The materials present in the river are for the most part a product of erosional forces wearing away the land. A river cuts both downwards (vertical erosion) and sideways (lateral erosion) to extend its channel. It may also extend back in the upstream direction through headward erosion. Four basic types of erosion operate in rivers: corrosion (erosion by chemical action), hydraulic action (i.e., water power), corrasion (the wearing away by abrasion) and crumbling or attrition(the cumulative result of repeated pounding). With all of these forms of erosion working together, even the most massive of rocks can be attacked and demolished over time.