Most of Mississippi is part of the East Gulf Coastal Plain, and the rest of the state is made up of a section of the Mississippi Alluvial Plain. The East Gulf Coastal Plain is generally composed of low hills, such as the Pine Hills in the south and the North Central Hills. Somewhat higher elevations are in the Pontotoc Ridge and the Fall Line Hills in the northeast. Yellow-brown loess soil is in the west, and a region of fertile black earth, part of the Black Belt, is in the northeast. The coastline, which includes large bays at Bay Saint Louis, Biloxi, and Pascagoula, is separated from the Gulf of Mexico proper by the shallow Mississippi Sound, which is partially enclosed by Petit Bois, Horn, Ship, and Cat islands. The Mississippi Alluvial Plain, known also as the Mississippi Delta, is narrow in the south and widens north of Vicksburg. The region has rich soil, partly made up of silt deposited by floodwaters of the Mississippi River.