The outer surface of the earth is not even or plain. It has high and low lands, valleys, very tall mountains, plateaus and plain areas. This irregular outer surface of the earth is called topography. The shape of surface with any size is called relief feature. The construction of this features or shape is not always similar. It keeps on changing. Such changes can be experienced. 30 million crores years ago, there was a big sea called the Tethys where there are our mountains and hills now days.
The earth surface changes due to several geological processes and elements that are active on earth’s surface . River, wind, glaciers, rainfall, plants and animals etc. are the external elements. As a result of this elements,new structure may form as eroded soil is collected else where. The rivers , wind and glacier erode and collect soil and bring changes on earth’s surface.The processes and elements inside the earth also create big hills. Internal elements like volcanoes and earth quakes bring immediate change on earth’s surface. In this way internal and external elements work to bring changes on earth’s surface.
The work done by such forces \ elements consists of denundation of high lands lands and deposition of materials on he low lands. Denundation is the process by which external forces wear away land. Actually, it is the summation of weather , transport and erosion. Both the Denundation and deposition process combined together and cause to bring the surface of lithosphere to a normal level.This process is called gradation.
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