Geologists regard the Teton Range as one of the most impressive known
ranges of the "fault block" type. Ages ago, along a great break in the
earth's crust (the "Teton Fault") a gigantic block was uplifted and
given a westward slant. Long-continued sculpturing of this tilted fault
block by many natural agencies--frost, streams, avalanches of rock and
snow, and glaciers--has produced the notable scenic features of the
Teton Range as we now see it.
[Illustration: --UNION PACIFIC RAILROAD PHOTO
_Mount Teewinot as seen from the Jenny Lake Museum._]
Streams on the east slope, having steeper gradients and therefore more
rapid flow than the other streams, cut spectacular canyons on this side
of the range. As these streams have worked back into the giant block,
they have caused the divide to migrate westward, diverting more and more
drainage to the east and leaving the great peaks standing like monuments
on the ever-widening east slope.
East of the Teton Fault, in the Jackson Hole area, another great earth
block lies deeply buried beneath debris brought down into the basin by
mountain streams and glaciers. Changes wrought by the great glaciers of
the Ice Age have given the region much of its distinctive character.
Glacial erosion is strikingly evident in the sharply chiselled peaks,
the U-shaped canyons and the profound basins ("cirques") at their heads,
and the numerous little alpine tarns (lakelets occupying ice-gouged
basins). The irregular wooded ridges of Jackson Hole, on the other hand,
are due to glacial deposition, being composed of bouldery debris heaped
up by the ice. Some of these moraines form the dams which enclose the
beautiful lakes at the foot of the Teton Range--Phelps, Taggart,
Bradley, Jenny, Leigh, and Jackson Lakes. The broad terraced plains of
Jackson Hole are for the most part great sheets of gravel spread out on
the valley floor by the glacial streams of the Ice Age.
Small glaciers still found among the Teton Peaks are now believed to be
youthful ice bodies only a few thousand years old, rather than the
dwindling remnants of great glaciers of the Ice Age, as was formerly
thought.
A fuller account of the geologic features is given on the reverse side
of the topographic map of Grand Teton National Park, for sale at the
museum.