plain, large area of level or nearly level land. Elevated plains are called plateaus, or tablelands, and very low, wet plains are called swamps. Plains have different names in different climates and countries. They include the tundras, steppes, prairies, pampas, savannas, llanos, floodplains of rivers, coastal plains, loess plains, arid plains (see desert), and lacustrine plains. The erosive action of water, glaciation, the draining of a lake, deposition of sediment, and the uplift of a continental shelf are some of the causes of the formation of plains. The extensive area comprising the western part of the Mississippi watershed, very gradually rising to the foothills of the Rockies, and having, largely, a steppe climate, is called the Great Plains region of the United States. The coastal plains region of the United States along the Gulf Coast and the Atlantic seaboard is widest in the south and southeast.