Sedimentary Rock


The word ‘sedimentary’ is derived from the Latin word ‘sedimentum’ which means ‘settling’. Sedimentary rocks are formed by the accumulation of sediment at a particular place over a long period of time. These rocks may be coarse or fine-grained, soft or hard. These are non-crystalline and commonly contain fossils of animals, plants, and other microorganisms.


Rocks (Igneous, Metamorphic, Sedimentary) are degraded by the process of weathering. It degrades and disintegrates rocks into smaller fragments. These fragments are transported and deposited from one place to another by various exogenic agencies like wind, karst, air, water, and even animals. The compaction of these sediments turns into sedimentary rocks. Therefore, this process is known as lithification. On the basis of the formation of sedimentary rock, it can be classified into three major groups-

MECHANICALLY FORMED- These rocks are formed by the accumulation of debris from other rocks and cemented together. For example, sandstone, conglomerate, limestone, shale, loess, etc. NOTE-

  • Sandstone is a mixture of sand grains and fragmented pieces of granite i.e., quartz.
  • When larger pebbles are cemented together to form rock is called conglomerate.

ORGANICALLY FORMED- These rocks are formed from the carcass of living organisms such as coral and shellfish. The calcareous type (limestone, chalk) of rock is formed in this type of process. Other than this, when trees are buried and compressed by loads of sediments over a million period of time then carbonaceous rocks (peat, lignite, or coal) are formed. These rocks are having great economic value.

CHEMICALLY FORMED- These rocks are precipitated chemically from solutions of one kind or another. For example, chert, limestone, halite, potash, etc.