Sedimentary rocks can be made up of clasts or detritus (rock grains) weathered from other rocks by river waters, wind, coastal sea waters, glaciers, living beings, etc. This kind of sedimentary rocks are called detrital rocks.
They're classified upon the grains that they are made up of. First up, grains may be all the same size, uniform, as in a sandstone, or different sizes, as in a conglomerate. Secondly, grains may be bigger or smaller. The following table shows roughly how rock grains are classified upon their size:
| Diameter | |
|---|---|
| Clay | < 0.004 mm |
| Silt | < 0.06 mm |
| Sand | < 2 mm |
| Gravel | < 6 cm |
| Cobble | < 25 cm |
| Boulders | > 25 cm |