Various external forces affect the earth's surface, such as different climates and the amount of rainfall. Freezing, thawing, and running water all contribute to weathering and erosion, processes that break rock down into tiny particles. These particles are then transported by water, ice, or wind as sediment. The processes of erosion reduce mountains to hills, create canyons, valleys, and soils, and deposit huge amounts of sediments that either become eroded again or are preserved and lithified into sedimentary rock.