Duval County is situated in southern Texas, 100 to 150 miles south of San Antonio and about midway between Corpus Christi, on the Gulf of Mexico, and Laredo, on the Rio Grande. The county lies on the Coastal Plain, which for the most part is low and relatively featureless. Between the Nueces River and the Rio Grande in this part of Texas the plain is interrupted by an erosion remnant, the Reynosa Plateau, which reaches a maximum altitude of nearly 1,000 feet above sea level and stands well above the areas to the east and west. The Reynosa Plateau includes most of Duval County and parts of Webb, Zapata, Starr, Jim Hogg, Jim Wells, McMullen, and Live Oak Counties. In Duval County the plateau is bounded on the west by the westward-facing Bordas escarpment, 75 to 150 feet high, which crosses the county with a southwesterly trend from about the middle of the north boundary to about the middle of the west boundary. On the east the plateau is bounded by a low seaward-facing escarpment, which passes through San Diego, trending a little west of south.
