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Clastic Rocks

Clastic rocks are sedimentary rocks that are made up of pieces of other rocks that have been deposited by wind, water, ice, or gravity. They are identified by the size of those pieces, and whether the pieces are loose or cemented together.

Grain Size

The individual pieces of rock are called grains. For sedimentary rocks have been carried and deposited by wind or water, the grains in each rock tend to be generally the same size. To understand why, try the Sorting Sediments activity.


If the grains are loose, then you have unconsolidated sediments. While most people do not think of sand or mud as a rock, to a geologist they are. If the grains have been cemented together by minerals, then you have a consolidated rock, something that fits into what most people think of as rocks.

Unconsolidated Grain Size Consolidated
Clay Less than 0.0625 mm Shale
Sand 0.0625 to 2 mm Sandstone
Gravel 2 mm to 64 mm Conglomerate/Breccia
Cobbles 64 mm to 256 mm
Boulders Greater than 256 mm