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.
[Illustration: Figure 31. _Paleozoic rocks on the west flank of the
Teton Range, air oblique view west. Ragged peaks in the foreground
(Buck Mountain on the left center, Mt. Wister, with top outlined by
snow patch on the extreme right), are carved in Precambrian rocks.
Banded cliffs in the background are sedimentary rocks. Alaska Basin
is at upper right. Teton Basin, a broad, extensively farmed valley
in eastern Idaho, is at top. Photo by A. S. Post, University of
Washington, 1963._]
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.
[Illustration: Table 2.—Paleozoic sedimentary rocks exposed in the
Teton region.]
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.
[Illustration: Figure 32. _Paleozoic marine sedimentary rocks near
south boundary of Grand Teton National Park. View is south from top
of Teton Village tram. National Park Service photo by W. E. Dilley
and R. A. Mebane._]
Madison Limestone
Darby Formation
Bighorn Dolomite
Gallatin Limestone