Clastic

In geology, the term clastic refers to sediments formed from fragments of pre-existing rock. Clastic sedimentary rocks are rocks composed predominately of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material composition, and texture.

Grain size varies from clay in shale; through silt in siltstone; sand in sandstone; and gravel, cobble, to boulder sized fragments in conglomerate or breccia. The Krumbein phi (φ) scale numerically orders these terms in a logarithmic size scale.

Composition includes the chemical and mineralogic make-up of the single or varied fragments and the cementing material holding the clasts together as a rock.

Texture refers to the degree of rounding or angularity as well as the amount of sorting, whether the rock is composed of a single sized or a variety of sized clasts.

An example clastic environment would be a river system, in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream.