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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.
In a clastic rock, the clasts are often of a different make-up from the ground rock or matrix
- Neuendorf, Klaus; Mehl, James; Jackson, Julia 2005. Glossary of Geology. 5th ed, American Geological Institute: Alexandria, VA.