ICSE Class 7 Answered

Sedimentary rocks are classified on the basis of their formation as:

Mechanically formed rocks: Old rocks are denuded and broken into fragments by running water or moving glaciers. When these rock fragments are carried away and deposited on new lands, sedimentary rocks are formed. For example, sandstone is formed by the deposition of sand. Similarly, rounded pebbles found in river beds get deposited and form a rock called conglomerate.

Chemically formed rocks: Chemical sedimentation generally takes place due to the evaporation of water from solutions containing salts. For example, in desert regions, the water from shallow lakes may evaporate leaving behind layers of salt on the surface. The layers of salt then get consolidated into sedimentary rocks. Example rock salt.

Organically formed Rocks: These rocks are formed when the remains of plants and animals get compressed forming a rock. For example, limestone is formed by the deposition and hardening of skeletons, remains of animals and shells. When the remains of plants and animals, buried under the soil for over millions of years undergo change, they result in the formation of coal.