The earth’s landscape is naturally shaped by the forces of weathering and erosion. Weathering refers to the breakdown of rocks at or near the surface of the earth, either by chemical processes (e.g. limestone being dissolved by water) or physical processes (e.g. rocks cracking because of temperature extremes). The movement of weathered rock fragments--also called sediment--by forces including wind, water or gravity is called erosion. Human activity may both accelerate erosion (for example by removing vegetation which stabilizes soil), or prevent it (through reinforcement of erosion-prone areas like beaches, riverbanks, and hillsides).