These phenomenons are determined by rain water, enriched of CO₂ during its fall, that corrodes rocks made mainly of CaCO₃ (insoluble), turning it into Ca(HCO₃)₂ (soluble in water).

CaCO₃+H₂O+CO₂ l Ca(HCO₃)₂

a) loss of a superficial hydrographic web

b) partial or total loss of soil forms caused by running water

c) irregular plateau with steep undulations morphology, flat as a whole, even if tormented in detail

d) presence of closed depressions, basin shaped

e) presence of karstic pits, swallow-holes, and caves

f) presence of underground watercourses, karstic springs and lakes.

Concerning a), peculiar is the hydrographic system of Postumia, with split watercourses, that emerge after an underground course, or formed on a different kind of rock to hide underground when meeting karstic rock (e.g. high Timavo).

Regarding b), there do not exist either valley connected -sets, or well defined watersheds, or alluvial deposits.

About c), the surface presents mostly tablelands or massifs, horizontally stratified or slightly inclined, but cut or levelled by a previous erosion cycle. More or less sunken sections of dead valleys, hollowed out before that the progressive fissuring of soil absorbed running water. There can also be seen blind valleys without superficial outlet, hollowed out in impermeable rock, and closed by a steep rock-wall at the foot of which water enter caves or is absorbed by fissures.

Rock surface exposed to rain water presents thick groovings called "Karrenfelder", that can be divided into three kinds:

1) Rillenkarren: little parallel drills separated by thin sharp ridges, 10-60 cm long, 1-3 cm wide, 0.2-2 cm deep.

2) Trittkarren: little hollows with flat bottom and semicircular contour (5-10 cm diameter)

3) Rinnenkarren: drills many centimeters wide and deep, and few meters long.

The closed depressions hollow-shaped (see point d)) are of three sorts:

1) Dolinas, rather little, oval shaped, and more or less symmetrical depending on the location of water drainage; their diameter is of 10-150 meters, and their deepness 2-20 meters. They may be caused by superficial dissolution forms (combined action of plant roots and bacteria spreading CO₂ around a fissure), or near a most fissured area, or by slow ground subsiding, or because of the collapsing of the vault of an underlying cave.

2) Karstic tiny valleys, with shape and contour rather irregular, caused by the karstic modification of pre-existing valleys.

3) Polje, karstic basins of kilometric extension and minimum deepness.

Other peculiar things are:

1) Karstic pits, small in diameter and very deep, not rarely branched, or bell shaped at bottom, where there may be a thick web of small fissures absorbing water, or a cave with an underground watercourse (e.g. Trebiciano abyss).

2) Swallow holes, irregular cavities too small to go through, that open on the bottom of karstic depressions (dolina + swallow hole = foiba).

3) Caves, underground cavities large enough to go through, and mainly horizontal.

If karstic massif ends in the sea, the wave labour on most wide fissures or on underground water outlets, helps in the creation of caves. The tumbling down of caves of this type creates coastal dolinas.

Concerning watercourses, they show themselves on surface either when they sink underground or when they emerge from karstic soil in the middle or at the end of their course. Underground river, if compared with surface ones, have a more irregular route and motion, depending on capricious progress of fissures widened by erosion. Added to a frequent alternation of jumps, rapids, and stagnations, there are also insertions of webs of still little fissures and upset siphons that flood the watercourse, causing a raising of water, that may be seen in peculiar karstic temporary springs, that emerge on the bottom of dolinas, creating so karstic tiny lakes. Cavities where water re-emerges are called "estavelle".

Where karstic limestone is the coast submarine springs are frequent, but a peculiar kind of these, called karstic re-sources, must be discriminated: these are the places where watercourses previously sunk re-emerge (e.g. Timavo mouths in Aurisina, near S.Giovanni di Duino).

The starting point is fissured rock, with its external and internal circulation. In karstic phenomenon we can distinguish three periods or phases.

1) Youth phase

It is characterized by the broadening of superficial fissures, with consequent creation of pits, caves, swallow holes, dolinas, and larger hollows.

Superficial watercourses grow poor, while their beds change into irregular karstic flutes, with depressions in which water sinks, which will develop into blind valleys.

2) Maturity period

In this period both karstic superficial and hypogean forms develop specifying and widening themselves.

3) Senile age

In this phase, quite rare indeed, the progress of demolition, both on and under the surface, tends to bring near and fuse the topographical surface with the horizontal fissures.

Often there is the renewal of the cycle, after a lowering of the local base karstic level: once the region is raised and the fluvial excavation is reactivated, the progressive sinking of the hydrographical net bring back to surface underground water at lower levels. A second underground circulation web forms and develop horizontally at a lower level than the previous one, while the absorbing cavities develop vertically.

Springs become inactive and are substituted by other ones at a lower height.

More than sometimes there is a sinking of dry land, mostly near the coast, with a consequent raising of karstic base level (e.g. Istrian, Dalmatian, and Apulian coast; or else Capri Blue Grotto, Emerald Amalfi Grotto, in which stalagmites emerge from water, showing how these caves once were higher than the sea.

               

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