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).
[Illustration: 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.