The Tetons are a still different kind—a fault block mountain range carved from a segment of the earth’s crust that has been uplifted along a fault. The Teton fault is approximately at the break in slope where the eastern foot of the range joins the flats at the west edge of Jackson Hole (see [map] inside back cover), but in most places is concealed beneath glacial deposits and debris shed from the adjacent steep slopes. The shape of the range and its relation to Jackson Hole have already been described. Clues as to the presence of the fault are: (1) the straight and deep east face of the Teton Range, (2) absence of foothills, (3) asymmetry of the range ([fig. 14]), and (4) small fault scarps (cliffs or steep slopes formed by faulting) along the mountain front ([fig. 15]).
Figure 14. Air oblique view south showing the width and asymmetry of Teton Range. Grand Teton is left of center and Mt. Moran is the broad humpy peak still farther left. Photo taken October 1, 1965.
Recent geophysical surveys of Jackson Hole combined with data from deep wells drilled in search of oil and gas east of the park also yield valuable clues. By measuring variations in the earth’s magnetic field and in the pull of gravity and by studying the speed of shock waves generated by small dynamite explosions, Dr. John C. Behrendt of the U. S. Geological Survey has determined the depth and tilt of rock layers buried beneath the veneer of glacial debris and stream-laid sand and gravel on the valley floor. This information was used in constructing the geologic cross section in the back of the booklet. The same rock layers that cap the summit of Mount Moran ([fig. 27]) are buried at depths of nearly 24,000 feet beneath the nearby floor of Jackson Hole but are cut off by the Teton fault at the west edge of the valley. Thus the approximate amount of movement along the fault here would be about 30,000 feet.
Anatomy of faults
The preceding discussion shows that the Tetons are an upfaulted mountain block. Why is this significant? The extreme youth of the Teton fault, its large amount of displacement, and the fact that the newly upfaulted angular mountain block was subjected to intense glaciation are among the prime factors responsible for the development of the magnificent alpine scenery of the Teton Range. An understanding of the anatomy of faults is, therefore, pertinent.
Figure 15. Recent fault scarp (arrows indicate base) offsetting alluvial fan at foot of Rockchuck Peak. View west from Cathedral Group scenic turnout. National Park Service photo by W. E. Dilley and R. A. Mebane.
A fault is a plane or zone in the earth’s crust along which the rocks on one side have moved in relation to the rocks on the other. There are various kinds of faults just as there are various types of mountains. Three principal types of faults are present in the Teton region: normal faults, reverse faults, and thrust faults. A normal fault ([fig. 16A]) is a steeply dipping (steeply inclined) fault along which rocks above the fault have moved down relative to those beneath it. A reverse fault ([fig. 16B]) is a steeply inclined fault along which the rocks above the fault have moved up relative to those below it. A thrust fault ([fig. 16C]) is a gently inclined fault along which the principal movement has been more nearly horizontal than vertical.
Normal faults may be the result of tension or pulling apart of the earth’s crust or they may be caused by adjustment of the rigid crust to the flow of semi-fluid material below. The crust sags or collapses in areas from which the subcrustal material has flowed and is bowed up and stretched in areas where excess subcrustal material has accumulated. In both areas the adjustments may result in normal faults.