Running water
It is an important agent of gradation. A river, like any living form, has a 'life cycle'. It takes birth in a mountain or a hill, becomes larger where it is met by a number of tributaries, flows over plains and finally ends when it reaches the sea.
In its early stage, the river is young. It flows over steep mountains where its rate of erosion is maximum. Due to down cutting work of a river, it forms striking youthful features like 'l' shaped and 'V' shaped valleys, waterfalls, etc. As the river enters into the plains, it comes to the stage of maturity. Here, the volume of water increases and the slope of land decreases, resulting in slow speed of the river. During this stage, the river is said to flow through meanders.
Before meeting the sea, the river becomes large and sluggish and gets divided into several distributaries. It now comes to its old stage. The river, thus, deposits silt, sand and sediments over a large area near its mouth. This large deposition helps in the formation of a delta. The delta of Ganga and Brahmaputra is the largest in the world.
Glacier
It is believed that a million years ago, the climatic condition of the earth's surface was much cooler than what it is now. The earth's surface was covered by ice but later, the temperature began to rise gradually. This rise in temperature resulted in melting of the ice. The mass of moving ice is called a Glacier. In India, the glaciers are found on high altitudes of the Himalayas where temperature is below freezing point.
Like running water, moving ice also erodes loose particles or parts of rocks. It takes them along and deposits them behind, when it melts. Huge masses of ice which cover large area of a continent are called Continental glaciers like Antarctica and Greenland. Other glaciers which occupy small areas over the mountains are called Mountain glaciers like Siachin glacier and Gangotri glacier of the Himalayas in India.
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