Sedimentary Rocks

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

Sedimentary Rocks

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.