sedimentary rock

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Limestone
(chemically precipitated)

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Sandstone
(clastic sedimentary)

Sedimentary rocks are rocks formed by the compaction or and cementation, along with some chemical alteration of sediments. There are four main categories of sedimentary rocks: clastic, organic, chemical, and evaporites.   Limestone, a chemically precipitated sedimentary rock shown in the photograph on the left, is formed by the compaction and lithification of the shells from marine organisms. Sandstone, on the right, is formed by the compaction of quartz grains.