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.