It is believed that much of the satin spar and selenite gypsum was originally _anhydrite_ (CaSO₄). Unlike gypsum, anhydrite does not contain water, but it can be changed to gypsum in the presence of moisture. There are two lines of evidence that indicate an anhydrite origin for the Quartermaster gypsum. First, microscopic examination of gypsum samples reveals the presence of residual anhydrite crystals embedded in the gypsum. Second, many of the gypsum beds have been squeezed into rather gentle _folds_. These consist of small _anticlines_, upfolds or arches, and _synclines_, downfolds or troughs (fig. 9). It has been suggested that this folding took place as the anhydrite underwent _hydration_, or took on water. As hydration occurred and the anhydrite was converted to gypsum, the gypsum expanded, thereby exerting both lateral and vertical pressure on the beds around it. This produced the crumpled, wave-like folding so characteristic of certain of the gypsum beds. However, there is not complete agreement that the folding in the gypsum is due to the hydration of anhydrite. Certain geologists attribute this deformation to slumping caused by solution cavities, for gypsum is relatively easily dissolved in water. As the gypsum was dissolved and carried away in solution, the removal of the supporting layers of gypsum permitted slumping and consequent deformation in the overlying shales and mudstones. Although some geologists believe that the folds were caused by expansion due to the hydration of anhydrite and others support deformation related to the removal of soluble gypsum, there is general agreement that the folding is local and not related to regional or widespread deformation. [Illustration: Fig. 9. Sagging beds of Quartermaster Formation have produced this gentle syncline, or downfolding, in the rocks. The “dome” on Capitol Peak can be seen in the background.] Not all of the red Quartermaster shales are uniformly colored. Some of them contain gray-green, circular spots called _reduction halos_ (fig. 10). These spots, which in places give the red shales a distinctive polka-dot appearance, have been produced as the result of chemical change of certain minerals within the shale.