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In this article we are discussing the types of rocks.

Sedimentary rocks:

a) Such rocks are formed of sediments accumulated over long periods, usually under water. 

b) Rocks on the Earth’s surface are exposed to denudational agents (wind, waves, glacial ice & animals) and are broken into small fragments. This process is called weathering. Such fragments are transported and compact under pressure when deposited (called soil compaction), thereby forming rocks. This whole process is called lithification.

c) They are distinct as they are deposited in layers or strata. Thus they are known as stratified rocks. 

d) They may be coarse/fine-grained or soft/hard.

e) They are non-crystalline and often contain fossils of animals, plants & other microorganisms. Thus, these rocks are varied in their composition vis-à-vis other rocks.

f) Sedimentary rocks can be further classified into the following 3 groups:

1) Mechanically-formed sedimentary rocks:

i) This type is composed of inorganic rocks and mineral fragments. Hence, they are also called clastic sedimentary rocks.

ii) They have been formed due to accumulations of bits from other rocks getting cemented together.

iii) When the layers of clastic sediments build up, the lower strata is pushed down by their weight. The resultant pressure compacts the sediment by squeezing out excess water. 

iv) The dissolved minerals recrystalize in between the sediment grains. This process is called as cementation. 

v) The texture, colour and composition of these rocks vary in a great amount.

vi) Sandstone, limestone, shale, loess are some examples of this type of rocks. A coarser type of sandstone is known as grit. 

vii) When big, rounded pebbles form a rock, it is called conglomerate. However, when angular pebbles form a rock, it is called as breccia.

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