The size of sediment is defined by the size of the particles that make up the sediment. The largest sediment size is called a boulder. Boulders have a diameter that is larger than 256 millimeters (about 10 inches). Cobbles are the next largest sediment, they are 64 - 256 mm in diameter (about 3-10 inches). Pebbles are next in size and are 4-64 mm in diameter (about 1/6-3 inches). The next sizes of sediments are very small, granules are 2-4 mm, sand 1/16-2mm, silt 1/256-1/16 mm, and the smallest sediment size is clay which is less than 1/256 of a millimeter in diameter.
Sedimentary rocks are formed in three ways from these different sized sediments.
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