Sedimentary Rocks

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. 
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.