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 origin for the Gulf of Mexico at the close of the Permian, which could have caused the Permian–Triassic extinction event. However, Gulf Coast geologists do not regard this hypothesis as having any credibility. Instead they overwhelmingly accept plate tectonics, not an asteroid impact, as having created the Gulf of Mexico as illustrated by papers authored by Kevin Mickus and others. This hypothesis is not to be confused with the Chicxulub Crater, a large impact crater on the coast of the Gulf of Mexico on the Yucatán Peninsula. Increasingly, the Gulf of Mexico is regarded as a backarc basin behind the