> THE KARST - The different areas of the Karst
The conditions of circulation of the water, which directly influences the mechanical process of erosion. This leads to recognisable differences in various areas of the karst.
Limestone is dissolved by water relatively slowly. For a substantial amount of limestone to be dissolved, water must stay in contact with the rock for a long time. Water flows from top to bottom of the limestone formation by gravity, and uses various conduits (fissures, faults, chimneys, galleries). When water flows quickly, it stays in contact with the rock for a short time, so little is dissolved. On the other hand, when it flows slowly much more is dissolved. In addition, the wider the space more quickly the water flows whereas it is slowed down in narrow conduits. Since a slow flow speed allows more limestone to be dissolved, narrow passages tend to be enlarged. In time, this creates larger spaces where the water will flow more freely. These phenomena of circulation and of dissolution therefore interact and their effects tend to cancel each other out, causing a phenomenon of self-adjustment which ultimately organised the drainage. This organisation from top to bottom (or from upstream to downstream) gives the karst system a vertical and horizontal structure. Several areas are identifiable in the karst: The epikarstic area - The area of infiltration - The area of drainage - The drowned area
Epikarstic area.
On the surface and in the immediate proximity, the water, having fallen through the atmosphere and the ground, is rich in carbon dioxide. It is therefore very corrosive and able to dissolve a large amount of the surface of the rocks. As the superficial erosion wears the earth away, the bare rock appears scored with cracks large and small, separated by often sharp edges. This type of morphology, characteristic of the surface rocks in karsts is called a limestone pavement. The term in Savoy is "lapiaz", meaning the particular surface appearance of the rocks, broken into blocks, scored with lacerations and hollowed by channels. The cracks are the direct result of dissolution: the dissolved minerals have been carried away by the run-off water leaving empty spaces hollowed into the rock. This double action of the water, dissolving and eroding, continues its slow work in the cracks, widening them and linking them together to form the typical karst landscape, with its strange formations and pillars of rock divided by deep ravines making tower-like structures. On a larger scale, the dissolution of the surface rocks and the washing away of the dissolved chemicals into in the fractures and the empty spaces below will create circular or oval depressions known as “dolines” or sink-holes. Often these depressions join up in larger groups, and the resultant depression is called a "polje'. Sometimes the dissolution enlarges the fissures into large openings, leading beneath the surface into substantial empty spaces, creating "potholes" or "swallow-holes", where the water runs down into the deeper areas. Sink-holes, poljes, rock towers, swallow-holes and potholes are the classic features of a Karst landscape.
Area of infiltration.
Directly under the superficial part, the water flows deeper, either taking advantage of larger channels caused by particularly active dissolution, or trickling through existing fissures that the corrosion will then enlarge. So, two methods of circulation are typical of this area:
- Rapid infiltration through wide channels where the water flows freely.
- Slow infiltration through a system of narrow cracks where the circulation of the water brings with it air and carbon dioxide.
Area of drainage.
Deeper still, the double action of the corrosion enlarging the cracks, and the circulation which scours them out, encourages communications between the empty spaces. The water-flow which was initially largely vertical now uses more horizontal routes creating local underground rivers which drain the groundwater towards the lowest level and the exterior of the formations.
Drowned area.
Around the areas of drainage there will still exist empty spaces full of water which flows very slowly, because of the narrowness of the channels linking them to the main watercourses. In these partially or totally drowned caves, dissolution can still continue its slow process of corrosion, forming vast reservoirs.
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