Limestone and Chalk are derived from the accumulation of corals & shells in the sea and are formed of sedimentary rocks of organic origin.
Karst Topography – Limestone & Chalk
In its purest form, limestone is made up of calcite or calcium carbonate, along with magnesium present in form of dolomite.
- Chalk is white & soft and is a pure form of limestone
- Limestone is soluble in rainwater
- It forms a weak acid, with carbon dioxide from the air
- Features of Karst Topography
- Streams generally cut their way along the joints & fissures of the rock
- It wears out a system of underground channels
- When the water penetrates to the base of the limestone, it meets the non-porous rocks
- When it meets with these rocks, it re-emerges onto the surfaces as a spring or resurgence
- Once the water has sunk into limestone, it etches out caverns & passages along joints
- When a number of swallow holes coalesce, a larger hollow is formed called a Doline
- Several dolines merge as a result of subsidence to form an even larger depression called an Uvala
- Polje are very large depressions as long as 100 square miles produced partly due to faulting
- Calcite pillars
- Sharp, slender, downward growing pinnacles that hang from the cave roofs are called Stalactites
- Calcium is deposited to form Stalagmites when moisture drips from the roof, it trickles down the stalactites & drops to the floor
- These are shorter, fatter & more round
- Human activities of Karst region
- Limestone vegetation in tropical regions is luxuriant because of heavy rainfall all year round
- Lead is the only mineral found in association with limestones
- Good quality limestone is used as building materials & quarried for the cement industry
- Chalk is covered with short turf
- It is used for pasture & sometimes for arable farming
A region with a large stretch of limestone possesses a very distinct type of topography, termed as Karst region.
The Karst regions have a bleak landscape, occasionally broken by precipitous slopes and there is an absence of surface drainage as most of the surface water percolates underground, hence surface valleys are generally dry.
The limestones are well jointed and rainwater finds its way into the underlying rock through these joints & cracks. These cracks enlarge into trenches by progressive widening by the solution & a most intriguing feature called limestone pavement is developed.
The enlarged joints are called Grikes & the isolated, rectangular blocks are termed as clints. There are numerous swallow holes on the surface of limestone that are small depressions carved out by solution where rainwater sinks into the limestone at the point of weakness, also known as sinkholes.
Caves & caverns that may contain ponds or lakes are formed by subterranean streams that descent through swallow holes to the underground passes. The most spectacular underground features that adorn the limestone caves are -
Solidified crystalline calcium carbonate is left behind when the lime-charged water evaporates as water carries calcium in the solution.
Over a longer period, the stalactite hanging from the roof is eventually joined to stalagmite growing from the floor to form a pillar.
Karst regions carry a thin layer of soil and are often barren. Limestone can usually support only poor grass as the porosity of the rocks & the absence of surface drainage makes vegetative growth difficult.
The landforms of chalk are different from other limestones. There is very little or no surface drainage. The valleys which once contained rivers are now dry often called Coombes.
There are no swallow holes & underground cave networks due to the friable nature of the chalk rocks.