Clastic rock

Conglomerate

Breccia. Notice the angular nature of the large clasts

Marble breccia from Naples.

Clastic rocks are composed of fragments, or clasts, of pre-existing rock.

Although, in principle, almost any rock made of eroded former rock is clastic (such as sandstone or mudstone), in practice the term is used for clasts of at least gravel size upwards. That means clasts are at least visible to the naked eye, and may be huge.

There are two types of clastic rock, by shape. conglomerates have rounded clasts, and breccias have jagged clasts.

In a clastic rock, the clasts are often of a different make-up from the ground rock or matrix.