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Thursday, August 19, 2021

Physical Geography (Part-8): Lime Stone & Chalk Landforms

Limestone and Chalk Land-Forms


Physical Geography (Part-8): Lime Stone & Chalk Landforms


Limestone and chalks are sedimentary rocks of organic origin derived from the accumulation of corals and shells in the sea.

In its pure state, limestone is made up of calcite or calcium carbonate, but where magnesium is also present it is termed dolomite. Chalk is a very pure form of limestone, white and rather soft. Limestone is soluble in rainwater, 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. It is then, termed a karst region, a name derived from the Karst district of Yugoslavia where such topography is particularly well developed.


Characteristics features of a Karst Region:

Karst region has a bleak landscape, occasionally broken by precipitous slopes. There is a general absence of drainage as most of the surface water has gone underground. Streams raising of the other rocks only flow over limestone for a short distance and then disappear underground. For the greater part of their course, they cut their way along the joints and fissures of the rock wearing out a system of underground channels. The surface valleys are therefore dry. When the water penetrates to the base of the limestone and meets non-porous rocks it re-emerges onto the surface as a spring or resurgence.

Limestones are well jointed and it is through these joints and cracks that rainwater finds its way into the underlying rock. Progressive widening by solution enlarges these cracks into trenches and a most intriguing feature called limestone pavement is developed. The enlarged joints are called grikes and the isolated, rectangular blocks are termed clints. The limestone pavements may have been formed beneath the soil and are now exposed by the removal of the soil cover.

On the surface of the limestones are numerous swallow holes, which are small depressions carved out by solution where rainwater sinks into the limestone at a point of weakness. They are also known as sinkholes. Example: Gaping Ghyll in Yorkshire. These holes grow in size through continuous solvent action.

Once the water has sunk into the limestone it etches out caverns and passages along joints or bedding planes. When the roof of an underground tunnel collapses, a precipitous limestone gorge such as the Cheddar Gorge is formed. Where several swallow holes coalesce a larger hollow is formed and is called a doline. Several Dolina may merge as a result of subsidence to form a larger depression called an uvala. Some of them are miles across, containing much clayey soil from the limestones, weathered after their subsidence.

In Yugoslavia, some very large depressions called polje may be as large as hundred square miles but these are partly due to faulting. During the rainy season, parts of the floor that are at or near the water table may become temporary lakes, but the drier areas are fertile and may support large villages.

Where subterranean streams descend through swallow holes to underground passages, the region may be honeycombed with caves and caverns, some containing ponds and lakes. The most spectacular underground features that adorn the limestone caves are stalactites, stalagmites, and pillars.

  • Stalactites: These are the sharp, slender, downward-growing pinnacles that hang from the cave roofs. The water carries calcium in the solution and when this lime-charged water evaporates, it leaves behind the solidified crystalline calcium carbonate.
  • Stalagmites: As moisture drips from the roof it trickles down the stalactite and drops to the floor where calcium is deposited to form stalagmites. They are shorter, flatter, and more rounded.
  • Pillar: Over a long period, the stalactite hanging from the floor forms a pillar. Such features are commonly seen in any well-developed limestone cave eg. Batu caves, Kuala Lampur, Mammoth caves, Kentucky and Carlsbad cave, New Mexico, in the USA, and Postojna caves, Yugoslavia.

The major Limestone Regions of the World:

  • Other regions include the Causses district of southern France, the Pennines of Britain, Yorkshire, and Derbyshire in particular, the Kentucky region of United States, the Yucatan Peninsula of Mexico, the Cockpit Country of Jamaica, and the limestone hills of Perlis.

Human activities of Karts Regions:

Karts regions are often barren and at best carry a thin layer of soil. The porosity of the rocks and the absence of surface drainage make vegetative growth difficult, so that limestone can usually support only poor and short turf, some sheep grazing is possible. Limestone vegetation in tropical regions, however, is luxuriant because of the heavy rainfall all year round. Settlement is scattered and the population is often sparse. The only mineral of importance is lead which occurs in veins in association with limestone. Besides this, good-quality limestones are often used as building material or quarried for the cement industry.

In West Malaysia, the limestone outcrops of the Kledang range and the Main range are quarried for the Pan-Malaysian and Tasek Cement Works.

Chalk:

The landforms of chalk are rather different from those of other limestones. There is little or no surface drainage and valleys that once contained rivers are now dry. These are often called coombes. The chalk forms low rounded hills in southern and southern eastern England, where they are called downs, and in Northern France. 

The chalk is covered with short turf, and in places with woodland, and is used for pasture and sometimes for arable farming. Because of the friable nature of the rock, swallow-holes and underground cave networks do not generally develop.


Previous Page: Physical Geography (Part-7): Arid and Desert Landforms

Next Page:Physical Geography (Part-11): Islands & Coral Reefs


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