Sentinel Peak is a 2,897 ft (883 m) peak in the Tucson Mountains southwest of downtown Tucson, Arizona. The valley’s first inhabitants grew crops at the mountain’s base, along the Santa Cruz River. The name “Tucson” is derived from the O’odham Cuk Ṣon [tʃʊk ʂɔːn], meaning “[at the] base of the black [hill]”. In the 1910s, University of Arizona students used local basalt rock to construct a 160-ft-tall (49 m) block “A” on the mountain’s east face, near its summit, giving the peak its other name, “A” Mountain. The peak is part of a 272-acre (110 hectares) park, the largest natural resource park in the City of Tucson. The fertile land at the base of Sentinel Peak was used for agriculture from circa 2000 BCE until the 1930s. Bedrock mortars found on the sides of the peak are believed to have been used to grind corn and mesquite beans into flour. In the 1690s, the O’odham people living in the area were visited by Father Eusebio Francisco Kino, who established the nearby Mission San Xavier del Bac. After Presidio San Augustin del Tucson was constructed in 1775, sentinels were stationed on the peak to watch for raiding Apaches.
Sentinel Peak is made up of several layers of igneous rock representing various types of volcanic activity, though the mountain itself is not a volcano. It is one of a cluster of outcroppings at the eastern edge of the Tucson Mountains that are primarily the remnants of 20–30 million-year-old (Ma) lava flows that once extended west towards the Tucson Mountains and east into the Tucson Basin, where the city is now. Erosion and faulting are responsible for the peak’s conical shape. The layer of rock at the peak of the hill is a basaltic andesite dating to 23–24 Ma. This caps a 30–36 meter-thick layer of tuff (compacted volcanic ash) above another layer of basaltic andesite, both dating to 26–28 Ma. Tuffs and andesite exposed at the base of the hill, on the south side, date to approximately 60 Ma. Volcanic ash and breccia, along with ancient lava beds, or lahars, can also be found on the mountain, further evidence of a once active volcanic field that formed the Tucson Mountain range.
On October 3, 2016, in the Arizona Daily Star newspaper, historian David Leighton explained the origin of the name Sentinel Peak: He wrote that in the very early days of Tucson, an Indian fortification was constructed at the top of a small mountain that would come to be called Sentinel Peak – also sometimes referred to as Picket Post Butte. This Indian fortification in time became known as the sentinel station because a sentinel or guard was posted there to watch for approaching enemies, likely Apache Indians. During the U.S. Civil War, armed guards or soldiers were posted at the sentinel station and a canvas was stretched across the stone fortification, to keep the sun from hitting directly on the men posted there. By 1883, only ruins of the fortification remained. The remains included, “A circular wall, about 3 feet thick and made of boulders, [that] enclosed an area about 8 feet in diameter. North of the circular structure was a small wall, roughly two feet high and about 10 feet long. To the east were traces of another, smaller circular wall.” The origin of the name Sentinel Peak, according to Mr. Leighton, “comes from the sentinel station and the sentinel stationed there.