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.
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?
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.