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Geology of Grand Teton National Park
EDITOR'S NOTE: In general Dr. Edmund's talk followed his publication "Structural Geology and Physiography of the Northern End of the Teton Range, Wyoming," Augustana College Library Publications, No. 23, 1951, Rock Island, Ill., and the following comments are a very curtailed digest of this publication. Any geologist going to the Tetons should take Dr. Edmund's publication with him. The high point of Dr. Edmund's talk was his startling collection of colored slides illustrating every item of geology, physiography and scenery across the northern part of the Teton Mountains mass and flanking country.)
The Teton Mountain Range is one of the several outstanding physiographic features of the Middle Rocky Mountains Province. On its west is the Teton Basin, on its east Jackson Hole and to the north and south the range loses itself respectively in the Pitchstone Plateau and the low Teton Pass Mountains adjoining the transverse Snake River Range.
At either end of the narrow belt of peaks of the Tetons the summits attain a maximum altitude of 11,000 feet; these heights increase toward the central portion and culminate in a group of jagged spires of which the highest, Grand Teton, has an altitude of 13,766 feet.
The major Teton Peaks and the canyons of the east slope are carved in Precambrian crystallines. At the north and south ends of the range the east slope in part is developed in Paleozoic and Mesozoioc sedimentary rocks. On the west slope the canyons are cut mainly in Paleozoic and Mesozoic sedimentaries and in the Tertiary volcanies of the Pitchstone Plateau at the north end.
The Teton Basin is an intermontane basin drained by the northward flowing Teton River which has cut a gorge in the volcanies lying across the northern portion of the basin. Jackson Hole is a narrow intermontane basin, with the Mount Leidy Highlands and the Gros Ventre Range to the east. On the north are uplands which become the Pitchstone Plateau and at the south end are the Hoback, Wyoming and Snake River Ranges.
The sedimentary rocks exposed in the northern portion of the Teton Mountains range in age from middle Cambrian to early Tertiary. The Paleozoic and Mesozoic part of this section thicken south-southwestward from 8,000 to 10,000 feet to more than 40,000 feet in the area around the southeast corner of Idaho. The accompanying indicates in part the generalized lithology of the sedimentary section in the northern portion of the Tetons.
The series of cross sections show also the character of the deformation in the northern Tetons. Extensive consideration of all the structural factors indicates that the Teton block originated by upthrust. This hypothesis seems best to explain the following conditions:
1. The high position of the Cambrian rocks.
2. The sharply defined Teton block.
3. The higher position of volcanic remnants at the north end of the range as compared to the Pitchstone Plateau.
4. The close folding of Cretaceous beds along the east shore of Jackson Lake.
5. The crescent pattern of the fault block and the break down of the main fault into branches at the extremities of the block.
6. The superior elevation or a Precambrian wedge between two major faults.
7. The westward tilt of the pre-volcanic erosion surface.
8. The dips in Paleozoic beds related to certain faulting.
The deposition of the thick Paleozoic and Mesozoic sediments was followed by the Laramide deformation in which structural axes were developed lying in a northwest-southeast direction. This folding was followed by extensive erosion and on the resulting low relief surface the earlier Tertiary volcanics were laid down. However, the details of the relationships of the depositional history following the erosion are confused.
Isolated volcanic remnants scattered through the northern Tetons at various elevations suggest that the Teton region was buried by this material. Other conditions, including apparent dips and erosion surfaces in the volcanics and different times of flows indicate that the Teton block was uplifted while volcanism was still active. Following the uplift of the Teton block erosion produced at least three "peneplains" or "surfaces". On this erosional surface the glaciation of the Pleistocene occurred, producing all the glacial effects which are present in the mountain regions of the west.
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