Red pebbles and cobbles, San Pedro River terrace

Red pebbles and cobbles mantle a late Pleistocene river terrace along the San Pedro River of southeastern Arizona. This surface lies topographically higher than the modern channel and hosts mostly catclaw acacia and creosote. Pebbles are well-rounded, reddened due to soil development and oxidation, and derived from a number of different rock sources, diagnostic of a river system with a large drainage area.