The parts of the rock cycle that occur only on the surface are weathering and erosion. Weathering is the process by which rocks are broken down into smaller pieces by physical, chemical, and biological processes. Erosion is the process by which these smaller pieces of rock are transported away from their original location by wind, water, or ice. Both weathering and erosion are essential for the formation of sedimentary rocks, which are formed from the accumulation and cementation of sediments that have been weathered and eroded from other rocks.