The outline of urban San Antonio, barely discernible in this photograph (upper right quadrant), shows Texas Highway 1604 (the thin line that encircles the northern part of the city) and parts of Interstate Highway 410 (the inner loop, which traverses rolling terrain, especially along the southern edge of the city). Within the center of these highway systems lies the central business district. North of the city (top of photograph) is the dark Balcones fault system (the uplifted balcony) with white spots marking several quarries that produce limestone for cement and road gravel. Visible are three of the city’s airports—Randolph Air Force Base with its dual runways (northeast); San Antonio International Airport, north of downtown; and Kelly Air Force Base southwest of downtown—and three lakes south-southeast of the city—large Calaveras Lake; Braunig Lake; and Mitchell Lake, the smallest. Apparent west of the city are rural Medina Valley, a region of rich soils and abundant crops, and the main channel of the Medina River as it loops around and turns eastward, and to the south is the low, rolling topography of sandy soils, sand hills, and mudstone beds.