The white, yellow, and orange lichen on the rock pillars at this location consist of a fungus and an algae in a sumbiotic relationship. Because they absorb nutrients from rainwater, lichen are particularly vulnerable to air pollution. Sulfates, in particular - the predominant form of air pollution in the Big Bend region - kill the algae, so dead lichen are an early warning sign of harmful air pollution. Studying lichen can tell us about the levels and types of air pollution. Look for more colorful lichen on the rocks and trees as you continue your hike.