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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.