Plain is a broad, nearly level stretch of land with no abrupt changes in elevation. Plains are generally lower than the land around them. They may be found along a coast or inland. Coastal plains generally rise from sea level until they meet higher land forms, such as mountains or plateaus. Inland plains may be found at high altitudes.
Plant life on plains is controlled by climate. Thick forests usually thrive on plains in humid climates. Grasslands cover fairly dry plains, such as the Great Plains in the United States. Plains are usually well populated because the soils and terrain are good for farming, and because roads and railroads are easy to build between rural towns and cities.
Coastal plain is a stretch of lowland along a seacoast, which slopes gently toward the sea. In many cases, such a plain may be an elevated part of the ocean floor. Solid materials carried off by rivers or waves form other coastal plains. These materials are deposited along the shore, extending the coast seaward.
The Atlantic Coastal Plain is a good example of a fertile and well-populated coastal plain. It lies along the eastern shore of North America from Canada to Florida. A zone called the Fall Line divides this low plain from the higher plateau known as the Piedmont. Along the Fall Line, waterfalls and rapids tumble from the plateau to the plain. Coastal plains generally have few and poor harbors. But the mouths of rivers along the Atlantic seacoast have produced some fine harbors.
Flood plain is the floor of a river valley, beyond the riverbed. A flood plain is formed of mud, sand, and silt left by the overflow of a river. These materials are carried off by the river as it erodes (wears away) the land upstream. A river in flood can carry a large amount of eroded material, which the overflow waters deposit onto the flood plain.
