When we talked about geological cycles we said that mountains can be worn away. You have already learned that hard rocks can become soft and rotten by weathering and may even dissolve completely. Eventually, all the materials which result from this weathering process may be transported from their place of formations by the Sun-driven carriers such as rain, wind, or sea. Let’s know more how are mountains worn away.
The Earth’s gravity can also play an important part in removing these weathered materials, particularly on hills and mountains. On hills, helped by the action of the rain, or even by plant and animals the soil can slowly keep downwards.
And of course, as the soil in removed, more rock is exposed to the elements so that weathering can once more take its toll of the bare surface.
In dry deserts where there is not enough moisture to hold the weathered material together, the wind may pick up particles of sand and hurl them against any rocks which may be still standing and slowly wear them away thus providing , even more, ammunition for the sandblasting wind.
Water, in its various forms, is probably the most important of the earth’s many transport systems. We have seen that rain can accelerate the process of soil creep, even though occasionally a particularly hard rock can protect what lies beneath it from further decay so that earth pillars result.
But much of the rock debris, as it is sometimes called, finds its way into the rivers and streams where it may travel great distances. It is carried by rolling along the river bed, by bouncing, or by completes suspensions in the water itself. The dissolved materials may move also in the form of solutions.
Particles of rock wear away by bumping into one another and hitting the sides and bead of the river. They are rounded and polished as they are carried along and may further widen and deepen the river valley itself.
As the river deepens its valley, gravity plays an increasingly important part in carrying material down the valley sides into the river. Ice, too, can trap rock fragments within it and carry them done valley as we shall see later. Most of the material eventually finds its way into the sea where the tides and currents may sweep it many kilometers from the original mo until or hill.
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