On a clear day you can see the Gila National Forest in New Mexico from here!
You are looking at the San Pedro River Valley. The San Pedro River 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 there was no San Pedro River and this basin had no outlet, so a large lake formed. Evaporation of the lake's salty waters resulted in a concentration of minerals such as gypsum, which is now mined to make sheetrock for the building industry.
The Galiuro Mountains, visible on the other side of the San Pedro valley, consist almost entirely of volcanic rocks that were produced by large volcanic eruptions between 20 and 30 million years ago. This was a period of great geologic unrest in southern Arizona, and the rocks that formed the Santa Catalina Mountains were uplifted from great depths at this time. The large copper deposits mined below in the valley are about 67 million years old and are related to intrusions of molten rock (magma).
In the valley below you can see the San Manuel copper smelter and tailings ponds.
Most of the copper produced by mining is used for electric wires in buildings and vehicles. A typical automobile, like the one you might have driven up
the mountain, contains about fifty pounds of copper.
This smelter uses technology designed to minimize impacts on the environment. It recovers 99% of the sulfur produced by smelting, and thereby helps keep the air clean.
The large rock you see behind you is called Barnum Rock, named after Willis E. Barnum, who became a Boy Scout leader in Tucson in 1916. During the early 1920s it was customary for honor scouts at Camp Lawton to be stationed on top of Barnum Rock to help the Forest Service as fire lookouts.
Funding for this sign was provided by the Tucson Section, Society for Mining, Metallurgy, and Exploration; Arizona Geological Society; and the Mining Foundation of the Southwest.