Headed back home from the Master Naturalist conference in Marble Falls... I decided to stop and explore a few place that I hadn't been before. :) I stopped at Longhorn Caverns State Park -- I didn't go down into the cave, but I did walk around a bit. This place was lichen-central! I think this is the species, based on: http://www.sbs.utexas.edu/quedensley/balcones.htm