The Gulf Coastal Plain extends around the Gulf of Mexico in the Southern United States and eastern Mexico. This coastal plain reaches from the Florida Panhandle, southwest Georgia, the southern two-thirds of Alabama, over most of Mississippi, western Tennessee and Kentucky, into southern Illinois, the Missouri Bootheel, eastern and southern Arkansas, all of Louisiana, the southeast corner of Oklahoma, and easternmost Texas in the United States. It continues along the Gulf in northeastern and eastern Mexico, through Tamaulipas and Veracruz to Tabasco and the Yucatán Peninsula on the Bay of Campeche. Geography The Gulf Coastal Plain's southern boundary is the Gulf of Mexico in the U.S. and the Sierra Madre de Chiapas in Mexico. On the north, it extends to the Ouachita Highlands of the Interior Low Plateaus and the southern Appalachian Mountains. Its northernmost extent is along the Mississippi embayment (Mississippi Alluvial Valley) as far north as the southern tip of Illinois. To the east the Gulf coastal plain meets the South Atlantic Coastal Plain in southern Georgia along the basin divide between the rivers flowing into the Gulf and those flowing to the Atlantic and south along the Apalachicola River through the Florida panhandle. The flat to rolling topography is broken by many streams, river riparian areas, and marsh wetlands. The Gulf Coastal Plain also extends into southern Mexico and up to the northern west coast states of the US. United States section The Gulf Coastal Plain is a westward extension of the Atlantic Coastal Plain around the Gulf of Mexico. It is only the lower, seaward part of this region that deserves the name of plain, for there alone is the surface unbroken by hills or valleys. The inner part, initially a plain, has been maturely dissected into an elaborate complex of hills and valleys, usually of increasing altitude and relief as one passes inland. The Gulf Plain features not found in the Atlantic coastal plain are: the peninsular extension of the plain in Florida the belted arrangement of relief and soils in Alabama and in Texas the Mississippi embayment or inland extension of the plain half-way up the course of the Mississippi River to its junction with the Ohio River at Cairo, Illinois, with the Mississippi flood plain there included. Florida peninsula A broad, low crustal arch extends southward at the junction of the Atlantic and Gulf coastal plains. The emerged half of the arch, constitutes