The Gulf Coastal Plain region of Texas is divided into 5 subregions. The Piney Woods subregion is heavily forested and located in eastern Texas. It contains the Big Thicket National Preserve. The Gulf Coastal Plain subregion near the coast contains many bayous and includes the cities of Houston and Corpus Christi. Houston is the largest city in Texas. The South Texas Plains subregion includes the Rio Grande Valley and its largest city is San Antonio. The Post Oak Belt subregion contains lignite and its largest city is Bryan-College Station. The Blackland Prairie subregion has rich black soil suitable for farming and contains the Dallas-Fort Worth metropolitan area.