A coastal plain is an area of flat, low-lying land adjacent to a seacoast. One of the world's largest coastal plains is located in eastern South America. The Gulf Coastal Plain of North America extends northwards from the Gulf of Mexico along the Lower Mississippi River to the Ohio River, which is a distance of about 500 miles (about 800 km). During the Cretaceous period, the central area of the United States was covered by a shallow sea, which disappeared as the land rose. Large fossilized aquatic birds called Hesperornis and Ichthyornis, found in western Kansas, indicate that the shallow sea was rife with fish.