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Geology Unit 6: Sedimentary Rocks Student Information The minerals formed deep within the earth combine in various ways to form the hard solids we call rocks. Sedimentary rock is a kind of rock formed when a layer of sediment becomes solid. This type of rock is formed by the weathering and erosion of existing rocks. Rocks exposed to air and water slowly wear away. During the process of weathering, small pieces, or particles, break away from the main rock. Water flowing over the earth's surface picks up sediment, or small pieces of rock, sand, clay, and other materials. The water flows into streams or rivers. As the flow slows down, some particles of rock and other materials fall to the bottom of the river and settle out of the water. Rock particles in flowing water settle out and layers form. The layers of sediment become covered by other layers.The upper layers press down on the lower layers.The weight of accumulated particles, along with mineral-laden water, cements everything together. After thousands of years, layers of sediment become solid and form rocks. Sandstone is an example of a sedimentary rock that was formed when layers of sediment became solid. Sedimentary rocks are known for their layered characteristics and often provide clues to the past in the form of fossils. Sedimentary rocks are classified by the source of their sediments. There are three classifications of sedimentary rocks. Chemical sedimentary rocks, such as rock limestone, halite, and gypsum, are formed from minerals left behind after water evapor a * e S - Clastic sedimentary rocks, such as conglomerate, breccias, and sandstone, are formed when little pieces of broken-up rock pile up over time and are pressed and cemented toaether. p » fxZI^ I gggg^ Limestone Q v | jirifeiiaVi I ™ ! Halite J^^^^K ^^^yMM, Gypsum v ^ r ^ i S f e sssS^Jlm^. ^frapsaHl v^^^Sf^L ^ ^ g ^ p g i ^ -,^||HK M ^1|||B8 .'••••-•..••;i;---,l Conglomerate Organic sedimentary rocks, such as coal r • and organic limestone, contain the remains $§SBk of materials that were once alive. Deposits of V" ) vf^k^\ Breccia , ( Js3BMd Sandstone — HH / ^|^«^KJP^B^^^^^ Coal -' -J ^^ '^^^^ Organic Limestone All sedimentary rocks are formed by two common processes: compaction and cementation. Compaction is caused by the weight of the overlying rocks squeezing the sediments together. Cementation is a chemical process through which water carries and deposits dissolved minerals in the small spaces between the sediment particles. In a sense, the particles are cemented together.