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.
[Illustration: 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.
Reverse faults are generally caused by compression of a rigid block of
the crust, but some may also be due to lateral flow of subcrustal
material.
Thrust faults are commonly associated with tightly bent or folded rocks.
Many of them are apparently caused by severe compression of part of the
crust, but some are thought to have formed at the base of slides of
large rock masses that moved from high areas into adjacent low areas
under the influence of gravity.
The Teton fault (see cross section inside back cover) is a normal fault;
the Buck Mountain fault, which lies west of the main peaks of the Teton
Range, is a reverse fault. No thrust faults have been recognized in the
Teton Range, but the mountains south and southwest of the Tetons (fig.
1) display several enormous thrust faults along which masses of rocks
many miles in extent have moved tens of miles eastward and
northeastward.