Downtown San Antonio is the point of convergence of a major highway system (center of photograph). East and west of the central business district are curving north-south-oriented Salado Creek and Leon Creek, respectively, both of which flow into the Medina River south of the city. The meandering Medina River and its floodplain are prominent features as they loop around the west and south sides of the city. Visible are the runways of Kelly Air Force Base (southwest) and Randolph Air Force Base (northeast) and Calaveras Lake and smaller Braunig Lake to the southeast. An edge of the deeply dissected Edwards Plateau, known locally as the Texas Hill Country, is visible north of the city. Highly reflective limestone quarries appear within the uplifted sedimentary rocks along the Balcones fault system. The mined limestone is used to make cement and road gravel.