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A calcium oxalate rock coating is ubiquitous on limestone surfaces inside dry rock shelters and under rock overhangs within the canyons of the southwestern Edwards Plateau in southwestern Texas. The oxalate was likely producted by epilithic lichens that flourished in these niches during dry climate regimes. During wet climate conditions the productivity of the lichen would be severely reduced due to physiological response to moisture regime. Thus, lichen productivity and the production of calcium oxalate may have changed through time in response to wet-dry climate fluctuations. Twenty-five AMS radiocarbon ages of rock crust samples collected from 14 sites demonstrate that oxalate was produced episodically during the middle and late Holocene. The occurrence of oxalate is correlated with periods of dry climate, whereas gaps in the record of oxalate deposition coincide with more mesic climate intervals. The results of this study demonstrate the potential for obtaining paleoclimate data from biogenic residues on rock surfaces.