Sedimentary Rocks and its Classification

Sedimentary Rocks and its Classification

The Latin word `sedere’ means ‘settling down’. Deposition (settling down) of eroded materials of pre-existing rocks by natural agents like river, glacier, wind at distant places below the river, lake, sea or oceans under water in layers and solidification of these sediments (deposited particles) in layers form Sedimentary Rocks. As these rocks are formed under water, so they are called Aqueous Rocks.

Sedimentary Rocks are also called Stratified Rocks (Latin ‘strata’ means ‘layers’) as they are formed in layers. Each layer is called ‘bed’ and the surface separating two adjacent beds is called ‘Bedding Plane’. Cracks developed at angles to this bedding plane due to unequal pressure are called Joints. Remains or impressions of plants and animals embedded in the sedimentary rocks are called Fossils. Fossils are commonly found in these type of rocks. For example, fossils found in the Himalayas, Alps, and the Atlas Mountain in Africa have led the scientists to conclude that all these mountains were formed by sediments

Classification of Sedimentary Rocks

Sedimentary rocks are classified, according to their origin as follows:

1. Mechanically formed sedimentary rocks: When sediments of various sizes are cemented together, this type of sedimentary rock is formed, e.g.,

  • Grit – When small pebbles are cemented.
  • Conglomerate – when rounded and bigger pebbles are cemented.
  • Breccia – When angular fragments are cemented,
  • Mudstone or Shale – When fine particles are cemented.

2. Organically formed sedimentary rocks: When the rocks are formed by the accumulation of decomposed plants or animals, this type of rocks is formed. They are called

  • Calcareous sedimentary rocks, e.g., limestone which are deposits of decomposed shells and skeletons of marine organisms.
  • Carbonaceous sedimentary rocks, e.g., coal which is accumulation of decomposed plants.

3. Chemically formed sedimentary rocks: These rocks are the deposits formed by the precipitation of dissolved salts in water after evaporation, e.g., gypsum and rock salt.

– Rahul Chopra.