In eastern Utah, east of the ancient Jurassic sea, the Entrada Sandstone
is entirely unfossiliferous, was partly water laid and partly wind
blown, and has been divided into three distinctive parts, which in
ascending order are the Dewey Bridge, Slick Rock, and Moab Members.[25]
In and near the Colorado National Monument, the long period of erosion
discussed in a preceding section probably continued well into the
Jurassic, so only the upper part of the Slick Rock Member and the
overlying Moab Member were deposited on the eroded surface of what
little remained of the Kayenta Formation (fig. 14).
The Slick Rock Member was named from its occurrence at and near the
mining town of Slick Rock, Colo., which originally was named after the
appearance of the rock because it generally forms slick, smooth cliffs.
It reminds one of the chicken and egg conundrum. The Slick Rock is
composed mainly of sand dunes that were piled up on the eastern shore of
the Jurassic sea by winds blowing from the northeast. Occasional rainy
spells created lakes and ponds in which some of the sand was laid down
in level beds. This pile of sand later hardened into the cliff-forming
Slick Rock Member, which looks something like the Wingate but is
generally only half as thick, weathers into less abrupt cliffs, is
mostly salmon red, and is almost free of joints. The joints in the
Wingate (fig. 11) probably resulted from the uplift and tilting of the
Plateau before the long period of pre-Entrada erosion; whereas the land
seems to have been more stable during Entrada time. The Slick Rock is
cemented with calcium carbonate (CaCO₃), which is soluble even in weak
acid, such as rain or snow water containing dissolved carbon dioxide.
For this reason solution openings or pits occur in some of the cliff
faces, the most striking of which are those shown near the top of figure
15.
The Slick Rock Member of the Entrada Sandstone forms a line of cliffs
and isolated monoliths that are second in height and grandeur only to
those of the Wingate. The Member is best displayed southwest of Rim Rock
Drive between the Visitor Center and the Coke Ovens and along the
western arm of Ute Canyon (fig. 16). It also forms the Saddlehorn just
south of the camp and picnic grounds near the Visitor Center (fig. 50).
Most of the smooth cliff faces show both the steeply dipping crossbeds
of the old sand dunes and the flat-lying beds of the lake or pond
deposits.
[Illustration: ENTRADA SANDSTONE, just above normally dry waterfall
in west arm of Ute Canyon. Note smooth unjointed cliff of Slick Rock
Member protected at left by overhanging basal bed of Moab Member,
which forms about lower half of slope in distance. Upper part of
distant slope is the Summerville Formation overlain by Salt Wash
Member of the Morrison Formation. Note Slick Rock at left resting
upon eroded crossbedded sandstone in Kayenta Formation, in which the
canyon was cut. (Fig. 16)]
The overlying Moab Member of the Entrada is a white level-bedded
sandstone that generally weathers into stairsteps or ledges. One of the
best exposures of the Moab Member is shown in figure 17, but good
exposures also are seen along the west side of Rim Rock Drive just
northeast of the Coke Ovens Overlook. In some places the Moab Member
forms cliffs continuous with those of the underlying Slick Rock Member.
It appears to consist of hardened beach or lagoon sand that was
deposited along the eastern shore of the sea, which suggests that the
sea extended farther east during Moab and Summerville times than it did
during Dewey Bridge and Slick Rock times. Like the Slick Rock, the Moab
also is cemented by calcium carbonate, but the lower sandstone ledges of
the Moab Member are more resistant to erosion than the Slick Rock
Member, so the Moab helps preserve the underlying cliffs. The top of the
Moab Member forms patches of bare pavement east of the Monument, known
as the “Slick Rim,” which may be observed from the Little Park Road.
[Illustration: MOAB MEMBER OF ENTRADA SANDSTONE, showing typical
steplike weathering. In west arm of Ute Canyon about a quarter mile
above the view shown in figure 16. Moab Member caps and protects
overhang of Slick Rock Member. Moab is overlain by unexposed slope
of Summerville Formation and lower part of Morrison Formation. (Fig.
17)]
Although the Slick Rock Member normally is salmon colored or pink, the
upper half of an outcrop just north of the highest point on Rim Rock
Drive at the head of main Ute Canyon has a distinctly mottled
appearance, wherein much of the color has been leached to white, but
irregular splotches of color appear in the dominantly white upper part,
and white splotches appear in the colored part, as shown in figure 18.
By way of contrast, in an outcrop of the two members of the Entrada
about 2 miles north of the Glade Park Store and Post Office (fig. 19),
the entire Slick Rock Member is as white as the Moab Member, and the
former is white for some distance to the east. Why is the salmon color
entirely missing from the Slick Rock near Glade Park, partly missing in
figure 18, but present virtually everywhere else in and near the
Monument? The answers to this seeming mystery involve events that
occurred long ago, so only the high points will be touched upon here.
[Illustration: MOTTLED SALMON-AND-WHITE SLICK ROCK MEMBER, overlain
by white level-bedded Moab Member, on west side of Rim Rock Drive
about four-tenths of a mile north of head of main Ute Canyon. (Fig.
18)]
It seems reasonable to suppose that the Slick Rock Member at both
localities originally was salmon colored or pink, as it is everywhere
else, but that later, the coloring agent, red ferric iron oxide (Fe₂O₃),
was chemically reduced, or leached to ferrous iron oxide (FeO), by
acidic ground water, and was carried away to the northeast by the slowly
moving ground water. But as I have already pointed out, the cliff
exposures of the sandstones are now bone dry, so what happened to the
ground water and why was it acidic here and not elsewhere?
[Illustration: WHITE ENTRADA SANDSTONE, in outcrop just east of
gravel road about 2 miles north of Glade Park Store and Post Office.
Reasons for absence of salmon color in Slick Rock Member are given
in text. (Fig. 19)]
Before the cutting of the deep canyons of the Monument, which followed
the last major uplifts of the region accompanied by bending and breaking
of the rocks, the now dry sandstones were saturated with ground water
that moved very slowly northeastward. Somewhere to the southwest the
Entrada Sandstone seemingly took in water containing dissolved hydrogen
sulfide gas (H₂S), changing the ground water to a weak acid. The H₂S
could have been produced by a type of anaerobic bacteria that has the
ability to reduce dissolved sulfates (SO₄⁻²) in water to the dissolved
hydrogen sulfide gas, thereby obtaining needed oxygen.
The next questions you might logically ask are (1) if the above
deductions have any merit, how do we know the acid water was caused by
dissolved hydrogen sulfide,[26] (2) what is the source of the sulfate
ions (SO₄⁻²) from which the H₂S was obtained, and (3) why is the color
of the Slick Rock Member in figure 19 completely reduced to white
whereas that in figure 18 is only partly reduced in the upper part?
Although the ground waters from artesian wells in the Grand Junction
area contain small amounts of sulfate as do most ground waters, the
amount needed for the results observed more likely came from solution of
the common mineral gypsum (calcium sulfate containing some water,
CaSO₄·2H₂O). The overlying Summerville and Morrison Formations contain
some gypsum in many places in Utah, so it is not improbable that these
formations contain gypsum locally in Colorado. If so, sulfate-bearing
water could have percolated downward into the Entrada at some point
southwest of Glade Park. But as this must have happened several million
years ago, the clues as to just where this occurred have grown quite
cold.
Seemingly, the color in the Slick Rock Member near and east of Glade
Park was entirely removed by the process described, but the very slow
moving ground water had time to leach only the upper part of the Slick
Rock (the most permeable part) in Ute Canyon before the process was
halted forever by the draining of water from these beds by canyon
cutting.
Shortly before the Jurassic sea to the west dried up, silt, mud, and
some sand were carried into either a shallow arm of the sea or a broad
bay or lagoon near it, and later the silt, mud, and sand hardened to
become the Summerville Formation. The Summerville is only 40 to 60 feet
thick in the Monument but is much thicker in Utah.
The Summerville Formation is so soft that it weathers very rapidly and
hence is exposed at only a few places. It is best displayed in the high
roadcut at Artists Point and along the road to the south for the next
mile (fig. 20), but it is also exposed in roadcuts along the west arm of
Ute Canyon. Even the thinnest beds of the Summerville can be traced for
hundreds of yards, and individual beds have a nearly constant thickness
for such distances. This greatly facilitated the detailed measurement of
a section of the Summerville[27] by my son Bill and me from Artists
Point to the base of the overlying Morrison Formation about a mile
south. Using a 6-foot folding steel rule we measured and described each
thin bed from some key bed at about ground level to one at eye level,
followed the upper key bed southward to ground level, then repeated the
process until the entire 54 feet had been measured and described.
[Illustration: SUMMERVILLE FORMATION, at Artists Point (fig. 3).
Base of formation rests upon Moab Member of Entrada just beneath the
pavement. Note geologist’s pick resting upon lower ledge of
sandstone just to the left of middle. Top of the Summerville here
has been removed by erosion. (Fig. 20)]
The Summerville at the type locality in the San Rafael Swell, Utah, is
much thicker than in the Monument and contains many chocolate-brown
beds; but the Summerville exhibits the same lateral continuity of even
the thinnest beds. Thin sedimentary beds of such uniform thickness are
thought to have accumulated in relatively quiet bodies of water. If you
look at the undersides of some of the blocks of hard light-gray
sandstone that have broken off, you may see corrugations like those on
some metal barn roofs. These are ripplemarks produced by wave or current
action while the sand was still loose, which indicates that the water
was not always entirely quiet. Although much of the Summerville is red,
you will see beds of many other colors including gray, blue gray,
greenish gray, chocolate brown, and reddish brown.