The San Pedro River Valley is part of a river system in which the San Pedro flows north to the Gila River, which ultimately meets the Colorado River and heads south to Mexico and the Gulf of California. The river supports riparian vegetation that provides wildlife habitat and recreation opportunities. Six million years ago, this basin had no outlet and no San Pedro River, so a large lake formed; evaporation of its salty waters concentrated minerals such as gypsum, which is now mined to make sheetrock for the building industry. The Galiuro Mountains across the valley consist almost entirely of volcanic rocks produced by large eruptions between 20 and 30 million years ago, during a period of great geologic unrest in southern Arizona when the rocks that formed the Santa Catalina Mountains were uplifted from great depths. Large copper deposits mined in the valley are about 67 million years old and are related to intrusions of molten rock. In the valley, the San Manuel copper smelter and tailings ponds reflect the importance of copper mining, much of whose output is used for electric wires in buildings and vehicles; a typical automobile contains about fifty pounds of copper. The smelter uses technology designed to minimize environmental impacts by recovering 99% of the sulfur produced by smelting and thereby helping keep the air clean. Barnum Rock is named after Willis E. Barnum, who became a Boy Scout leader in Tucson in 1916, and during the early 1920s honor scouts at Camp Lawton were stationed on top of it to help the Forest Service as fire lookouts.