Sentinel Peak is a basaltic dike that rises to the west of the Santa Cruz River. The underground ridge of rock, running to the east, once forced groundwater to the surface. The floodplain was used for agricultural fields from about 4,000 years ago until the 1930s. Residents of the nearby village of Schook-schon (Tucson) were visited by Father Eusebio Francisco Kino in the 1690s. These residents likely used bedrock mortars found on the sides of the peak to grind mesquite beans and corn into flour. When the nearby Presidio of Tucson was constructed in 1775, a sentinel stood on the top of the peak searching the horizon for raiding Apache warriors.
A Mountain features a large, painted, man-made letter "A" basalt rock formation, built by University of Arizona students. The "A" is maintained by student organizations at the school, and has been traditionally painted white. After September 11, 2001, the "A" was painted red, white, and blue, which are the school colors, but primarily was a show of patriotism. Since then, the "A" has not returned to its previous white. On St. Patrick's Day, the "A" is often painted green.
The idea for the "A" began in 1914 after the Arizona team defeated Pomona College in a big football game. A civil engineering student on the team convinced one of his professors to include the project of creating the "A" atop Sentinel Peak as a class assignment.
On March 4, 1916, the "A" was whitewashed onto the mountain, measuring 70 feet wide and 160 feet tall. Basalt rock removed from Sentinel Peak was incorporated into foundations and walls throughout historic neighborhoods in Tucson and in the walls surrounding the west side of the University of Arizona campus.
Arizona State University has a more recently-created "A" Mountain (Tempe Butte) near the school's football stadium. During the week of the ASU-UA football game, rival fans and students try to paint the "A" of the opposing school with their own school colors.