Sedimentary or detrital rocks :
They Formed as a result of denudation (weathering and erosion).These deposits through compaction turn into rocks. This process is called lithification. Sedimentary rocks Cover 75 per cent of the earth’s crust but volumetrically occupy only 5 per cent. They are layered or stratified of varying thickness. Example: sandstone, shale etc. Till or Tillite are Ice deposited sedimentary rocks. Loess are Wind deposited sediments.
There are three types of sedimentary rocks:
- Mechanically formed by agents like running water, wind ocean currents, ice etc. eg sandstone and shale.
- Chemically formed sedimentary rocks : Water containing minerals evaporate at the mouth of springs or salt lakes and give rise to Stalactites and stalagmites (deposits of lime left over by the lime-mixed water as it evaporates in the underground caves.
- Organically formed sedimentary rocks: The remains of plants and animals are buried under sediments and due to heat and pressure from overlying layers, their composition undergoes a change. Coal and limestone are well-known examples.
Chief characteristics of sedimentary rocks:
- Rocks are present in strata form. Stratification is a key feature of the rock.
- These rocks have fossils of plants and animals.
- These rocks are generally porous and allow water to percolate.
- These rocks are characterised by marks left behind by water currents and waves etc.
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