Sedimentary rocks are rocks made by sediment that has accumulated over millions of years. This sediment can accumulate on the surface of the Earth or under water. Sedimentary rocks tend to resemble the sediment that created them, so they are usually gray or brown.
There are three types of sedimentary rocks. Fragments of other rocks and seashells that have migrated and cemented together make clastic rocks. When minerals and other substances bubble out of a solution, chemical sedimentary rocks form, such as when rock salt forms. Age-old remains of plants and animals form organic sedimentary rocks, including coal.
The sediment that goes into these rocks forms flat, broad layers called beds. These layers are different in color and texture. The weight of the accumulating layers make them compact, and water oozing through the layers helps to form the cement that binds the grains, minerals and rock fragments together. Sedimentary rock often contain fossils, and fossil hunters can tell the age of the fossil if they know the age of the rock layer in which they are found. When there is an earthquake or other geologic event, the layers of these rocks sometimes crack open.
Examples of sedimentary rocks are breccia, siltstone, flint, chert and limestone.