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.
Consolidated/Unconsolidated
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 |