Sentinel Peak, more commonly known as "A" Mountain, is a prominent ridge in the Tucson Mountains west of Tucson, Arizona.Sentinel Peak rises 2,897 feet into the air to the west of the Santa Cruz River. The underground ridge of rock, running to the east, once forced groundwater to the surface. The resulting floodplain was used for agricultural fields from about 4,000 years ago until the 1930s.
Sentinel Peak is made up of a series of volcanic rocks that represent different types of volcanic activity, though the mountain itself is not a volcano. These layers once extended out west towards the Tucson Mountains
and east into the Tucson Basin, where the city is now. Erosion and faulting are responsible for the mountain’s place in Tucson and its conical shape.