Answer: Abrasion
Abrasion is a geological phenomena in which weathering and erosion of the rocks or other hard surfaces by physical agents like water and wind. In this process, the surface material is scrapped, scratched and wear down by the frictional force exerted by the physical agents.
The sawing and grinding action of the rock fragments in the water is called as abrasion because here the water is the physical agent affecting the physical structure of the rock.