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A calcium oxalate (whewellite)-rich crust occurs on exposed limestone surfaces in dry rock and open air shelters in the Lower Pecos region of southwest Texas. The crust, which also contains gypsum and clay, formed over silica-rich limestone during the Holocene. SEM and optical photomicrographs reveal similarities between whewellite microstructures and the lichen Aspicilia calcarea. This desert lichen is known to produce calcium oxalate, and has been found in several sites in the region. The ubiquity of the whewellite-rich crust in the Lower Pecos shelters suggests that the lichen flourished in the past. Since A. calcarea is a desert species, the virulence of the organism likely peaked during xeric climate episodes then waned during mesic periods. Thus, radiocarbon ages of whewellite would correspond to dry climate periods experienced in the region, while periods with few or no C data would indicate wet climate episodes. A preliminary paleoclimate reconstruction based on fourteen AMS C dates indicates the Lower Pecos experienced dry to wet climate fluctuations during the late Holocene. This reconstruction generally agrees with other models established for Texas.