Kinds of mountains
There are many kinds of mountains. Some are piles of lava and debris erupted from a volcano. Others are formed by the bowing up of the earth’s crust in the shape of a giant dome or elongated arch. Still others are remnants of accumulated sedimentary rocks that once filled a basin between preexisting mountains and which are now partially worn away. An example of this type is the Absaroka Range 40 miles northeast of the Tetons (figs. [1] and [52]).
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.