Many of the world’s vineyards are located on sediment that is now distant from its source, while in others the soil is derived from the local bedrock, which may be visible as outcrops protruding through the loose material. All exposed bedrock is under constant chemical and physical attack (weathering), breaking it down into the smaller fragments generally called stones (see stones and rocks). Smoothed pieces a few centimetres or so across are referred to as pebbles, or cobbles if larger. As the material is further fragmented, into sand, silt, and clay, it becomes soil. Loose material on hillsides tends to slip down the slope due to gravity (see colluvium). The world’s largest vineyards are on flat plains, carved by rivers over millennia and now covered by alluvium. All these fragmented products and the bedrock itself are composed of minerals.