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
Both images courtesy of Martin Miller, University of Oregon

What Is a Sedimentary Rock?

Sedimentary rocks make up about 75% of the rocks that are at the Earth’s surface. They preserve a chronicle of the environments and events that happened at the surface throughout Earth history, as well as the creatures that lived there.

Millions of little fragments of rock called sediment are the building blocks of a type of sedimentary rock known as clastic rocks. Sediment is found almost everywhere. It 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 particles are carried in streams, other waterways, glaciers, or the wind. They eventually settle out of the air or water and form a rock if they are cemented together.

But not all sedimentary rocks are made in this way. Chemical sedimentary rocks are not made of particles that come from somewhere else. They are made of mineral crystals formed by chemical processes. Organic rocks are made from the remains of living things such as clamshells, plankton skeletons, dinosaur bones, and plants.

Last modified June 11, 2009 by Jennifer Bergman.

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