Exogenetic forces are constantly working to bring about levelling or the gradation of land.
They attempt to achieve a condition of balance between erosion and deposition which mean a graded position.
The above forces operate through the process called the process of gradation.
Agents of gradation like rivers, glaciers winds, sea waves and underground water perform their task with the help of the triple action of weathering, erosion and deposition.
- The levelling down of elevated portions of the earth’s surface is done by erosion.
- The filling up of depressions is done by deposition of the eroded material transported by the external agents of gradation.
Endogenetic forces of the earth give rise to major landforms on the earth surface and the exogenetic forces level them down.
The work of gradation has two components –
- (a) degradation
- (b) aggradation
When rocks are removed by scraping, scratching and cutting as a result of the process of erosion, thereby lowering the elevation of the land, it is called degradation.
Degradation, first of all includes the work of weathering that is the movement of scarped and scratched material aided by the great force of gravity. It also includes the work of erosion implying the transportation of the rock material by an agent of gradation.
The increase in the movement of rock-debris increases both its erosional and transportational capacities.
Filling up of low-lying areas of depression by eroded material is called deposition.
Deposition starts when the agents of gradation lose their force or have obstruction in their way. As a result eroded material is deposited in depressions which not only creates new landforms but also modifies the existing ones.
The above figure explains the total process of gradation and its two components-degradation and aggradation.
It shows the elevated portions continuously being lowered by weathering and erosion. The debris consisting of the eroded material is transported and deposited in the low lying areas. The surface of the lower areas on the other hand is raised through deposition of this debris. Finally, the position of a uniform or near uniform level is achieved. The process of gradation is not performed by a single agent. It is rather a result of the work of all agents of gradation acting simultaneously. It is however possible for a single agent of gradation to be more active in particular area or at a particular time.
Levelling and smoothening of land surface is called gradation; it includes both degradation and aggradation.
Bibliography : NIOS Geography Book
Reference : http://www.nios.ac.in