Explanation: The earth’s surface is being continuously subjected to by external forces originating within the earth’s atmosphere and by internal forces from within the earth.

• The external forces are known as exogenic forces and the internal forces are known as endogenic forces.
• The actions of exogenic forces result in wearing down (degradation) of relief/elevations and filling up (aggradation) of basins/ depressions, on the earth’s surface.
• The phenomenon of wearing down of relief variations of the surface of the earth through erosion is known as gradation.

Source: NCERT – Fundamental of Physical Geography