The relatively flat and low-lying vast expanse of land is called a plain. While some plains are extremely level, others may be rolling or undulating. Most of these plains are formed by major rivers and their tributatries which have been  bringing and depositing        silt, stones and sand along their course for ages together. The great Northern plain of India is formed by the deposits brought by the river Ganga and her tributaries. The Hwang-Ho Plain in China and the Nile Valley Plain in Egypt are also formed by the rivers.

How Plains are useful
    People find plains useful for a wide range of activities. The soil is fertile here. The gentle slopes and valley floors have the earth's most important farmlands. Some of these valuable farmlands include the North European Plains, the Ganga-Brahmaputra Plain      of India and the North China Plain. Plain and valleys are also places for settlements with sprawling town and cities facilitating farming and industrial activities. Communication is smooth as construction of road and railways is easy here. This is the reason that      plains are thickly populated.

                         Key Words
GLACIER :         A slowly-moving river of ice on land.
HILL :                 A raised area or mound of land.
FAULT LINE :    The place where two tectonic plates meet forming a weak line.
HORST :            A block of high ground that stands out because it is flanked by faults on two Sides.
GRABEN :         The portion of the earth's crust, bounded on at least two sides by faults, that has subsidised.