A coastal plain is a flat, low-lying piece of land next to the ocean. Coastal plains are separated from the rest of the interior by nearby landforms, such as mountains.

In western South America, a large coastal plain lies between the Andes Mountains and the Pacific Ocean. In the United States, coastal plains can be found along the Atlantic Ocean and the Gulf of Mexico.

Coastal plains can form in two basic ways. Some start as a continental shelf, a flat piece of land located below sea level. When the ocean level falls, the land is exposed, creating a coastal plain. Sometimes, these coastal plains can extend far inland. Fossils of marine organisms have been found in the landlocked U.S. state of Kansas, for instance. Kansas was part of a vast coastal plain that formed when the Western Interior Seaway was forced to the Gulf of Mexico about 100 million years ago. The Western Interior Seaway was a large sea that split the continent of North America from what is now the Gulf of Mexico to the Arctic Ocean during the Cretaceous period.

A coastal plain can also develop when river currents carry rock, soil and other sedimentary material into the ocean. Layers of this deposited sediment build up over time, creating a flat or gently sloping landscape.