The earth's surface is constantly being eroded. This means that rocks are broken up into smaller pieces by weathering agents such as wind, water, and ice. These small pieces of rock turn into pebbles, gravel, sand, and clay. They tumble down
rivers and streams. These pieces settle in a new place and begin to pile up and the sediments form flat layers. Over a long period of time, the pieces become pressed together and form solid rock called sedimentary rock. Most sedimentary rocks form under water. Most of the earth has been covered by water some time in the past. 70% of
the earth is covered by water now. So sedimentary rocks are common all over the world. Sedimentary rocks are often rich in
Sediments can harden into sedimentary rock in two ways.
pressure-As layer after layer of sediments are deposited, the lower layers are pressed together tightly under the weight
of the layers above.
cementing-Some sediments are glued together by minerals dissolved in water.
Some examples of sedimentary rocks are sandstone, limestone, conglomerate, and shale. Sandstone is formed from grains of
sand pressed tightly together. Sandstones are very common rocks. They are formed from the sand on beaches, in riverbeds, and sand dunes. Sandstones are usually made of the mineral quartz. Limestone is formed from tiny pieces of
shells of dead sea animals that have been cemented together. Conglomerate contains sand and rounded pebbles that have also
been cemented together. Shale is formed from mud or clay that has been pressed together. Shale forms in quiet waters such
as swamps and bogs. Sedimentary rocks are easy to identify because you can actually see the layers.