The oldest formation exposed in the canyon is the Quartermaster Formation of Permian age (_see_ fig. 6) which is named from exposures along the banks of Quartermaster Creek in Roger Mills County, Oklahoma. One of the more colorful formations in the park, the Quartermaster is composed primarily of brick-red to vermilion shales which are interbedded with lenses of gray shales, clays, mudstones, and sandstones. Averaging about 60 feet thick where exposed in the park, the Quartermaster forms the floor and lower walls of the canyon. The rocks of this formation are easily examined at many places throughout the canyon and in them can be seen a number of interesting geologic phenomena. Probably the most noticeable of these features are the shining white veins of _gypsum_ that lace the face of the red shale outcrops (fig. 8). A soft, transparent to translucent mineral that can be scratched by a fingernail, gypsum is hydrous calcium sulfate (CaSO₄·2H₂O). Three varieties of gypsum are found in the canyon: (1) _satin spar_, a fibrous variety with a silky sheen; (2) _selenite_, a colorless, transparent variety which commonly occurs in sheet-like masses; and (3) a fine-grained massive variety called _alabaster_. Satin spar is the most common variety of gypsum present and it commonly occurs in thin bands interbedded with the mudstones and sandstones. It is much more noticeable in the shales, however, for it is typically seen in narrow veins which criss-cross the surface of the outcrop and intersect the bedding planes at various angles. Although normally white, some of the satin spar has a soft pink or bluish hue due to the presence of impurities in the mineral. [Illustration: Fig. 8. Veins of selenite gypsum (top arrow) in Quartermaster Formation. Notice diagonal joint to left of geologist’s hand (lower arrow).] The presence of gypsum in the Quartermaster red beds is of special significance to the geologist, for it provides valuable information about the geologic history of the Palo Duro area. It is known, for example, that when a landlocked body of sea water in an arid climate becomes separated from the ocean, one of the most common salts to precipitate is hydrous calcium sulfate, or gypsum. Gypsum may also be precipitated when a lake without an outlet evaporates in an arid climate. Geologic evidence suggests that the sediments which gave rise to the rocks of the Quartermaster Formation were deposited in a landlocked arm of the sea during the latter part of the Permian Period. As evaporation continued and the sea water was reduced to approximately one-third of its original volume, gypsum was precipitated. There must have been periodic influxes of silt- and mud-bearing waters entering the ancient Permian sea, for layers of shale and mudstone are interbedded with the gypsum.