GRAND TETON NATURE NOTES
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Vol. II |
Autumn, 1936 |
No. 4. |
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THE GEOLOGICAL HISTORY OF THE TETON RANGE, A SUMMARY
by Howard R. Stagner
How were the Tetons formed? Why does this mountain range have such an
abrupt, precipitous eastern front? How were the deep, cliff-bound canons
formed? what is the explanation of the conspicuous black dike that
appears on the eastern slopes of Mount Moran? These are but a few of the
many questions that thrust themselves before the attention of the
visitor to Grand Teton National Park. An understanding and the full
appreciation of these wonders of Nature demands a knowledge of the
events that have occurred in this area during the geologic past.
The limited size of Nature Notes precludes the telling of the
geological story in one or in a number of issues. In order to treat the
story in the detail necessary to give a true conception of the
geological story of the range it will be necessary to relate each phase
of the story and each episodes eparately in different issues of this
bulletin. An attempt shall be made to make each episode complete in
itself, but each will form a part of a series that will tell the story
of the geology of the Tetons as far as it is now known.
With this purpose in mind, an effort has been made in tho following
account to summarize and to date the major events of Teton geological
history. The reader of future articles can, perhaps, by reference to the
following, better understand the more detailed descriptions in their
relation to the complete story.
I. An Ancient Landscape
Looking backward in geologic time, the duration of which is as
impossible of human conception as is the extent of astronomic space, one
can see but the most indistinct suggestions of the beginnings of the
earth and of its earliest history. Many events certainly took place-
volcanic eruptions, earth movements, atmospheric and climatic changes,
the beginnings of life- events of which the rocks bear very little
record that can be recognized and interpreted.
After assigning these vague things to the unknown geologic past, we
come to a time when it is known that no mountains such as are now so
prominent in the Rocky Mountain region, existed. In place of high
mountains and mountain-girt basins, western United States was a low
subdued land, presenting a landscape of low hills, shallow valleys and
gently rolling plains. The relief was low and the average elevation was
but little above sea level. The most ancient of rocks composed this
land. These were chiefly the metamorphic rocks gneiss and schist into
which granites, pegmatites, diorites and other igneous rocks had been
intruded.
II The Ancient Sea-
At the time when our known geological story begins, this ancient land
was subsiding along broad, elongated areas to permit the waters of the
oceans to flood these portions of the continent. At various times such
continental seas extended from Lower California northward and
northeastward across western United States to connect with similar seas
coming from the Arctic regions. Other invasions of the seas began in the
Gulf of Mexico area and connected across the Rocky Mountain area with
the Arctic seas. Many times the seas advanced and retreated over the
Rocky Mountain region, and many times the Grand Teton area and
surrounding areas were alternately covered by the seas and exposed as
dry land.
Rivers, flowing from neighboring highlands, brought millions of tons
of sand, silt and limy ooze into the seas and millions of plants and
animals of the seas, upon dying, left their calcareous hard parts to add
to the material accumulating on the sea floor. All of this material was
distributed in nearly horizontal layers on the floor of the sea. During
the time since then, pressure and natural cements have hardened these
deposits to form the sedimentary rocks sandstone, shale and limestone.
The total thickness of sedimentary rocks in the Grand Teton region is in
excess of 6,000 feet.
Occasionally plants and animals which inhabited the seas or the land
were buried with the sediments in such a manner that they were preserved
in the rocks as fossils.
III. The Building of the Mountain Highlands
Finally the continent emerged permanently from the seas, and soon
"wrinkles" or upward folds began to appear in the rocks as the continent
was squeezed by tremendous forces. These folds grew to form the highland
areas that are now the mountain ranges of the Rocky Mountain region. In
the Teton area such an upward fold, about fifty miles long north to
south, and fifteen miles wide, fractured along its eastern side to form
a great fault. The total upward movement of the Teton Range along this
fault was at least 7,000 feet, and may have been as much as 12,000 to
14,000 feet. The displacement was not the result of a single
catastrophic movement, but was doubtlessly attained as the result of
innumerable small movements occurring over a long period of geologic
time.
While the mountain area was rising, great volcanic outpourings and
eruptions developed in the areas to the west, north and east of the
Tetons. Some of the breccia and ash from the Yellowstone explosive
volcanoes were without doubt carried into Jackson's Hole. Great rhyrlite
lava flows from Yellowstone submerged the north end of the Tetons, and
thegreat basalt lavas of Idaho flowed against the western slopes of the
range. However, very little if any evidence of actual outpourings of
lava or of eruptions of breccia and ash have been found in Jackson's
Hole or in the Teton Range.
IV. The Sculpturing of the Tetons
Immediately after the uplift of the mountains, or more truly, while
this process was going on, running water and moving ice began to carve
canons into the uplifted fault block. Running water has continued to be
very active in the erosion of the mountains and in the removal of much
sedimentary, igneous, and metamorphic rock material from the mountains
area. Periodically glaciers formed in the mountains and moved down the
steep slopes to deepen and to render more rugged the existing canons.
Three times did glaciers form, enlarge, and move down the canons onto
the floor of Jackson's Hole.
V. The Tetons Today
The Teton Range is now in a post-glacial period, and the great
glaciers have long since melted hack until they are confined to the
cirques at the heads of the canons. Seven such small glaciers and
numerous permanent ice fields still exist- remnants of the once great
alpine glaciers.
So, streams and glaciers have combined forces to carve from an
uplifted fault block the present rugged Teton Range The deep,
steep-sided, "U"-shaped canons and the deep, cliff-bound
amphitheatre-like cirques at their heads; the sharp, saw-toothed ridges
and the "horn" peaks; the cascades, falls and numerous lakes all owe
their origin to the mighty glaciers of the past.
Erosion by streams and glaciers has removed from the high portion of
the range its former covering of sedimentary rocks. So, again, the
ancient rocks are exposed to form the high Teton peaks. Only on the
western side of the range, and in its lower areas to the north and to
the south of the culminating peaks do the sedimentary rocks still
remain.
We know not what lies ahead. Erosion is still going on at a rapid
rate, and in neighboring areas the tremors of earth movements are still
felt. Thus, the endless war between the forces of building and the
agents of destruction continues. Nature is never idle, and "time, which
means everything to our idea, and is often dificient to our schemes, is
to Nature endless and as nothing".
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Chronology
era | period | events |
Cenozoic |
Quaternary |
Glacial erosion predominated |
Tertiary |
Erosion by streams began with the uplifting of the mountains, and has
continued to the present.
Volcanic activity to the west, north and east accompanied some of the
earth movements.
Mountain uplifting continued at intervals during this period.
The emergence of the continent from the sea, and the first building
of the mountains began with this period.
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Mesozoic |
Cretaceous Jurassic Triassic |
Paleozoic and Mesozoic were the eras of sea invasions and of the
formation of the sedimentary rocks. Not all periods are represented in
the Teton Range, but in surrounding areas rocks of all periods except
the Silurian are represented. There were some intervals of erosion
between successive advances of the sea.
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Paleozoic |
Permian
Pennsylvanian
Mississippian
Devonian (Silurian)
Ordovician
Cambrian |
Proterozoic Archeozoic |
(pre-Cambrian) |
During these eras the old igneous and metamorphic rocks were formed,
and the surface of those rock formations was eroded to form the ancient
landmass.
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CROSS SECTIONS THROUGH THE TETON RANGE
(click on image for a PDF version)
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