The oldest Precambrian rocks in the Teton Range are layered gneisses and
schists exposed over wide areas in the northern and southern parts of
the range and as scattered isolated masses in the younger granite that
forms the high peaks in the central parts. The layered gneisses may be
seen easily along the trails in the lower parts of Indian Paintbrush and
Death Canyons, and near Static Peak.
The _layered gneisses_ are composed principally of quartz, feldspar,
_biotite_ (black mica), and _hornblende_ (a very dark-green or black
mineral commonly forming rodlike crystals). Distinct layers, a few
inches to several feet thick, contain different proportions of these
minerals and account for the banded appearance. Layers composed almost
entirely of quartz and feldspar are light-gray or white, whereas darker
gray layers contain higher proportions of biotite and hornblende.
Some layers are dark-green to black _amphibolite_, composed principally
of hornblende but with a little feldspar and quartz. In many places the
gneisses include layers of _schist_, a flaky rock, much of which is
mica. At several places on the east slopes of Mount Moran thin layers of
impure gray marble are found interleaved with the gneisses. West of
Static Peak along the Alaska Basin Trail a heavy dark rock with large
amounts of _magnetite_ (strongly magnetic black iron oxide) occurs as
layers in the gneiss.
In some places the gneiss contains dark-reddish crystals of garnet as
much as an inch in diameter. Commonly the garnet crystals are surrounded
by white “halos” which lack biotite or hornblende, probably because the
constituents necessary to form these minerals were absorbed by the
garnet crystals. In Death Canyon and on the slopes of Static Peak some
layers of gray gneiss contain egg-shaped masses of magnetite as much as
one-half inch in diameter (fig. 20). These masses are likewise
surrounded by elliptical white halos and have the startling appearance
of small eyes peering from the rock. Appropriately, this rock has been
called the “bright-eyed” gneiss by Prof. Charles C. Bradley in his
published study (Wyoming Geological Association, 1956) of this area.
What were the ancient rocks from which the gneisses of the Teton Range
were formed? Most of the evidence has been obliterated but a few
remaining clues enable us to draw some general conclusions. The banded
appearance of many of the gneisses suggests that they were formed from
sedimentary and volcanic rocks that accumulated on the sea floor near a
chain of volcanic islands—perhaps somewhat similar to the modern
Aleutians or the islands of Indonesia. When these deposits were buried
deep in the earth’s crust the chemical composition of some layers may
have undergone radical changes. Other layers, however, still have
compositions resembling those of younger rocks elsewhere whose origins
are better known. For example, the layers of impure marble were probably
once beds of sandy limestone, and the lighter colored gneiss may have
been muddy sandstone, possibly containing volcanic ash. Some dark
amphibolite layers could represent altered lava flows or beds of
volcanic ash; others may have resulted from the addition of silica to
muddy magnesium-rich limestone during metamorphism. The magnetite-rich
gneiss probably was originally a sedimentary iron ore.
[Illustration: Figure 20. _“Bright-eyed” gneiss from Death Canyon.
The dark magnetite spots are about ¼ inch in diameter. The
surrounding gneiss is composed of quartz, feldspar, and biotite, but
biotite is missing in the white halos around the magnetite._]
Minerals that were most easily altered at depth reacted with one another
to form new minerals more “at home” under the high temperature and
pressure in this environment just as the ingredients in a cake react
when heated in an oven. Rocks formed by such processes are called
_metamorphic rocks_; careful studies of the minerals that they contain
suggest that the layered gneisses developed at temperatures as high as
1000°F at depths of 5 to 10 miles. Under these conditions the rocks must
have behaved somewhat like soft taffy as is shown by layers that have
been folded nearly double without being broken (fig. 21). Folds such as
these range from fractions of an inch to thousands of feet across and
are found in gneisses throughout the Teton Range. In a few places folds
are superimposed in such a way as to indicate that the rocks were
involved in several episodes of deformation in response to different
sets of stress during metamorphism.
When did these gneisses form? Age determinations of minerals containing
radioactive elements show that granite which was intruded into them
after they were metamorphosed and folded is more than 2.5 billion years
old. They must, therefore, be older than that. Thus, they probably are
at least a billion years older than rocks containing the first faint
traces of life on earth and 2 billion years older than the oldest rocks
containing abundant fossils. How much older is not known, but the
gneisses are certainly among the oldest rocks in North America and
record some of the earliest events in the building of this continent.