San Antonio Missions National Historical Park - Geology

San Antonio Missions National Historical Park is situated near the base of the Edwards Plateau, adjacent to the Balcones Escarpment and directly within the overlap of the Rio Grande Plain (South Texas Brush Country) and the Blackland Prairie. This area consists of level prairie and undulating hills dissected by the San Antonio River and its tributaries. Elevations on the plain range from 450 to 700 feet above sea level, with a general slope to the southeast.

Regional topography is primarily influenced by the Balcones Escarpment; a broad area of faulted limestone that forms the southwestern edge of the Edwards Plateau. The escarpment rises 1,000 feet above the prairie to the south and east, and reaches from Del Rio, Texas (160 miles to the west), through northern Bexar County, to Austin (70 miles to the northeast). The escarpment follows a northeast/southwest alignment through northern Bexar County. The park is approximately 15 to 20 miles south of the Balcones Escarpment). Topography within the park is relatively flat.

A thick sequence of sedimentary rocks and unconsolidated material (4,500 to 5,000 feet thick), ranging in age from the early Cretaceous to Quaternary, underlies the San Antonio area. The Edwards Aquifer consists of three limestone formations, and is the principal water supply source for the San Antonio area.