Valley. Kinds of valleys. How valleys are formed

Valley is a natural trough in the earth's surface. Systems of valleys extend through plains, hills, and mountains. Rivers and streams flowing through valleys drain interior land regions to the ocean. The bottoms of many valleys have fertile soil, which makes excellent farmland.

All valleys are similar in shape. The bottom of a valley is called its floor. Most floors slope downstream. Mountain valleys usually have narrow floors. But in low-lying plains, a floor may be several miles or kilometers wide. The part of the floor along riverbanks is called the flood plain. When the river overflows its banks, it floods the flood plain. In some cases, flooding is helpful because it adds nutrients (nourishing substances) to the soil. But severe flooding can damage crops and buildings and even kill people. A valley's sides are called valley walls or valley slopes. The ridge formed where the walls of neighboring valleys meet is a divide.

Kinds of valleys. Various kinds of valleys are named according to their appearance. A deep valley with steep walls is called a canyon. One of the most famous canyons is Grand Canyon in Arizona. Along coastlines, valleys flooded by the ocean are called drowned valleys.

Chesapeake Bay and Delaware Bay are drowned valleys. Where a valley joins a larger valley from the side, the two floors usually meet at the same level. But sometimes the floor of the side valley is higher than that of the main valley. The side valley is then called a hanging valley. A river flowing through a hanging valley may form a waterfall where the water enters the main valley.

Not all valleys are on land. Many deep submarine canyons are found on the slopes leading up from the ocean floor to the edge of the continental shelf. Hudson Canyon is a submarine canyon. It extends south-eastward down the continental shelf to the Atlantic Ocean floor from a point near New York City.

How valleys are formed. Most valleys on dry land are formed by the running water of streams and rivers, and by the erosion of slopes leading to them. Erosion moves material down the slopes to the valley floor where the stream carries it to a lake or to the ocean. In addition, the stream may erode its channel deeper. Hanging valleys are usually formed when erosion is greater in the main valley than in the side valley.

A rift valley may form when a long, narrow section of the earth's crust sinks. One system of rift valleys extends from the Sea of Galilee south through the Red Sea, and into southeastern Africa.

Glaciated valleys are valleys enlarged by the action of glaciers. They are often found high in mountains and are U-shaped rather than V-shaped.