The following are the erosional landforms that characterize a karst region

The following are the erosional landforms that characterize a karst region:

(i) Terra Rossa:

The solvent action of rainwater dissolves a certain part of the surface rock and percolates into the ground leaving behind a residue of a red, clayey soil on the surface. This red soil may extend down into the opened joints.

This kind of residual material is not found on steep slopes. But it is present on gentle or moderate slopes. Its thickness may vary from a few centimeters to many meters. Sometimes the rock surface is completely covered by it.

Terra rossa resembles in appearance the lateritic soils found in tropical regions. It is found in tropical or subtropical regions, but it is also encountered in certain parts of southern Europe and southern Indiana.

(ii) Lapies:

In German lapies is known as Karren, and bogaz in Yugoslavia.

In areas of high relief with steep slopes when limestone is exposed in the absence of terra rosa, the surface water or the soil water cuts small solution furrows. It is to the rugged surface of limestones in the karst region that the term lapies is applied.

Closely spaced small solution channels are known as rill lapies. According to Civic, lapies is found mainly on outcrops of bare rocks. According to him, the following are the important factors which influence lapies formation: composition and texture of the rock and structure and surface slope.

Natural vegetation also exercises some control on its formation. The lapies is of various sizes and forms. Its depth varies from 1 cm to about 1 meter. There is also a great variation in its length and breath. Lapies refers to the extremely rugged surface developed on the surface of pure limestone.

Sinkholes:

Sinkholes are the most common features formed by the work of rainwater on limestone. Topographically, a sinkhole is a shallow depression or a pit that varies in depth from a few meters to a maximum of 30 meters or more.

Majority of sinkholes range in depth from 3 to 10 meters. They also differ in their real extent (from a few square meters to several hectares).

Sinkholes never occur in isolation, rather they are found by hundreds and thousands where a limestone bed is being attacked by solution. Sinkhole is a funnel shaped depression in the ground surface of a limestone formation.

A sinkhole is formed by subterranean collapse of a cave or by surface solution and it is roughly cylindrical in shape. Thus, sinkholes are of the following two categories:

(i) Those that develop slowly downward by solution beneath a soil covering. In this case there are no disturbances of the rock.

(ii) Those that are produced by the falling down of rock above an underground cave. The sinkholes are, therefore, referred to as solution sinks and collapse sinks.

Doline:

The term ‘doline’ is used for solution sinkhole. Solution of limestone along intersecting joints produces circular depressions.

These depressions gradually develop and join so that the whole land is lowered giving rise to a landform called ‘doline’ which comes from the Serbian terminology ‘dolinas’. The shape of a doline is circular and its sides are steeper.

Swallow holes:

Swallow holes represent the surface an opening through which surface runoff enters into dolines. This is the point at which a surface stream disappears underground in the karst region.

It is called water sink in the U.S.A., embut in France, and Ponor in Yugoslavia. Some swallow holes are merely a point where a surface stream gradually dries up as it percolates through its gravels.

Surface streams that disappear down a swallow hole may reappear again at the surface in another location a few kilometers away. Swallet is another name for a swallow hole.

Uvala:

Dolines which are in close proximity, sometimes, merge together as a result of which a large surface depression is produced. Uvala is characterized by having an irregular floor which is the result of former elevations, and character of the degraded slopes of the dolines. The floor of an uvala is different from that of a polje whose floor is smooth.

Karst Window:

Collapse of the upper surface of sinkholes or dolines accounts for the formation of karst windows. It is through these open parts that the subsurface features formed by the underground drainage are visible. So these open parts are called karst windows.

Polje:

The term polje refers to an elongated depression in karst terrain. The long axis is developed over a lengthy distance parallel to the structural grain. This kind of depression is thought to have been produced by the merger of collapsed cave systems. The floor of polje is invariably covered by alluvium.

Cave or Cavern:

Caves or underground caverns present the most important land forms produced by the solution of limestone. One of the most important factors for cavern development is the jointing of the limestone strata. Ground water solution works along jointing planes.

A cavern may be in the form of a passage or a gallery, its shape depending in part on the joint pattern or structure of the rock, and partly on the type of process involved in its excavation during the course of solution of limestone by ground-water extensive cave systems develop.

In India such caves are found in the Meghalaya Plateau near Cherrapunji, near Dehra Dun in Uttaranchal, in southwestern Bihar, and in Chitrakut in Madhya Pradesh. In the U.S.A. the Plateau of Kentucky is said to have a large number of complicated cave-systems.

Of these, famous Mammoth Cave is the most important example of an underground cave. It may be pointed out that the underground features of karst include, among other things, caves or caverns and their connecting tunnels. The chambers and tunnels associated with these caverns run vertically and horizontal y depending on rocks structure.

Of course, their size and shape are controlled by their location, above or below the water table. There is great variation in cavern forms. This indicates variation in the mode of origin.

Generally large caverns appear to have formed either at the water table, where rate of solution is most rapid, or below the water table in the phreatic zone. Many smaller caverns appear to have developed in the vadose zone entirely above the water table.

In fact, civern development is a very complicated process which involves many variables like rock structure, groundwater chemistry, hydrology and tectonic and erosional history of the region.

It may be pointed out that caves generally form just beneath the water table, where a later lowering of the water level exposes them to further development.

The dripstones that form under such conditions are produced as water containing dissolved materials gradually drips from the ceiling of the cave; Calcium carbonate precipitates out of the evaporating solution and accumulates at a point below on the cave floor.

Depositional features called stalactites grow downward from the ceiling and stalagmites build upward from the floor. Sometimes these features grow until they connect and form a continuous column.

These features are depositional in nature. There are large caverns in the United States which occur in Mississippian limestone formations around the margins of the Appalachians including Western Virginia and Kentucky.

The origin of limestone caverns has been the most controversial, and various theories regarding their mode of formation have been propounded.

The Two-cycle theory of Davis, Water-table theory of Swinnerton, and the Invasion theory of Malott have shed lights as to how the underground caverns were formed. But no single theory could get universal acceptance.