The Mississippi Sound is a sound along the Gulf Coast of the United States. It runs east-west along the southern coasts of Mississippi and Alabama, from Waveland, Mississippi, to the Dauphin Island Bridge, a distance of about 145 kilometers (90 mi). The sound is bordered on its southern edge by the barrier islands - Cat, Ship, Horn, Petit Bois and Dauphin Islands - which are part of the National Park Service's Gulf Islands National Seashore. Those islands separate the sound from the Gulf of Mexico. The sediment of the islands was created partly by the ancient Mississippi River when the St. Bernard Lobe of the Mississippi Delta was active over two thousand years ago. The expansion of the St.