Chalk is a good example of an organically formed sedimentary rock. It is made up of the microscopic skeletons of marine plankton that lived millions of years ago. These skeletons settled on the ocean floor and over time, were compacted and cemented together to form the soft, white rock that we know as chalk. The process of formation of chalk is an example of biological sedimentation. Chalk is often found in areas that were once covered by shallow seas, such as the White Cliffs of Dover in England.