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Sedimentary Rocks S edimentary rocks form from particles deposited by water and wind. Sand grains, mud, and pebbles are some of the sediments that form sedimentary rock. Sediment is small, solid pieces of material that come from rocks or living things. Erosion occurs when running water or wind loosens and carries away fragments of rock. Eventually, the moving water or wind slows and deposits the sediment. Deposition is the process by which sediment settles out of the water or wind carrying it. After sediment has been deposited, the processes of compaction and cementation change the sediment into sedimentary rock. At first, the sediments fit together loosely. However, gradually, over millions of years, thick layers of sediment build up. These layers are heavy and press down on the layers beneath them. Then compaction occurs. Compaction is the process that presses sediments together. Year after year more sediment falls, creating layers. The layers often remain visible in the sedimentary rock. While compaction is taking place, the minerals in the rock slowly dissolve in the water. Cementation is the process in which dissolved minerals crystallize and glue particles of sediment together. There are three major groups of sedimentary rocks: clastic rocks, organic rocks, and chemical rocks. Most sedimentary rocks are composed of broken pieces of other rocks. A clastic rock is a sedimentary rock that forms when rock fragments are squeezed together. Clastic rocks are grouped by the size of the rock fragments of which they are made. One common clastic rock is shale, which forms from tiny particles of clay. Sandstone is a clastic rock formed from the compaction and cementation of small particles of sand. Some sedimentary rocks contain a mixture of rock fragments of different sizes, including conglomerate and breccia. Organic rock forms where the remains of plants and animals are deposited in thick layers. Two important organic rocks are coal and limestone. Coal forms from the remains of swamp plants buried in water. Over millions of years, they slowly change into coal. The hard shells of living things produce limestone. These shells collect on the ocean floor. Over millions of years, compaction and cementation of shell particles produce limestone. Chemical rock forms when minerals that are dissolved in a solution crystallize. For example, a type of limestone can form when calcite dissolved in water comes out of solution and crystallizes. Chemical rocks can also form from mineral deposits left when seas or lakes evaporate. For thousands of years, people have used sandstone and limestone as building materials. Both types of stone are soft enough to be easily cut into blocks or slabs. Limestone is also used in making cement.