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Geological and Geographical Survey of the Territor.

Preliminary report of the field work of the U.S. Geological and geographical survey of the territories for the season of 1877 online

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Pike's Peak. There is a remarkable similarity in the general geological features of
all the mountains on the eastern slope. The more lofty elevations, as Long's and Pike's
Peaks, with other ridges and peaks scarcely less lofty than those just mentioned, are
composed of the same coarse feldspathic granite before alluded to, but the lower ridges
are formed to a great extent of a ferruginous feldspathic granite which easily yields
to atmospheric agencies, and the surface of the country is paved with crystals of feld-
spar in consequence of its decomposition. All along" the base, and often extending up
to the crest of the mountains, we see the outcropping edges of the fossiliferous rocks
inclining at greater or less angles, and on crossing over into the Laramie Plains we
find the corresponding strata leaning from the opposite side. The granitoid nucleus
varies from 8 to 20 miles in width. No indications of true eruptive rocks were observed



GEOLOGICAL SURVEY OF THE TERRITORIES. 971

in tliis range. The Medicine Bow and Sweet Water Mountains appear to be of the
same character for the most part, but 011 the east side of the Sweet Water River the
evidence of igneons action is shown on a large scale. The ancient volcanic material
would seem to have been elevated to a great height in but a partially fluid condition
and then to have gradually cooled, affecting to a greater or less extent the fossilifer-
ous strata in contact.

Near the junction of the Popo Agie with Wind River we came in full view of the
Wind River Mountains, which form the dividing crest of the continent, the streams on
the one side flowing into the Atlantic and those on the other into the Pacific. This
range is also composed to a large extent of red and gray feldspathic granite, with the
fossiliferous rocks inclining high upon its sides. After passing the sources of Wind
River the mountains appear to be composed entirely of eruptive rocks. Even the '
Three Tetons, which raise their summits 11,000 feet above the ocean level, are formed
of very compact basaltic rock.* The Wasatch and Green River Ranges, where we ob-
served them, have the same igneous origin, and the mountains all along the sources
of the different branches of the Columbia exhibit these rocks in their full force. In
Pierre's Hole, Jackson's Hole, and other valleys surrounded by upheaved ridges, these
ancient volcanic rocks seem to have been poured out over the country and to have
cooled in layers, giving to vast thicknesses of the rocks the appearance of stratified beds.

The mountains about the sources of the Missouri and Yellowstone Rivers are of erup-
tive origin, and in the valley of the Madison Fork of the Missouri are vertical walls of
these ancient volcanic rocks 1,000 to 1,500 feet in height, exhibiting the appearance
of regularly stratified deposits, dipping at a considerable angle. As we pass down the
Madison we find some beds of feldspathic rocks and mica and clay slates beneath the
eruptive layers, dipping at the same angle. After passing the divide below the three
forks of the Missouri we see a number of partially detached ranges which appear to be
of the same igneous character. In the Belt and Highwood Mountains, and indeed all
along the eastern slope in this region, we find continual evidence of the outpouring of
the fluid material in the form of surface beds, or in layers thrust between the fossilif-
erous strata. These igneous beds thin out rapidly as we recede from the point of effu-
sion. A large number of these centers of protrusion may be seen along the slope of
the mountains west of the Judith Range. The erupted material sometimes presents
a vertical wall 300 feet high, then suddenly thins out and disappears. The Judith,
Bear's Paw, and Little Rocky Mountains seemed to be composed for the most part of
granite and other rocks, with igneous protrusions here and there. I have in a former
paper expressed the opinion that the central portions of our mountain ranges are com-
posed of feldspathic granite, and to a certain extent this is true in regard to the more
eastern outliers, but more recent observations have convinced me that these rocks
which I have defined by the term " eruptive" compose by far the greater portion 01
the mountain masses of the West.

-ROFT

II. POTSDAM SANDSTONE (SILURIAN). ,/ w ;

The discovery of this formation in its western extension has already been announced
in a former paper.t It was first made known as occurring in the Black Hills and rest-
ing upon the upturned or nearly vertical edges of the schists, clay slates, and granit-
oid rocks, and the inference was drawn that the same rocks would be found forming an
outcropping belt along the eastern slope of the Rocky Mountains. After leaving the
Black Hills we next observed it along the margins of the Big Horn Range near the
summit, holding the same relative position and exhibiting the same lithological char-
acters. A few thin layers of fine calcareous sandstone were observed filled with fossils
characteristic of this period. At the head of La Boiite Creek, in the Laramie Range, I
noticed a bed resting discordantly upon Azoic slates, 50 to 100 feet in thickness, hold-
ing the same position and possessing the same lithological characters which it reveals
at other localities. I could discover no fossils in it at this point, but I am confident
that this bed represents the Potsdam sandstone. The same bed seems to occur a
along the mountains from Laramie Peak to Cache la Poudre Creek, underlying the well-
known Carboniferous strata, and resting upon the decomposing granitoid rocks wlm-h
form the nucleus of the first ridge. This rock (the Potsdam) is more or less changed
by heat from beneath, but I was able to trace it continuously from the source of the
Chuo-water Creek to the source of Cache la Poudre. a distance of over II
was also seen along the eastern slope of the Wind River Mountains, but did not con-
tain any organic remains.

The above facts show very clearly that in its western extension, the primordial zoi
of Barrande is represented only by a thin bed of sandstone never exceeding 150 feet
thickness, and that it is seen only in a very narrow outcropping belt near the margins
of the mountain crests. The stratified Azoic rocks upon which it rests discort lai 1 1 1 \ . s, i
far as my observations have extended, never reach a very great thickness

*This is an error. The Tetons are composed mostly of gneissic granite. 1878.

tAm. Jour. Sci. and Arts, vol. xxvi, 276.



972 REPORT OF THE SECRETARY OF THE INTERIOR.

III. CARBONIFEROUS ROCKS (INCLUDING PERMIAN ?).

f On both sides of the divide of the Rocky Mountains, so far as our explorations have
extended, a series of calcareous, areno-calcareous, and arenaceous beds are seen, which
we have referred to the Carboniferous epoch. They vary in thickness at different
points. Without specifying localities it will be sufficient to remark that all along the
margins of any of the mountain elevations in the far West, these rocks are seen in a
more or less inclined position.

Sometimes they are not visible for a short distance (as between the Laramie and
Platte Rivers, 20 or 30 miles), but it is plain that they have either been removed by
erosion or concealed by more recent deposits. Along the Big Horn Mountains there
are alternate layers of sandstone, arenaceous, andmaguesian limestones, many of which
show oblique laminae and other indications that their deposition took place in shallow
and perhaps turbulent waters. They are here developed to a thickness of 1,000 to
1,500 feet, and incline high upon the sides of the mountains at an angle of 50 to 70.
They contain few fossils, but these indicate rocks of the same age as those in the Black
Hills. Along the Laramie Mountains, from the Red Buttes to Pike's Peak, apparently
the same limestones are see"n inclining against the sides of the elevated ridges at greater
or less angles, and on the opposite side of the axis, sloping down to the Laramie Plains,
the corresponding strata are seen, though leaning at much smaller angles, usually
from 9 to 15. Along the Sweet Water and Wind River Mountains these rocks are
highly developed and incline against the sides of the ridges of elevation, as heretofore
described. The corresponding portions are also seen on the west slope of the main
range at the sources of Green and Snake Rivers, but not as conspicuously developed,
the eruptive rocks predominating. Crossing back over the dividing crest near the
sources of the Madison, Jefferson, and Gallatin Forks of the Missouri, we find similar
limestones largely developed and covering a considerable area on the eastern slope.
Near the junction of the three forks and along Smith's or Kainas River we find them
reaching a thickness of 800 to 1,000 feet, often partially changed by contact with igne-
ous rocks beneath. They were also observed around the Judith Mountains, and also
about the Bear's Paw and Little Rocky Mountains.

Nowhere in the Rocky Mountain Range, so far as my observations have extended, do
the Carboniferous rocks seem to abound in organic remains, and the few usually seen
are generally found in a bad state of preservation and comprise a limited number of
species. The precise period to which these rocks belong which are so persistent in all
disturbed regions is not positively known, the evidence from organic remains pointing
to the age of the Coal Measures and sometimes to that of the Lower Carboniferous pe-
riod; probably both members of the system occur there.

At the foot of the Big Horn Mountains, near the head of Powder River, I observed
at one locality a series of beds which indicated the presence of Permian rocks. These
beds, which are composed of cherty magnesian limestone, are very much like those
already described in Northeastern Kansas, and contain in great abundance some of
the same species of fossils as Myalina perattenuata and others. I have also seen similar
limestones in other localities, out no fossils were detected, and though having a Per-
mian appearance they may belong to the upper portion of the Carboniferous.

The evidence is clear in many localities that prior to the deposition of the Red Marls
succeeding the supposed Permian a very great erosion of the surface of the Carbonif-
erous rocks took place. We find, for example, in many localities only a thin repre-
sentation of the Carboniferous rocks, and again a full development, 1,000 to 1,500 feet
in thickness.

IV. RED ARENACEOUS DEPOSITS.

Overlying the Carboniferous rocks, and equally persistent with them, is a series of
red arenaceous marl beds or gypsum-bearing marls, which are coextensive with the
upheaved sedimentary formations along the Rocky Mountains. The largest develop-
ment of these beds which I have observed occurs on the northeastern side of the Big
Horn Mountains and on the west slope of the Wind River Mountains, near the source
of the Gros Ventres Fork of Snake River. From the Red Buttes, on the North Platte,
to Pike's Peak these beds are often removed by erosion or concealed by superficial de-
posits, but their appearance in numerous places shoAvs very clearly that beneath the
surface they occupy a considerable area throughout the country bordering the mount-
ain ranges, possibly extending entirely over the eastern slope. Passing over into the
Laramie Plains we find that the red marls constitute the surface formation of the plain
country. It has also been shown from Mr. H. Engelinann's explorations that these
beds are revealed along the Wasatch Mountains, even south of Lake Utah, furnishing
undoubted evidence that they belong to the same great deposit. The fact also that
1,000 to 1,500 feet of red arenaceous beds are seen near the sources of Green River,
leads to the inference that they continue southward far down the Green River Valley
to that portion which takes the name of Colorado, and are in fact a continuation of
the extensive red deposits described by various explorers in New Mexico.



GEOLOGICAL SURVEY OF THE TERRITORIES. 973

These red beds are also seen under similar circumstances highly developed alone the
mountains at the sources of the Missouri. There seems to be a change in the Htho-
logical characters below the Gate of the mountains, the peculiar red deposits disap-
pearing for the most part, and a series of irregular' layers of siliceous limestone with a
reddish tinge, and with oblique laminse, ripple-mark and other indications of shallow-
water deposition. It is through these layers of rock that the Missouri River cuts its
way from the foot of the mountains to the mouth of High Wood Creek, about 10 miles
below the falls. They are also distinctly revealed around the Judith Mountains
Along the Big Horn Mountains thick layers of gypsum occur, but the gypsum beds
are by no means coextensive with the red deposits, and indeed are present in but few
localities. Near the head of Powder River the aggregate thickness of the ypsum
strata is about 100 feet, while near the source of Snake River there is a thickness from
50 to 80 feet. It also occurs to a considerable extent at the foot of the mountains on
La Bonte Creek, a branch of the North Platte.

V. JURASSIC ROCKS.

These rocks are everywhere revealed, overlying the red deposits just mentioned and
possessing an equal geographical extension. Their fullest development and most fos-
siliferous condition seems to be along the margins of the Black Hills, where they have
furnished the most satisfactory evidence of their age. Along the northeastern slope
of the Big Horn Mountains this group of rocks presents its usual appearance of gray
and whitish calcareous and arenaceous layers, with indurated, somewhat variegated
beds of more or less laminated marls, containing in great abundance Belemnites densus,
Pentavrlnus asteriscus, a new species of Ostrea, Pecten, &c.

At Red Buttes we find a fair development of these beds with the same fossils, but as
we proceed southward toward Long's Peak, the intercalated laminated marls disap-
pear and the whole formation seems to be reduced to a thickness of 50 to 100 feet, with
very few fossils. Along the southwest side of the Big Horn Mountains and the north-
east side of the Wind River Mountains we have a thickness of Jurassic rocks from 800
to 1,000 feet containing organic remains in the greatest abundance. Crossing the Wind
River Mountains we observed the strata corresponding to those upon the eastern side,
with B. densus, Ostrea, &c. Returning to the eastern slope at the sources of the Mis-
souri, we see occasional indications of their existence, but not so conspicuous as to be
readily identified. The age of this group of rocks may be now considered as thoroughly
established, so great a number of fossils which appear to be of undoubted Jurassic
forms having been obtained.

I have remarked that the older fossiliferous beds doubtless pass beneath the more re-
cent Cretaceous and Tertiary deposits, and occupy a greater or less area underneath
the prairie country east of the "divide" of the Rocky Mountains. I have made this
inference from the fact that where any elevations occur the complete series of fossilif-
erous beds are exposed around the axis of upheaval. That I may not be misunderstood
by those geologists who have colored large areas Triassic and Jurassic on geological
maps of the West, I would say that I liave never seen any of the older fossiliferous
rocks, from the Potsdam to the Jurassic, inclusive, exposed except in narrow outcrop-
ping belts around the margins of the mountain elevations. The Carboniferous rocks
occupy a belt from one to two miles wide, and the red arenaceous deposits are exposed
over about the same area, while the Jurassic form a zone never more than one-fourth
of a mile to three miles in width.

VI. CRETACEOUS ROCKS WITH SUBDIVISIONS.*

The various subdivisions of the Cretaceous Group in the West were observed at nu-
merous localities. The strata in many places occupy large geographical areas, holding
a horizontal position, in others forming a belt or zone of greater or less width around
the mountain elevations. No. 1 is a well-marked and distinct division along the Mis-
souri River from De Soto to a point above the mouth of the Big Sioux River in the
eastern portions of Kansas and Nebraska and in the South and Southwest. But when
we come into the vicinity of the mountain ranges in the Northwest its typical form is
wanting, and apparently an increased development of No. 2 only is seen. Along the
Big Horn Mountains, No. 2 is 800 to 1,000 feet in thickness, composed of black, plastic
clay with several layers of gray and yellowish calcareous sandstones 10 to 50 feet in
thickness. Along the Laramie and Wind River Mountains the same characters are
shown. After leaving the Missouri, near the mouth of the Niobrara River, No. 3 is
never seen presenting its typical marly character. In the vicinity of the Black Hills
we saw a series of beds composed of alternate thin layers of arenaceous and argilla-

* The Cretaceous rocks of the West have been divided into five formations, num-
bered 1, 2, 3, &c. A more careful study of No. 1 ? may render it necessary to make
other divisions.



974 REPORT OF THE SECRETARY OF THE INTERIOR.

ceous sediments with Ostrea congesta and Inoceramus problematicns which may possibly
represent No. 3. Along the Big Horn Mountains and from Red Biittes to Cache la
Poudre Creek the same fossils were pften found and some other indications of its ex-
istence, but no well-marked typical beds were seen. It is now well known that O.
congesta and I. problematicus range down into No. 2, so that No. 3 in the West and
Southwest may give place to an increased development of No. 2. Nos. 4 and 5 are
largely developed everywhere, when not concealed by the overlying Tertiary deposits,
especially along the Laramie Mountains and in the valley of Cache la Poudre. In the
valley of Wind River all the Cretaceous rocks down to No. 2 appear to have been re-
moved by erosion prior to the deposition of the Tertiary beds, and the characteristic
fossils of No. 2 are quite abundant. As we pass over the mountains, we have, inclining
from the western slope, 600 to 800 feet of alternations of black plastic clays, arenace-
ous marls, and beds of sandstones and limestones, with a few seams of carbonaceous
matter passing up into calcareous and arenaceous compact rocks. In some arenace-
ous limestones near the middle of the series, and extending upward, quite abundant
fossils were observed, among them a large Inoceramus, two species of Oslrea, a large
Pinna, four inches in length, a Cardium, and a number of undetermined species,
with fragments of silicified wood. The general dip of these rocks is about 20.
These well-marked Cretaceous beds pass up quite imperceptibly into an enormous
thickness of Lignite Tertiary. Passing over the dividing crest to the headwaters
of the Missouri, we did not observe any indications of Cretaceous rocks until we had
descended below the three forks, where we find traces left after erosion. They do not
reveal themselves conspicuously until we arrive within twenty or thirty miles of Fort
Benton, where the black, plastic clay begins to overlap the Jurassic rocks, with its
characteristic fossils, and on reaching Fort Benton the plastic clay is quite homogeneous,
and is developed to a thickness of 800 feet. As we proceed toward the mouth of
the Judith River and near the Judith Mountains, we find quite thick beds of concre-
tionary sandstone which form the " Stone Walls," "Citadel," &c. It is from these
beds that we have obtained a group of fossils which 'we have referred provisionally to
No. 1, but which seem to be specifically distinct from all others in the West. It may
be that when this group of beds now referred to Nos. 1 and 2, comprising a thickness
of 1,500 to 2,000 feet in this region, are more carefully studied, that several subdivisions
will be made, having equal importance with the others. During the past season our
route led us along the "divide" between the Missouri and Yellowstone rivers, south of
the Judith Mountains, so that we passed outside of any good exposures of No. 1, as well
as beyond the limits of the estuary beds at the mouth of the Judith. We must await a
more thorough and detailed exploration of this region before we can state with entire
confidence the succession of the beds.

VII. TERTIARY DEPOSITS.

In speaking of the Tertiary deposits of the Northwest, so far as known at; the present
time, I propose to separate them into four divisions, which will be sufficient for our
immediate purposes : 1st. Estuary deposits. 2d. True Lignite beds. 3d. Wind River
Valley deposits. 4th. White River Tertiary deposits.

The estuary deposits, of which the Judith Basin may be regarded as the type, are
quite remarkable and of a most interesting character. ' Opinions of a somewhat con-
flicting nature have been entertained in regard to them, owing to the peculiar character
of the organic remains ; but recent observations have convinced me that they are all
of Tertiary age, and that they are quite widely distributed throughout the for West.
The lithological characters of the Judith deposit have already been sufficiently de-
scribed, and it has yielded many important fossils. A thin series of beds is also found
near the sources of the Moreau, Grand, and Cannon Ball Rivers; and at the mouth of
the Big Horn River we have a group of beds 800 to 1,000 feet in thickness, with fossils
of the same character as those occurring at the mouth of the Judith. The researches
of Mr. H. Engelrnann, in Utah, have also established the existence of an estuary de-
posit in the country bordering upon Green River, scarcely less interesting than that of
the Judith. These deposits pass up into the true lignite beds without any perceptible
line of separation, gradually losing their estuary character and ever after containing
only land and fresh-water shells. The lignite strata are chiefly remarkable for yielding
in the greatest abundance finely preserved vegetable remains. A few fragments of
leaves of Dicotyledonous trees and silicified wood, with very impure lignite beds, are
formed in some of the estuary deposits, but no groups to indicate the great luxuriance
of vegetation which must have existed during the accumulation of the lignite strata.

The geographical extension of the lignite deposits of the West is now a matter of the
highest interest, and, from what is already known, I am convinced that they will yet
be found to cover a greater or less area on both sides of the main "divide" of the Rocky
Mountains, from the Arctic Sea to the Isthmus of Darien. The estuary and lignite
beds seem also to have partaken equally with the older fossiliferous rocks of the influ-
ence which elevated the mountain chains. Along the Laramie Mountains and from



GEOLOGICAL SURVEY OF THE TERRITORIES. 975

the Red Buttes to the "divide" between Platte and Wind Rivers, along the Big Horn
Mountains, the strata incline at very high angles, 40 to 80, and in some instances are
very nearly vertical. The true lignite strata seem to conform to the older fossiliferous
rocks and to have been disturbed by the same influences that elevated the mountain
ranges in the vicinity. These Tertiary beds extended over all the plain country to the
north, and east of the Laramie Mountains, far to the northward, beyond the limits of
our explorations. Crossing the Wind River Mountains, we find them largely developed
high upon the western slope, dipping at a high angle, from the Wind River Range on
the one side and the Wasatch and Green River Mountains on the other.

Throughout the Wind River Valley is a series of beds, of great thickness, which seem
to be intermediate in their character between the true lignite beds and the White
River Tertiary deposits. We first observed them gently inclined near Willow Springs
on the North Platte, and thence westward toward the Sweet Water Mountains and
near the "divide" between the North Platte and Wind River they reach a thickness of
400 feet. From this "divide" throughout the Wind River Valley they occupy the
greater portion of the country, and, though inclining in the same direction with the
older strata, the beds do not dip more than 1 to 5. They diifer from the other de-
posits in the great predominance of arenaceous sediments and in the absence of vege-
table remains, but they contain fragments of turtles and numerous fresh-water and
land shells of the genera Helix, PlanorMs, Vivipara, &c. The entire thickness of these
deposits may be estimated at from 1,500 to 2,000 feet. From the fact that these de-
posits do not conform to the true lignite beds and that detached portions are seen


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