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strongly with the deep red matrix.

The northwest-southeast ridge of sedimentary rocks,
which the fault-depression places vis a vis with the abrupt
border of the granite, is, within a distance of about one
and a half miles from Iron Spring Station, divided by
transverse gulches into four hills (1, 2, 3, 4). On pass-
ing down either of these gulches we see that the high
northeast dip due to the fault quickly gives way to the
normal dip of the region (S. E. 10°-15*^), and this rising
of the formations to the northwest brings the i)asal beds
above the present surface at the northwest end of the
fourth hill.

ESSKX INST. BUI.LEriX, VOL. XXN'II 12



118 BULLETIN OF THE ESSEX INSTITUTE.

Opposite the middle of the second hill, a few feet
southwest of the trail (6), is a small prospect shaft about
fifteen feet deep on a sandstone dike three to four feet
wide. It trends northwest-southeast, approximately, and
hades southwest about 5°. The rock is much crushed,
with evidences of shearing along the walls. On the
trail, opposite the northwest end of the second hill, a
dike at least six or eight feet wide is exposed between
disintegrated granite walls. The sandstone of the dike
is white on the northeast side and red (highly ferruginous)
on the southwest. These two outcrops make the sand-
stone dikes unquestionable for this locality ; and the
appearances suggest their origin in the sheeting of the
formations along the great fault-line.

On the second spur of granite opposite the third hill,
and several hundred feet southwest of the probable line
of the fault, are two large sandstone dikes ( 7 ) . The
southwest dike is fifty to one hundred feet wide and
separated by about twenty feet of granite from the nar-
rower northeast dike. The larger dike gives ofl' a branch
one to two feet wide on the southwest side. Toward the
northwest end of the third hill the granite appears
beneath the Potsdam beds, the downthrow being no longer
sufficient to conceal the base of the Potsdam ; and it is clear
that from this point northwest the fault now lies wholly
in the granite. The Potsdam sandstone is non-glauconitic
here and not unlike that of the dikes, except that it is,
in the main, rather coarser. Some of it is blotched and
spotted with white in the manner so characteristic of the
dike rock.

From this point and the last sedimentary outcrops
northwestward an occasional small fragment of sand-
stone in the disintegrated granite shows that the dikes are
not wholly wanting. About half way to Cascade (8)



SANDSTONE DIKES OF UTE PASS. 119

and about three hundred feet west of the trail, a dike of
sandstone, which may be ten or fifteen feet wide, out-
crops quite plainly for a few rods. Beyond this is the
front of the great moraine which stretches quite across
the valley — an immense accumulation of granite bowl-
ders and debris, and beyond this, as far as Cascade, no
farther traces of the sandstone dikes were observed.
Between Cascade and Green Mountain Falls I have not
searched for dikes, assuming that Cross had covered this
ground. Although it appears probable that thorough
search would reveal traces of the sandstone dikes along
the entire distance from Manitou to Green Mountain
Falls, it is a very significant fact that they are practically
coterminous with the sedimentary rocks, alike of the
Manitou and the Manitou Park basins.



SANDSTONE DIKES SOUTHEAST OF MANITOU, BETWEEN
RUXTON CREEK AND BEAR CREEK.

Having demonstrated that northwest of Manitou the
great fault is bordered by sandstone dikes on the south-
west as far as it is by the sedimentary rocks on the north-
east, and somewhat farther, I naturally anticipated that
the same relation would l^e found to hold southeast of
Manitou ; but I was not prepared for the great develop-
ment of sandstone dikes in that direction which my
observations disclose.

Immediately south of Ruxton Creek the structure is
rather complicated ; and it appears to me that the best
explanation of this complexity is found in a transverse
fault in the valley of Ruxton Creek, as shown on the
map (9) and more in detail in Fig. 1. This transverse
displacement, which may be called the Ruxton fault,
evidently breaks the great Ute fault ; and it affords the
simplest and most natural explanation of the fact that the



120



BULLETIN OF THE ESSEX INSTITUTE.



Potsdam sandstone and Manitou limestone, which have
such a prominent development north of Ruxton Creek,
are wholly wanting south of this line, the throw of the
Ute fault south of the Ruxton fault being great enough
to conceal all the sedimentary formations below the
Fountain series. The Ruxton fault not only breaks
and displaces the Ute fault, but south of the former the
latter has a greatly increased southwest hade (inclination
to the vertical).

Immediately east of the bridge over Ruxton Creek and




Granit






\

Hade \



\






Avenue the railroad passes through a tunnel about one
hundred feet long, which with the open cuts at either end
gives a continuous section of nearly four hundred feet.
The western end of this section shows a red and gray
mottled and banded sandstone, which, although non-
glauconitic or nearly so and closely resembling the sand-
stones of the dikes, must be correlated with the Potsdam.
The prevailing dip of this sandstone is southwest 60°-
80° ; but it is highly disturbed, crushed, and mineralized



SANDSTONE DIKES OF UTE PASS. 121

in consequence of its proximity to the faults. Above the
tunnel is a craggy and highly brecciated mass of quartz-
ite, which extends down through the tunnel. On either
side of this, and extending twenty to fifty feet beyond
the ends of the tunnel are soft, decomposed, ochrey
rocks, apparently ferruginous and manganiferous clays
occurring as residuary impurities of a crushed limestone
(base of the Manitou limestone). East of the tunnel,
sandstone like that of the west end of the section dips
northeast 60° at first and then changes abruptly to dips,
away from the railroad on either side, of 10° to 30°.
East of this tunnel (Fig. 1) is another about two hundred
feet long with open cuts giving again a continuous section
of about four hundred feet, the distance between the two
sections being about four hundred and fifty feet, following
the railroad. The second section shows in the cut west
of the tunnel, or for about one hundred feet, the varie-
gated and structureless dike sandstone. The tunnel and
first fifty feet of the eastern cut are in coarse granite
with numerous small (one to four feet) dikes of sand-
stone. A very regular dike cuts the tunnel at the east
portal, widening downward from one to three feet, with
a westerly hade of 10° to 20°. Most of the dikes are
transverse to the tunnel, but they run in all directions
and are extremely irregular. Midway of the eastern cut
the granite overlies the soft, red and white sandstones
of the Fountain series, the contact hading southwest about
45°, while the sandstone dips northeast 70°, approx-
imately (Fig. 2). Both rocks, and especially the granite,
show much crushing near the contact. This is very
obviously the continuation of the Ute fault, crossing the
railroad obliquely and trending in a southeasterly direc-
tion, with the Fountain series on one side and the coarse
granite with sandstone dikes on the other.



122



BULLETIN OF THE ESSEX INSTITUTE.



Higher up on the spur intersected by the western
tunnel sandstone dikes outcrop obscurely ; and the south
wall of what appears to be the most southerly dike is
exposed in two prospect holes, showing, like the greut
fault, a strong hade to the southwest. All along this
line of contact the granite has a finel}'^ brecciatedor semi-
crushed appearance, with much slickensiding along the
joints. On the spur above the second tunnel this contact,
not well exposed, is found again in the same direct line.
Between this contact and the tunnel eight hundred feet
to the north is granite with occasional inclosed masses
of foliated diorite and numerous dikes of sandstone.
About three of the dikes are large (10 to 50 feet) and




Granite



W



■\ + + -^'^ + + 4- + +



Saiidstone



these are clearly parallel with the Ute fault. In several
of the dikes the sandstone is much coarser than usual and
practically indistinguishable from the ordinary brown
sandstone of the Potsdam.

The next spur, which terminates below near the junc-
tion of Ruxton and Manitou Avenues (10), is all drift
in the first bench south of the railroad, but at the head of
this bench, about eight hundred feet from the railroad, the
Fountain beds can be seen dipping gently to the east ; and
on the steep front of the next bench a large sandstone
dike outcrops obscurely. The same conditions were noted



SANDSTONE DIKES OF UTE PASS. 123

on the next spur (11), which rises directly above the
Barker House. A small prospect hole at the head of the
first bench shows the coarse, soft Fountain beds in slti'i.
The sandstone dike, one hundred feet or more in width and
coarser than the normal, outcrops more prominently here,
giving the second bench an almost precipitous front. These
breaks in the profiles of the spurs are directljMn the course
of the Ute fault as followed from the second tunnel ; the
topography and geologic structure are evidently in agree-
ment ; and the sandstone dikes closely accompany the
fault.

The spur running southwest from the Midland Depot
(12) is drift underlaid by Fountain beds to the head of
the first main bench, about one thousand feet (estimated)
from the railroad. Here, on the steep slope or front
of the seccmd bench the great sandstone dike has a width
of fully one hundred and fifty feet. Both the north and
«outh granite walls are obscurely exposed, fifty to one
hundred feet of granite separating the dike from the
great fault. Some of the dike rock is quite coarse and
indistinguishable from the normal Potsdam ; and the dike
is divided by some very prominent slickensided shear
planes. As usual, the prospect openings afl^ord valuat)le
exposures of the geologic structure.

The first important gulch east of Ruxton Creek is that
running south-southwest from the Midland Depot. On the
western slopes of this gulch,* above the main detrital
cones, the dike saudstone outcrops very strongly on
several spurs, extending about one-fourth mile up the
gorge from its mouth, or approximately one half mile
from the railroad. Good exposures are aftbrded by
road-cuttinsfs ; and the indications are tavorable to the
view that the great dike turns slightly on reaching the
ffulch and extends with a breadth of one hundred to two



124 BULLETIN OF THE ESSEX INSTITUTE.

hundred feet obliquely along the side of the gulch. On
the uphill side, at least, the dike is very much branched,
surrounding and enclosing many large masses of granite ;
and there appear also to be large dikes of sandstone
extending southwest into the granite. The sandstone
is mostly fine, but some of it is decidedly coarse ; and
in general it is much mottled. Traces of a true bedding
are indicated at various points in alternating layers of
coarse and fine material. The numerous shear planes
show a strong southwest hade. On the east side of
the gulch the sandstone outcrops strongly about one
hundred yards below its southern limit on the west side ;
and extends thence eastward along the steep slope (13)
above the more gently sloping bench of the Fountain
series. The dike sandstone seems to extend to the
bottom of the slope, and its lower edge is concealeil by
drift ; but on the uphill side the boundary can be traced
at intervals, although the fine grained dark brown sranite
occurring here is easily mistaken for the sandstone. If
all the sandstone on this slope is to be referred to one
dike, it must be from two hundred to three hundred feet
wide. A quarry-like excavation afibrds a good exposure
of the sandstone, whicn is of entirely normal character.
The distribution of sandstone in the slide material indi-
cates branching along the upper edge of the dike ; and
one dike four feet thick is clearly exposed, hading south-
west about 45°. Near this is a dikelet two to four inches
wide of a very dark brown sandstone which is not easily
distinguished from the inclosing granite ; and this is
but one of many instances where the brown color of the
sandstone is most marked next the granite ; suggesting
the biotite of the granite as a source of a part at least of
the cementing and coloring iron oxide of the sandstone.
In its eastern extension this great dike appears to split



SANDSTONE DIKES OF UTE PASS. 125

up. Thus, on the high, steep and smooth hill (14)
directly south of the Denver and Rio Grande Depot
I found that a broad sandstone dike crosses the lower
northern slope of the hill ; all the upper and main part of
the hill, including the crest, is granite; the col leading
from this hill to the higher summit on the southwest
shows two sandstone dikes, probably fifteen to twenty
feet wide and separated by twenty to thirty feet of
granite, the more southern dike, especially, showing a
strong southerly hade; and, finally, several hundred feet
farther southwest up the ridge is a small dike (10 feet?).
Below the sandstone dikes and the fault-line the Foun-
tain beds form as usual a gently sloping plain or terrace
and exhibit near the mountains low east to southeast dips
(5° to 10°). But farther down the slope toward the
railroad, in the numerous excellent outcrops and monu-
ments, the beds dip due south 20° to 25°, seeming thus to
show in the diminished dips toward the fault-line the
lifting eflfect or upward drag of this great displacement.
In the long deep cut on the railroad, however, the dip is
east-southeast as usual and not exceeding 10° or possibly
15°. The Ute fault is clearly, in all this part of its
course, a thrust fault ; and hence this pronounced roll
of the Fountain beds, and the general disturbance of
the stratification, may be regarded as normal features.
Returning toward the mountains by the spur or mesa that
slopes up in a southwest direction from the limekiln
(15) east of Manitou, and leaving the gulch containing
the little cemetery on the west, the following dips were
observed. Near the railroad the Fountain beds dip E.
by S. about 10°. Farther south this changes to S. or S.
by E. about 20°. Quite clearly a low anticline runs
about southeast and pitches in that direction one-third to
one-half mile from the fault and the base of the moun-

ESSEX INST. BULLETIN, VOL. XXVII 12*



126 BULLETIN OF THE ESSEX INSTITUTE.

tains. Nearer, the beds change gradually to a north dip
3° to possibly 5° ; and this very gentle dip continues to
within about two hundred feet of the fault, when the beds
are suddenly flexed up to a nearly vertical northeast dip.
At the fault, which is quite clearly exposed in the
bottom of the gulch on the east side of the mesa, the
vertical or overturned Fountain beds are in direct con-
tact with a large dike of sandstone. The north edge of
the dike shows a trend N. W. -S. E. and a southwest
hade of at least 30° and possibly 45°. The dike is mainly
fine white sandstone, but abundantly mottled with red.
It is exposed almost continuously in the bottom of the
gulch for a breadth of about seventy-five feet. On the
west side of the gulch the upper or south contact may be
easily traced, showing the same southwest hade as the
lower contact. The dike is here, however, much branched,
inclosing large masses of granite and penetrating the
granite in numerous sharply defined dikelets one-fourth
inch to one foot thick. In some parts of the sandstone
numerous small angular fragments of granite are inclosed.
About one hundred yards above this sandstone dike is
another with the same southwest hade, and a surface
breadth of twenty to thirty feet. These two sandstone
dikes cross the ridge or spur on the east side of this little
gulch to the next gulch beyond, in which the fault is not
clearly exposed ; but the Fountain beds are seen within
forty feet of the north dike dipping north about 10°.
The north or fault dike is here nearly if not quite one
hundred and fifty feet wide, and seems to be nearly ver-
tical. The sandstone is light gray blotched with red, as
usual, with many highly polished and striated slicken-
sides or shear planes. There are also some indica-
tions of faulting along the south side of this dike. Two
to three hundred feet south of this dike is the other,



SANDSTONE DIKES OF UTE PASS. 127

which is probably not more than ten to fifteen feet thick
and seems to retain its strong southwest hade.

The south dike does not seem to cross the next spur,
but across its steep north end is some float that may be
referred to the great north dike ; and on passing around
to the northeast corner of this spur, where it slopes down
to the next little gulch, there is a great development
of sandstone. It meets the sfranite alons^ a northeast-south-
west line and quite certainly extends under the granite
at a low angle. In part the sandstone is quite distinctly
stratified, in beds one to two feet thick, and these are
sometimes minutely laminated. On the south side of
the outcrop and near the granite, the dip is toward the
granite, indicating that the sandstone underlies the gran-
ite ; but toward the north side of the dike the beds dip
slightly in that direction. In all cases the dip is low —
5° to 15°. Apparently we have here the original stratifica-
tion of the sandstone. The granite has been thrust
-obliquely over it, crushing nid disturbing the (then) halt
consolidated sandstone, but not wholly obliterating its
bedding. The sandstone is traversed in all directions,
but especially parallel to the bedding planes, by highly
perfect slickensides.

In the bottom of the gulch, below the dike sandstone,
the Fountain beds are seen tilted 90° or more. Farther
down, both in this gulch and the next one, the Fountain
beds quickly subside to a northerly dip of 10° to 15°.
Tlie great fault evidently hades southwest at a very low
angle, so far as the southwest wall of the dike is con-
cerned, but in the bottom of the gulch the dike sand-
stone, where it comes nearest to the Fountain beds
(the nearest outcrops being thirty to forty feet distant),
is inclined at the same angle (S. W. 85°). The stratifica-
tion is very distinct, and the sandstone beds are evidently



128



BULLETIN OF THE ESSEX INSTITUTE.



beat sharply down along the fault plane, which presum-
ably hades southwest 5° (from vertical) at this point.

Crossing the next spur brings us to the valley of
Sutherland Creek (16), a living stream which] supplies
the reservoir near the limekiln. On the west of the
valley where it issues from the mountains, the Fountain
beds, as before, are tilted to a S. W. 85° dip. This out-



a?, a




\M .



crop must be near the fault, for within a few yards south
of it the dike sandstone is seen also dipping S. W. 85°,
while higher up the dike sandstone dips S. W. only 5° to
10°. These outcrops are thus in perfect harmony with
those of the preceding gulch, and the accompanying
section (Fig. 3) may be regarded as expressing the



SANDSTONE DIKES OF UTE PASS. 129

general relations of the formations for both localities. It
will be observed that the flexing of the beds is entirely
normal — upward on the downthrow side of the fault and
downward on the upthrow side. From the top of the
curve the dike sandstone dips gently (5° to 10°) into the
hill. The outcrop, in part nearly horizontal, can be
followed right around the hill to the extreme south side.
The sandstone seems to become gradually thinner and
to die out finally in the bottom of the lateral gulch. The
upper contact is well exposed in a prospect hole and is
seen to be much broken by minor transverse faults, and
both sandstone and granite are much crushed and slicken-
sided along the contact. Below the main body of sand-
stone the granite is a complete network of sandstone
dikes from three inches to several feet wide running in
all directions. Apparently, the granite has overridden
the sandstone bed, and the unconsolidated portions were
forced down into the cracks in the granite below. Some
of the sandstone is quite coarse and gritt}', with many
feldspar grains ; and there are slickensided surfaces all
through it. In this hill and the preceding, the sandstone
is mainly brown. Gray sandstone is most characteristic
of vertical or highl}' inclined strata, probably because
that position is most favorable to the circulation of the
meteoric waters which have bleached the strata. The
stratification is in part as perfect as in any of the sand-
stones of the region — a fine and even lamination. This
dike is exposed again, with essentially the same char-
acters, on the east side of the creek ; but before gaining
the crest of the first main ridge it ends abruptly, appar-
ently cut ofi" by a transverse fault.

Going up the west side of the valley, we find south of
the lateral gulch, first granite and then at least two hun-
dred feet of sandstone of the usual dike character. Its



130 BULLETIN OF THE ESSEX INSTITUTE.

southern contact with the granite is exposed and hades
steeply to the southwest ; and some small branches pene-
trate the granite. The dike trends about E. S. E. and
re-appears in force on the east side of the valley, at the
base of a dark cliff of granite (17). This dike has here
a maximum width, though possibly enclosing some
granite, of about four hundred feet. On the crest of the
spur it is narrowed down to about two hundred feet ; and
six hundred feet of granite separate it from the fault,,
beyond the abrupt eastern termination of the fault dike.
All the indications favor a strong southwest hade ; and on
the east slope of this spur it is seen very clearly that the
fault hades southwest at a very flat angle (45° or more).
The granite passes obliquely up over the edges of the
Fountain beds, which for a breadth of several hundred
feet are overturned about 10°.

On the first branch of the next main spur, we cross,
from the fault southward, nearly eight hundred feet of
granite, and then, high up on the united spur, come to
about one hundred feet of sandstone. A few yards
farther east, on the second branch of the spur, this sand-
stone seems to broaden out to four or five hundred feet.
The next spur (18) is a short one which the fault cuts
low down on its end slope ; and, immediately south of
the fault and the outcrops of the Fountain beds, are four
hundred, and possibly five or six hundred, feet of sand-
stone. The south Avail is cut by a prospecting tunnel ;
and it can be clearly seen, both in the tunnel and in the
ledge above it, that four feet of granite separate the main
body of sandstone from a parallel two-feet dike of sand-
stone. The hade is S. W. about 45°. This tunnel spur
is directly at the head of the little or western Ked Rock
valley. All the way down the west side of the valley
the Fountain beds dip E. S. E. about 30°; while on



SANDSTONE DIKES OF UTE PASS. 131

the opposite (east) side the Red Beds (Triassic) dip
in the same direction 40° to 60°, the dip increasing rapidly
eastward.

On the next main spur east of the tunnel the southern
boundary of the dike sandstone is found in the same direct
line and evidently hades southwest. Northward from this
contact the sandstone is exposed continuously for a breadth
of nearly if not quite five hundred feet, or to the extreme
end of the spur, with no distinct appearance of bedding.
The Ute fault clearly forms the northern boundary of this
great dike, every exposure on this line showing the dike
sandstone in contact with either the Fountain series or the
Red Beds. In following this boundary eastward it is found,
on the west side of the last spur referred to, to be shifted
to the northward about three hundred feet by an obliquely
transverse fault, the course of which is readily traced by
a superb zone of crush breccia. This zone, fifteen to twenty
feet wide and vertical, is a complete breccia of the dike
sandstone in angular fragments of all sizes, mingled with
the quartz pebbles and cobbles of the Fountain conglom-
erate, and with the finer part of the Fountain series as
a paste. Near the re-entrant angle where the fault should
enter the dike, the crush breccia terminates abruptly
against a transverse fissure hading S. E. about 20° and
containing a thin seam of reddish brown clay and sand-
stone. It is probable, however, that the fault continues
obliquely across the spur along some offset parallel line


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