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LIMESTONE AND CHALK LANDFORM
- Limestone & chalk are sedimentary rocks of organic origin derived from the accumulation of corals & shells in the sea.
- In its pure state, limestone is made up of calcite or calcium carbonate; along with magnesium present in form of dolomite.
- Chalk is pure form of limestone, white & soft.
- Limestone is soluble in rain water, which, with carbon dioxide from the air, forms a weak acid.
- A region with a large stretch of limestone therefore possesses a very distinct type of topography, termed as Karst region.
Features of Karst Topography
- Generally, Karst regions have a bleak landscape, occasionally broken by precipitous slopes.
- General absence of surface drainage as most of the surface water percolate underground, hence surface valleys are generally dry.
- Streams generally cut their way along the joints & fissures of the rock wearing out a system of underground channels.
- When the water penetrates to the base of the limestone & meets the nonporous rocks, it re-emerges onto the surfaces as a spring or resurgence.
- Limestones are well jointed & it is through these joints & cracks that rain water finds its way into the underlying rock.
- Progressive widening by the solution enlarges these cracks into trenches & a most intriguing feature called limestone pavement is developed.
- The enlarged joints are called Grikes & the isolated, rectangular blocks are termed as dints.
- On the surface of limestone are numerous swallow holes, which are small depressions carved out by solution where rainwater sinks into limestone at the point of weakness, also known as sink holes. Once water has sunk into limestone, it etches out caverns & passages along joints.
- When a number of swallow holes coalesce, a larger hollow is formed & is called a Doline.
- Several dolines may merge as a result of subsidence (gradual caving) to form an even larger depression called an Uvala
- In Yugoslavia, some very large depressions called Polje, may be as large as 100 square miles, but produced partly due to faulting.
- Subterranean streams which descent through swallow holes to the underground passes leads to the formation of caves & caverns which may contain ponds or lakes.
- The most spectacular underground features that adorn the limestone caves are
(iii) Calcite pillars
- Water carries calcium in solution & when this lime charged water evaporates, it leaves behind solidified crystalline calcium carbonate.
- Stalactites are sharp, slender, downward growing pinnacles that hang from the cave roofs.
- When moisture drips from the roof, it trickles down the stalactites & drops to the floor, where calcium is deposited to form Stalagmites, which are shorter, fatter & more round.
- Over a longer period, the stalactite hanging from the roof is eventually joined to stalagmite growing from the floor to form a pillar.
Human Activities of Karst Region
- Karst regions are often barren & at best carry a thin layer of soil.
- The porosity of the rocks & the absence of surface drainage make vegetative growth difficult, hence limestone can usually support only poor grass.
- Limestone vegetation in tropical regions is luxuriant because of heavy rainfall all the year around.
- The only mineral found in association of limestones is lead.
- Good quality limestone is used as building materials & quarried for cement industry.
- Landforms of chalk are rather different from other limestones.
- There is little or no surface drainage & valleys which once contained rivers are now dry often called as Coombes.
- Chalk is covered with short turf & is used for pasture & sometimes for arable farming.
- Because of the friable nature of the chalk rocks, swallow holes & underground cave networks do not generally develop.