Gulf of Mexico numerous bays and smaller inlets. A number of rivers empty into the gulf, most notably the Mississippi River and Rio Grande in the northern gulf, and the Grijalva and Usumacinta rivers in the southern gulf. The land that forms the gulf's coast, including many long, narrow barrier islands, is almost uniformly low-lying and is characterized by marshes and swamps as well as stretches of sandy beach. The Gulf of Mexico is an excellent example of a passive margin. The continental shelf is quite wide at most points along the coast, most notably at the Florida and Yucatán Peninsulas. The shelf
Gulf of Mexico basin on top, by siliciclastic, eolian, playa and other sediments characteristic of arid terrestrial environments closer to shore, implying that at certain times during deposition, sediments accumulated almost up to sea level. The salt structures, which include anticlines, pillows, plugs and walls, in addition to extensive sheets, were created long after deposition by the rapid overlying sedimentation. Nearer the depocenter, which for the Gulf of Mexico basin was towards the spreading center, terrestrial sediments grade into marine shales and limestones. Post salt deposition, the strata recorded millions of years of marine flooding sediments, carbonates from coral reefs, thick sandstones, and eventual