Rock tripe is an edible lichen found in eastern parts of the US that is commonly found on rock faces and cliffs. It is very sensitive to air pollution, so when you see it growing abundantly, chances are good that the air around you is unpolluted.
Anne Gunther, a naturalist I know, told me that rock tripe was used by the American Indians as a food source, but only after extensive soaking to remove its tannins. It was also purportedly eaten by George Washington’s soldiers in Valley Forge to prevent starvation.
Nutritionally, rock tripe has one third more calories per weight than honey or corn flakes.
In Japan, it is considered a delicacy and is called “iwa-take” for rock mushroom and is often served in salads or deep fried.