The present chapter discusses weathering and mass movement, leaving other processes that produce earth’s varied landscapes to be examined in the next chapters.
Endogenic forces build and create initial landscapes, whereas exogenic processes operate to wear them down, so the exogenic processes, which level down the raised land by the tectonic processes, are called gradational processes.
In fact, gradation is an ongoing process, and its agents such as running water, wind, waves and glaciers are at work at all the time and at all the places.
Gradation comprises picking up and removal of loose material, its transportation to another location, and its deposition there. Degradation or the wearing of the land is performed by the combined action of erosion and transportation.
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At this point, it is worthwhile to remember that there is basic difference between gradation and aggradations. Aggradations are the filling in of depressions or the raising of land elevation.
The combined effect of gradation and degradation is the gradational reduction of irregularities in elevation. The final result of gradation is to reduce the land surface to base level – a surface that the erosional forces can no longer affect it.
The ultimate level is the level of the sea. American geologist, J.W. Powell, put forward the idea of base level or a level beyond which a stream cannot erode its valley further. Ideally, this is the lowest practical level for all denudation processes.
However, there are other base levels also. A local base level, may be a temporary one, might be a river, a lake or an artificial manmade dam.
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Keeping in view the concepts of base level, landmass denudation and slope development, let us examine specific processes that are active in reducing landforms.