Lynn Williams, age 10, of Brownsville, Tex., for her question:70
HOW ARE VALLEYS FORMED?
Valleys are natural troughs in the earth's surface. 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 ocean. In addition, the stream may erode its channel deeper.
A valley may also be formed when a long, narrow section of the earth's crust sinks below the surrounding area. A valley formed in this way is called a rift valley. One system of rift valleys extends about 4,000 miles from the Sea of Galilee south through the Red Sea and into southeastern Africa.
Glaciated valleys are valleys that were formed and enlarged by the action of glaciers. These valleys are often found high in mountains. They are U shaped rather than V shaped.
Systems of valleys extend through plains, hills and mountains. Streams and rivers flowing through valleys drain interior land regions. Many valleys have fertile soil and make excellent farmland. All valleys are similar in shape. The bottom is often called its floor. The floor of a valley usually slopes gradually in one general direction. Mountain valleys usually have narrow floors but valleys in low lying plains are often many miles wide.
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 that are 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 the floor of the main valley where they join. 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.
The sides of a valley are called valley walls or valley slopes. The ridge formed where the walls of neighboring valleys meet is called a divide.
The Shenandoah Valley is one of the nation's most beautiful valleys. Located in northwestern Virginia, it inpludes the seven counties drained by the Shenandoah River and much of the area drained by the James River west of the Blue Ridge Mountains. The Massanutten Ridge and other prominent hills rise above the valley.
Across the nation in California is the San Joaquin Valley, one of the richest farming areas in the world.