objective: 3240-0503

You probably think of rocks as being uniform. However, rocks on the earth's surface are subject to the processes of erosion which are continually breaking down and rearranging them. Small rocks, fragments and organic remains that have been moved by water, wind or other agents of weathering and erosion are called sediment. Over a period of time, sediment is pressed or cemented together to form sedimentary rocks. Sedimentary rocks can be formed from metamorphic, igneous or other sedimentary rocks that have been broken down or broken up by weathering and erosion. Sedimentary rocks account for 75% of the rocks exposed at the earth's surface. In nature sedimentary rocks are usually layered.

CLASTIC

Sedimentary rocks that have been formed from the fragments of other rocks are clastic rocks. The word clastic comes from the greek word klastos which means "broken." Conglomerate and Sandstone are both examples of clastic rocks.

ORGANIC

Organic rocks are formed throught the life processes of living organisms or the remains of the organisms. Limestone that is formed from the shells of clams and other organisms is organic. When you write with chalk on the chalkboard, you are actually using the shells of organisms that lived long ago!