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Pieces of rocks, minerals, remains of living things, and dissolved minerals that come out of water (such as lime) are all kinds of sediments. Sedimentary rocks are rocks formed when sediments are pressed and cemented together naturally, over millions of years. Sedimentary rocks may be formed from small pieces of rock, shells, soil, fossils, and the remains of plants. Rocks are broken down by a process called weathering. The small parts of rock may be carried by wind or water to a new location during the process of erosion. The remains of living things may be deposited at the bottom of a lake, river, or pond. Sedimentary rocks often contain fossils. Layers of different types of sediment build up over many thousands of years. These layers are pressed together. The pressing down is called compaction. Over many thousands of years, water is pressed out and the layers stick together. The process that results in the layers being stuck together is called cementation. Sometimes, it is possible to identify the sediments in sedimentary rocks. Some large particles of rock or shell may be visible. Sometimes the particles are too small to see without a microscope. Fossils or imprints of plant and animals can be seen in some sedimentary rocks. Most of the rocks found in the world are sedimentary rocks. Examples include sandstone, limestone, and halite.