<P> The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life . The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology . Sedimentology is part of both geology and physical geography and overlaps partly with other disciplines in the Earth sciences, such as pedology, geomorphology, geochemistry and structural geology . Sedimentary rocks have also been found on Mars . </P> <P> Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, volcanism, and other minor processes . </P> <P> Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals . Clastic rocks are composed largely of quartz, feldspar, rock (lithic) fragments, clay minerals, and mica; any type of mineral may be present, but they in general represent the minerals that exist locally . </P> <P> Clastic sedimentary rocks, are subdivided according to the dominant particle size . Most geologists use the Udden - Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel (> 2 mm diameter), sand (1 / 16 to 2 mm diameter), and mud (clay is <1 / 256 mm and silt is between 1 / 16 and 1 / 256 mm). The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of the finest material . This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature . </P>

4 examples of sediment that might become sedimentary rock