Solution (or corrosion) is when acidic water dissolves rocks such as chalk or limestone.

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Vertical erosion involves the wearing away and deepening of the river bed. This is mostly by hydraulic action. It is most common in the upper course of the river.

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Abrasion, also known as corrasion, is when boulders and stones wear away the river banks and bed. Angular rocks that have entered the channel recently are useful tools of abrasion because they are more angular. Abrasion is responsible for both lateral and vertical erosion of the river channel. 

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Hydraulic action is when the force of fast-flowing water hits the bed and banks and forces water and air into cracks in the bedrock. The repeated changes in air pressure cause the river bed to weaken.

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It becomes more rounded and smaller.

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