Fort Bridger is a government military post, situated on the high
southern plateau of western Wyoming Territory, in the midst of one of
the most interesting geological regions of the world.
The country on all sides was once the bottom of a great eocene lake,
the water of which was probably slightly brackish. Whether this lake
district had direct communication with the ocean, is undetermined as
yet, but there is a possibility that it had.[1]
[Footnote 1: Ichthyic fauna of the Green River shales, Hayden's
Surveys, vol. iii., No. 4, p. 819.]
The tributaries of the Green River, which drain this plateau, render
the valleys along the edges of the streams green and wooded. Beyond
this fertile strip, wide, barren plains extend, covered by a dense
growth of short "sage brush" (Artemisia tridentata).
From the fragmentary debris lying scattered over the surface of the
ground, it would seem as if the various streams formerly were of much
greater size and volume than they now are, and that long after the
eocene lakes had been drained-rivers of considerable size ploughed up
the lake bottoms, excavating an immense area. The formation known as
"Mauvaises Terres" rises abruptly from the valleys, and extends in a
series of plateaus, one above the other, on either side.
The high land shows the effect of violent erosion in two forms;
first, the irregular and jagged cones that appear upon the sides of
the high benches; and, second, the isolated butte structure, rising
directly out of the plain.
The bad lands of Cottonwood Creek, Henry's Fork, Dry Creek, etc., are
examples of the first, and Bridger Butte the best known example of
the second.
Bridger Butte, six miles to the south-west of Fort Bridger, rises to
a height of over a hundred feet, and is about two miles long; its
sides slope steeply up, and its level top serves as a landmark that
can be seen miles away.
The stratification throughout this whole formation is nearly
horizontal, and across the valley can be distinctly noted, owing to
the color and appearance of the various layers.
No satisfactory explanation has been given of the causes which
occasioned the removal of the waters of these lakes, nor of the
agencies necessary to accomplish the great excavations that now
show the former bottoms of the basins. These and like points future
investigation will undoubtedly solve.
It is certain that the level of the lakes varied at different times,
and also that great stretches of marshy land surrounded their borders.
The first fact is proved by the characters of the different layers of
strata; the second by the fossil remains entombed. A careful study
of the formation of the beds of Cottonwood Creek, at a point about
fourteen miles south of Fort Bridger, yielded the following result,
which will serve as an example illustrative of the regular formation
in this section.
Three distinct lines of bluffs are to be noticed, the first rising to
a height of one hundred and fifty feet; from these extends a plain,
gently sloping south-west to the foot of the second line of cliffs;
these seem high, owing to the downward slope of the plain, but they
really rise only fifty feet higher than the first.
On the top of the second, but less broad than the first, extends a
level plain, with a slight dip to the south-east; at the end of this,
the third line rises two hundred feet above the top of the second
line of bluffs, making a total height, in the series, of four hundred
feet above the level of Smith's Fork.
The strata throughout are nearly horizontal, and are of different
color and composition.
Specimens of the rocks and clays from this section were gathered,
and submitted for analysis to Professor Cornwall, of the Scientific
School at Princeton, who has kindly furnished us with the following
notes:
* * * * *
No. 1, very friable, light greenish-gray sedimentary rocks,
consisting chiefly of crystalline grains of quartz, orthoclase, and
hornblende (this often in slender crystals), with a little dark
mica, and irregular fragments of a light-greenish, transparent, not
dichroitic mineral. The above are not perceptibly affected by hot
hydrochloric acid. The whole is loosely cemented with a calcareous
clay, containing considerable phosphoric acid. These rocks might
result from the disintegration of a neighboring hornblendic granite.
The greatest mass of the strata is made up of this kind of rock, and
it is this which gives the peculiar color to the "Mauvaises Terres."
Above this in places is found a second kind, which is a light gray
indurated clay, with a slight greenish tint. It contains much fine
crystalline quartz, with considerable carbonate of lime, and a little
phosphoric acid.
It appears to be of similar origin with the first, but was deposited
in quieter waters. This mineralogical evidence is strengthened by the
fact that no remains of mammals were found in strata of this kind,
but only shells regularly deposited in layers one above another.
The third kind is found in thin layers, overtopping the highest line
of buttes; it consists of very fine-grained dark-brown sandstones,
containing a considerable proportion of carbonate and phosphate of
lime. They are hard and tough, and are mechanically deposited, and no
fossils are found in them.
In No. 2 the indurated clay is often found above a coarser sandstone
than No. 1, but of the same general appearance; with the exception
that it contains smooth, rounded pebbles, which were deposited either
on a beach or in running water. In this stratum the fossils found are
separate bones, often showing marks of having been broken before they
were silicified. This would prove that the lake level was changing
continually.
Several skeletons of animals have been found in a standing position,
with their legs slightly stretched out. They were probably mired,
and, being unable to extricate themselves, died in an erect position.
This fact affords evidence that extensive swamps surrounded the
borders of the lake.
The snow-water and the spring rains wear deep gulleys through the
lines of cliffs, and wash down fragments of bones into the dried-up
water-courses. These pieces can generally be traced up to the spot
from which they came, and the rest of the skeleton can so be secured.
The fossils found in the eocene of Wyoming are entirely petrified,
presenting a darker appearance than their matrix; but loose bones
washed out, and subjected to the influence of the sun and rain, often
become bleached so as to resemble in color modern bones.
The state of preservation of the fossils differs according to the
matrix in which they are found. Generally speaking, the remains found
in the lower lines of buttes have been considerably distorted by
pressure; while those from the highest line have suffered very little
from this cause.