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A whewellite-rich rock crust covers vast areas of limestone inside dry rock shelters and under rock overhangs throughout the Lower Pecos Region in southwestern Texas (USA). The natural rock crust, composed primarily of whewellite and gypsum with lesser amounts of quartz and silicates, encapsulates the paints of the extraordinary pictographs at more than 250 rock art sites in the region. The authors propose a model that describes the origin of each crust constituent and the evolution of these surfaces. Furthermore, they describe the relationship between the ancient paints and crust matrices, information that is necessary for the development of sound conservation strategies.