Most sedimentary rocks are clastic sedimentary rocks, formed from cemented sediment grains that are fragments of preexisting rocks. The rock fragments can be either identifiable pieces ofrock, such as pebbles of granite or shale, or individual mineral grains, such as sand-sized quartzand feldspar crystals loosened from rocks weathering and erosion. In most cases, the sedimenthas been eroded and transported before being deposited. During transportation the grains may have been rounded and sorted.