Ground-water is that part of the sub-surface water which fully saturates the porespaces of the bed-rock and regolith. Surface water, after infiltration, becomes a part of the underground water and it is also known as sub-surface water. The -ground-water occupies the saturated zone; above it is the zone of aeration. The upper surface of the saturated zone is the Water-table.
Mechanical process of erosion is absolutely insignificant in case of underground water. It brings about erosion only through chemical processes, i.e., by the solution action of the underground water. This process is particularly effective in regions of soluble rocks like limestone, dolomite etc.
Topography developed due to the action of ground-water is known as ‘Karst-topography’, after the occurrence of the typical limestone topography in the Karst area of Yugoslavia. Karst topographic features develop on both over and under the ground.
Karst topography develops very well where the following two essential conditions are fulfilled:
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(i) The soluble rocks are located near or at the earth’s surface.
(ii) The rocks are dense, highly jointed, and thin-bedded.
Two secondary conditions also favour these developments which are: (a) the presence of a deeply entrenched valley of a master stream, and (b) a moderate amount of rainfall.
Erosional Features:
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(i) Lapies:
The leaching action of the ground-water as it passes through the limestone region, produces a highly ruined topography. The ground water may enlarge the joints of the limestone into a conjugate pattern of clefts and ridges, this surface is called a limestone pavement or lapies surface. The clefts in such a pavement are called grikes, and the ridges clints.
(ii) Sink:
It is a conical depression in the limestone which may be several metres in Diameter. It is also called ‘doline’
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(iii) Uvala. An uvala is a very large elongated depression formed by the convergence of two or more sink-holes.
(iv) Caverns:
These are hollows underground with their roofs intact.
(v) Galleries:
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These are horizontal linking passages of connecting caverns,
(vi) Shaft:
These are vertical or inclined linking passages of connecting caverns.
(vii) Some characteristic underground or surface features occur in limestone caverns. These features including stalactites and stalagmites, which together constitute what is known as ‘drip stone
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Stalactites are columns of limestone that hang from the ceiling downwards and stalagmites are the columnar features which rise up vertically from the floor of the cavern.
(viii) Polje:
Larger depressions in the landscape (covering tens of square kilometres) are known as poljes and are characterised by steep sides and flat floors If the water-table is high enough lakes may form and they are known as ‘polje lakes’.
(ix) Hums or pepino hills:
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These are small residual hills found on the floors of polje.
(x) Stylolite:
It is irregular suture-like boundaries developed in some lime stones, in which less soluble portions of any two consecutive beds project into each other, and thus forms a zig-zag line of junction of the beds concerned.
(xi) Natural bridges:
It represents the remnant of the roof of a natural tunnel or subterranean cut off.
(xii) Swallow holes:
Sometimes sink-holes become so numerous that the sides begin to touch one another. Surface drainage becomes limited to short sinking creeks, those that disappear into the ground. Along some such streams there are small holes where water swirls into small openings leading into caverns. Such holes are called swallow holes.
(Xiii) Blind valleys:
These are valleys that lead into a hill side or gradually lose the characteristics of a valley as the water from their streams is lost to sub-surface channels.
(xiv) Karst valley:
It is a very deep valley, formed by the solution process and occurs in limestone rocks.
Deposition Features:
(a) Geode:
It is a cavity in a rock lined with quartz crystals projecting towards the centre.
(b) Sinter:
Deposits of silica or calcium carbonates by ground water are known as silicious or calcareous (inters.
(c) Ranker:
These are loose gravel and alluvium which have been subjected to cementation and compaction.
Karst cycle:
It consists of four stages, of which the youthful stage is characterised by progressive expansion of the underground drainage. The mature stage displays lakes, uvalas, and caverns. The late maturity stage shows decline of karst features. The old stage reveals the reappearance of streams and entrenched valleys on the surface.