What Is a Sedimentary Rock?

Ripples in beach sand , such as those in the upper photograph (A) may someday become a rock like the sandstone in the lower photograph (B). This sandstone was part of a beach over 200 million years ago in the Triassic period. Click on image for full size (143 Kb) Both images courtesy of Martin Miller, University of Oregon

Have you ever been to the beach and nestled your toes in the sand? Over thousands of years that sand might become part of a sedimentary rock!

Sedimentary rocks make up about three-quarters of the rocks at the Earth’s surface. They form at the surface in environments such as beaches, rivers, the ocean, and anywhere that sand, mud, and other types of sediment collect. Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed through Earth’s history. Fossils of ancient living things are preserved in sedimentary rocks too.

Many sedimentary rocks are made from the broken bits of other rocks. These are called clastic sedimentary rocks. The broken bits of rocks are called sediment. Sediment is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment may, in time, form a rock if the little pieces become cemented together.

There are other types of sedimentary rocks whose particles do not come from broken rock fragments. Chemical sedimentary rocks are made of mineral crystals such as halite and gypsum formed by chemical processes. The sediment particles of organic sedimentary rocks are the remains of living things such as clamshells, plankton skeletons, dinosaur bones, and plants.