Sedimentary Rocks


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  • These rocks are formed by successive deposition of sediments.
  • These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock.
  • Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks.
  • Generally, these rocks have some type of fossil between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks.
  • Sedimentary rocks are widely spread on the earth surface but to a shallow depth.

The individual rock particles are first broken from rocks and then transported by running water, ocean currents, glaciers or even by wind from one place to another.

The process by which rock-forming material is laid down is called sedimentation or deposition. It may settle in calmer waters of lakes or oceans or at places where the transporting agent has no longer enough energy to carry them farther. These are identified as riverine, lacustrine (formed by lake), glacial or aeolian (formed by wind) sedimentary rocks with reference to their deposition near rivers, lakes, glacier or deserts respectively.

The sediments are often loose, unconsolidated, soft rock material, in the beginning like sand and clay, but in course of time they get hardened to a compact material by excessive pressure and cementation to form sedimentary rocks. The deposition of sediments in the beginning is generally horizontal but it may get tilted afterwards due to movements in the earth’s crust. Sandstone, shale, limestone and dolomite are examples of sedimentary rocks.

Sediments get sorted by the transporting agents. Sediments of different sizes may get bound by cementing material under suitable conditions. Conglomerate is an example of such a sedimentary rock. This type of formation of consolidated material is termed as mechanically formed sedimentary rock.

The consolidation of organic matter derived from plants and animals forms sedimentary rocks of organic origin. Coal and limestone are organic sedimentary rocks.

The sediments may also result from chemical reaction. Direct precipitation of minerals from their solution in water may give rise to sedimentary rocks of chemical origin. Gypsum, rock salt and nitre are examples of such sedimentary rocks.

  • Huge folded mountains of the world like Himalayas, Andes etc. are made up of sedimentary rocks.
  • All the alluvial deposits of the world are also due to sedimentary accumulations.
  • All river basins, particularly their plains and deltas, e.g. Indo-Gangetic plain and Ganga-Brahmaputra delta are good examples of sedimentary accumulations.

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Bibliography : NIOS – Geography

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