Basic Concept of Sedimentary Rocks

Definition and Formation :-

 Sedimentary rocks are formed by the worn-out sediments of previous rocks. The primary rocks are worn down by the different natural agents such as sun,rain,running water, wind , moving ice etc. The fragment of rocks and sediments are brought down by the glacier, river, stream, wind and wave etc.and they are subsequently deposited in layers on the sea-bed.In time,owing to pressure of their own layers, heat of the interior and the chemical reaction of lime , these layers of sedimentary undergo solidification and cementation to form hard rocks.Thus the sedimentary rocks are formed in layers of sediments and they are also know as stratified rock.

A layer of sedimentary rock rest upon another layer . The plane or the surface which separates one layer of sedimentary rock from another is know as bedding plane.

Fossil :-

Fig :- Fossil

Fig :- Fossil

  With the sediments,remains of animals and plants are also carried down and these undergo solidification to from rock.Thus organic rocks are formed ; they are also know as fossils.Thus fossils are the recognizable parts trace or imprint of a particular pre-historic animal or planet once buried in sediment and subsequently hardened like of rocks.

Example :-

sandstone, mud stone, limestone, conglomerate and shale are the examples of sedimentary rocks. These rocks and some fossils are found in the layers of the sedimentary rocks of the Himalayas.

Fig :- Sandstone

Fig :- Conglomerate

Fig- Lime stone

Fig :- Shale

Classification of sedimentary rock according to its mode of formation :  

 According to its mode of formation,sedimentary rocks can be divided into the following three types: (1) mechanically formed rocks, (2) organically formed rocks and (3) chemically formed rocks.

Mechanically formed rocks :-

These rocks are consolidated in a mechanical manner. They are coarse grained.

Example :- 

 Conglomerate, Mudstone and Breccia are of this type of sedimentary rock. Large rounded boulders, sand and fragments of rock together from conglomerate. Breccia is formed by triangular from of rock-particles. Minute particles of sedimentary constitute the Mud stone.

Fig-Breccia

Fig :- Mudstone

Organically formed rock   :-

Hard particles of once living organism may accumulate to from the organically formed rocks.

Example :- 

Limestone and Coal are the example of organically formed rocks.Calcareous Limestone are composed of shells and skeletons of animals largely consisting of calcium carbonate.Coal is carbonaceous rock.It is formed from plant accumulation beneath the surface of the earth .

Fig-Limestone and Coal

Chemically formed rocks :-

These rocks are mostly precipitated from solution of certain salts and other chemical compounds.

Example-

Rocks salt , Dolomite, Calcite, Gypsum borax etc. are the examples of chemically formed rocks.

Fig :- Dolomite

Fig :- Calcite

Characteristics and uses of sedimentary rocks :-

(1) The sedimentary rocks are formed by sediments and remains of the primary rocks. (2) They are formed mainly by the action of water and are, therefore, called aquatic rocks. (3) They are formed in states of horizontal layers and are,therefore, know as stratified rocks. (4) They contain particles of various sizes and shape. (5) They are less hard than igneous and metamorphic rocks. (6) They cover nearly 70% of the total surface of the earth.(8) They contain several kinds of fossil. (9) They are not resistant to erosion.Sedimentary rocks are very useful.Sandstone and limestone are of great economic value. Sandstone are useful as building and road materials. They are used to raw materials in the glass factory. Limestone are used in cement industry. The fossils are very useful to trace out the past history of the earth.