporation of shallow bodies of salt water cut off from the open sea.

As the Triassic Period gave way to the Jurassic, salmon-red windblown sand (Nugget Sandstone) spread across the older red beds and in turn was buried by thin red shale and thick gypsum deposits of the Gypsum Spring Formation. Then down from Alaska and spreading across most of Wyoming came the _Sundance Sea_, a warm, muddy, shallow body of water that teemed with marine mollusks. In it more than 500 feet of highly fossiliferous soft gray shale and thin limestones and sandstones were deposited. The sea withdrew and the Morrison and Cloverly Formations (Jurassic and Lower Cretaceous) were deposited on low-lying tropical humid flood plains. These rocks are colorful, consisting of red, pink, purple, and green badland-forming claystones and mudstones, and yellow to buff sandstones. Vegetation was abundant and large and small dinosaurs roamed the countryside or inhabited the swamps.

Age Formation Thickness Description Where exposed
(feet)

CRETACEOUS
Harebell 0-5,000 Sandstone, olive Eastern and
Formation drab, silty, drab northeastern parts
siltstone, and of Jackson Hole.
dark-gray shale;
thick beds of
quartzite pebble
conglomerate in
upper part.
Meeteetse 0-700 Sandstone, gray to Spread Creek area.
Formation chalky white,
blue-green to gray
siltstone, thin
coal, and green to
yellow bentonite.
Mesaverde 0-1,000 Sandstone, white, Eastern Jackson
Formation massive, soft, thin Hole.
gray shale, sparse
coal.
Unnamed 3,500± Sandstone and Eastern Jackson
sequence of shale, gray to Hole and eastern
lenticular brown; abundant margin of the park.
sandstone, coal in lower 2,000
shale, and feet.
coal.
Bacon Ridge 900-1,200 Sandstone, light Eastern Jackson
Sandstone gray, massive, Hole and eastern
marine, gray shale, margin of the park.
many coal beds.
Cody Shale 1,300-2,200 Shale, gray, soft; Eastern and
thin green northern parts of
sandstone, some Jackson Hole.
bentonite; marine.
Frontier 1,000 Sandstone, gray, Eastern and
Formation and black to gray northern parts and
shale, marine; many south-western
persistent white margin of Jackson
bentonite beds in Hole.
lower part.
Mowry Shale 700 Shale, Gros Ventre River
silvery-gray, hard, Valley, northern
siliceous, with margin of the park,
many fish scales; and southern part
thin bentonite of Jackson Hole.
beds; marine.
Thermopolis 150-200 Shale, black, soft, Gros Ventre River
Shale fissile, with Valley, northern
persistent margin of the park,
sandstone at top; and southern part
marine. of Jackson Hole.
Cloverly and 650 Sandstone, light North end of Teton
Morrison(?) gray, sparkly, Range and Gros
Formations rusty near top, Ventre River Valley.
underlain by
variegated soft
claystone; basal
part is silty
dully-variegated
sandstone and
claystone.
JURASSIC
Sundance 500-700 Sandstone, green, North end of Teton
Formation underlain by soft Range, Blacktail
gray shale and thin Butte, Gros Ventre
highly River Valley.
fossiliferous
limestones; marine.
Gypsum Spring 75-100 Gypsum, white, North end of Teton
Formation interbedded with Range, Blacktail
red shale and gray Butte, Gros Ventre
dolomite; partly River Valley.
marine.
Nugget 0-350 Sandstone, North flank of Gros
Sandstone salmon-red, hard. Ventre Mountains,
southern Jackson
Hole.
TRIASSIC
Chugwater 1,000-1,500 Siltstone and North flank of Gros
Formation shale, red, Ventre Mountains,
thin-bedded; one north end of Teton
thin marine Range, southernmost
limestone in upper Jackson Hole.
third.
Dinwoody 200-400 Siltstone, brown, North flank of Gros
Formation hard, thin-bedded; Ventre Mountains,
marine. north end of Teton
Range, southernmost
Jackson Hole.

Drab Cretaceous strata

The youngest division of the _Mesozoic_ Era is the Cretaceous Period. Near the beginning of this period, brightly colored rocks continued to be deposited. Then, the Teton region, as well as most of Wyoming, was partly, and at times completely, submerged by shallow muddy seas. As a result, the brightly variegated strata were covered by 10,000 feet of generally drab-colored sand, silt, and clay containing some coal beds, volcanic ash layers, and minor amounts of gravel.

The Cretaceous sea finally retreated eastward from the Teton region about 85 million years ago, following the deposition of the Bacon Ridge Sandstone (fig. 40). As it withdrew, extensive coal swamps developed along the sea coast. The record of these swamps is preserved in coal beds 5 to 10 feet thick in the Upper Cretaceous deposits. The coal beds are now visible in abandoned mines along the east margin of the park. Coal is formed from compacted plant debris; about 5 feet of this material is needed to form 1 inch of coal. Thus, lush vegetation must have flourished for long periods of time, probably in a hot wet climate similar to that now prevailing in the Florida Everglades.

Sporadically throughout Cretaceous time fine-grained ash was blown out of volcanoes to the west and northwest and deposited in quiet shallow water. Subsequently the ash was altered to a type of clay called _bentonite_ that is used in the foundry industry and in oil well drilling muds. In Jackson Hole, the elk and deer lick bentonite exposures to get a bitter salt and, where the beds are water-saturated, enjoy “stomping” on them. Bentonite swells when wet and causes many landslides along access roads into Jackson Hole (fig. 17).

The Cretaceous rocks in the Teton region are part of an enormous east-thinning wedge that here is nearly 2 miles thick. Most of the debris was derived from slowly rising mountains to the west.

Cretaceous sedimentary rocks are much more than of just scientific interest; they contain mineral deposits important to the economy of Wyoming and of the nation. Wyoming leads the States in production of bentonite, all of it from Cretaceous rocks. These strata have yielded far more oil and gas than any other geologic system in the State and the production is geographically widespread. They also contain enormous coal reserves, some in beds between 50 and 100 feet thick. The energy resources alone of the Cretaceous System in Wyoming make it invaluable to our industrialized society.

ABSOLUTE TIME (Millions of years ago) INCHES

{submerged} 85-585 ⅝-4⅝
CENOZOIC 0-80 0-½
MESOZOIC 80-180 ½-⅞
PALEOZOIC 180-570 ⅞-4⅞
PRECAMBRIAN 570- 4⅞-

As the end of the Cretaceous Period approached, slightly more than 80 million years ago, the flat monotonous landscape (fig. 41) which had prevailed during most of Late Cretaceous time gave little hint that the stage was set for one of the most exciting and important chapters in the geologic history of North America.

Birth of the Rocky Mountains

The episode of mountain building that resulted in formation of the ancestral Rocky Mountains has long been known as the _Laramide Revolution_. West and southwest of Wyoming, mountains had already formed, the older ones as far away as Nevada and as far back in time as Jurassic, the younger ones rising progressively farther east, like giant waves moving toward a coast. The first crustal movement in the Teton area began in latest Cretaceous time when a broad low northwest-trending arch developed in the approximate area of the present Teton Range and Gros Ventre Mountains. However, this uplift bore no resemblance to the Tetons as we know them today for the present range formed 70 million years later.

One bit of evidence (there are others) of the first Laramide mountain building west of the Tetons is a tremendous deposit of quartzite boulder debris (several hundred cubic miles in volume) derived from the _Targhee uplift_ (fig. 42). Nowhere is the uplift now exposed, but from the size, composition, and distribution of rock fragments that came from it, we know that it was north and west of the northern end of the present-day Teton Range. Powerful streams carried boulders, sand, and clay eastward and southeastward across the future site of Jackson Hole and deposited them in the Harebell Formation (table 4). Mingled with this sediment were tiny flakes of gold and a small amount of mercury. Fine-grained debris was carried still farther east and southeast into two enormous depositional troughs in central and southern Wyoming. Most of the large rock fragments were derived from Precambrian and possibly lower Paleozoic quartzites. This means that at least 15,000 feet of overlying Paleozoic and Mesozoic strata must first have been stripped away from the Targhee uplift before the quartzites were exposed to erosion.

Remains of four-legged horned ceratopsian dinosaurs, possibly _Triceratops_ (fig. 43), reflecting the last population explosion of these reptiles, have been found in pebbly sandstone of the Harebell Formation in highway cuts on the Togwotee Pass road 8 miles east of the park.

Near the end of Cretaceous time, broad gentle uplifts also began to stir at the sites of future mountain ranges in many parts of Wyoming. The ancestral Teton-Gros Ventre arch continued to grow. Associated with and parallel to it was a series of sharp steepsided elongated northwest-trending upfolds (_anticlines_). One of these can be seen where it crosses the highway at the Lava Creek Campground near the eastern margin of Grand Teton National Park.

During these episodes of mountain building, erosion, and deposition, the dinosaurs became extinct all over the world. The “Age of Mammals” was about to begin.

TERTIARY—TIME OF MAMMALS, MOUNTAINS, LAKES, AND VOLCANOES

STRATIGRAPHIC SCALE THE LAST INCH OF ABSOLUTE TIME (million
THE YARDSTICK years ago)

CENOZOIC
QUATERNARY
Recent and 0 0
Pleistocene
TERTIARY
Pliocene 0 0
Miocene ⅛ 12
Oligocene ¼ 25
Eocene ⅜ 40
Paleocene ⁷/₁₆ 55
MESOZOIC
CRETACEOUS ½ 65

The Cenozoic (table 1), last and shortest of the geologic eras, comprises the Tertiary and Quaternary Periods. It began about 65 million years ago and is represented by only the final one-half inch of our imaginary yardstick of time (fig. 19). Nevertheless, it is the era during which the Tetons rose in their present form and the landscape was sculptured into the panorama of beauty that we now see. In order to show the many Tertiary and Quaternary events in the Teton region, it is necessary to enlarge greatly the last part of the yardstick (fig. 44). There are two reasons for the extraordinarily clear and complete record. First, the Teton region was a relatively active part of the earth’s crust, characterized by many downdropped blocks. The number of events is great and their records are preserved in sediments trapped in the subsiding basins. Second, the geologically recent past is much easier to see than the far dimmer, distant past; the rocks that record later events are fresher, less altered, more complete, and more easily interpreted than are those that tell us of older events.

Age Formation Thickness Description Where exposed
(feet)