Slovenian caves and their telliness

Photo: Darinka Mladenovič

September 2011

Volcanoes are a dream of every young geologist; in Slovenia, limestone, dolomites and Karst may be found almost everywhere, since carbonate rocks cover over 40% of the territory.

According to the Slovenian karstic terminology, a cave is an underground cavity that is passable by a human being and is longer than five metres. Currently (June 2011), 10,250 caves are registered in the Cave Registry of the Karst Research Institute at ZRC SAZU and the Speleological Association of Slovenia. The Postojna cave system is the longest with 20,570km, followed by the Kačna cave with 13,250km and the Predjamski system with 13,092km. The deepest caves in Slovenia are the Kanin massif caves, of which the Čehi II cave with 1,502m is the deepest, next is Mala Boka with 1,319m and the Renejevo brezno cave, with 1,242m.

Water forms various cave passages

Markov spodmol. Photo: Nadja Zupan Hajna

Karst caves are usually the result of the corrosive extension of cracks in carbonate rocks, which occur along ruptures, cracks and lesions.  Caves are actually underground channels, a product of rainwater running through the Karst.  The tunnels may have been formed in the phreatic (saturated), epiphreatic or vadoze (unsaturated) zone.  Each manner of running of the water forms various cave passages; during Karst development, these zones move downward, which results in a complex system of caves of various ages and forms.  Exceptions are caves that, for example, result from the water running into the Karst from large depths, are warmer and contain H2S or CO2 in higher concentrations; such caves are called hypogenic caves.

Vertical abysses that are formed by rainwater are usually cracks extended by corrosion, and are some ten metres deep.  On high plateaus, level abysses that may reach great depths are frequent.  Large horizontal caves were formed at the Karst water level and have been stable for some time.  In this way, large sinking or outlet caves were formed in the river basin of the Karstic Ljubljanica and along the underground current of the Reka River.

Photo: Darinka Mladenovič

The Postonjska cave is the longest and the most attractive Slovenian cave, as well as being one of the most famous Karstic caves in the world. The cave was formed at the contact of cretaceous limestone and Eocene flysch.  The Pivka river stream disappears into the cave. It has an extensive tunnel system and abundant stalactite decoration, which has been attracting crowds of tourists for several centuries.  Data obtained by means of various geological analyses indicates that Postonjska cave has gone through various periods of sediment depositing and erosion.  Certain parts of the cave or tunnels were filled up and emptied several times during the rather long development of the cave; this is proved for example by cement alluvia on the walls of Stara cave, and elsewhere.  The lengthy (assumed to be about 2 million years old) hydrological system, that was more or less stable, between Pivška kotlina – Postojnska cave – Planinska cave and Planinsko polje provided for the development of an extensive cave system with a complicated morphology in the relatively thin rock package.

The Škocjanske caves are 5,800m long and a 250m deep-water cave at the SE part of the Karst. On account of their huge channels, underground canyon, several collapsed dolines and extraordinary history of cave researchers, in 1986 the caves were included in the UNESCO list of natural heritage sites. The caves and the above surface are also preserved as a Regional Park

Križna cave. Photo: Nadja Zupan Hajna

For me, cave sediments are the most interesting. In general, they may be divided into autochthonous, which originate in caves (calcareous sinters, sinking materials) and allochtone, which are brought into caves from the outside.  Their composition, graininess and sedimentation method reflects the climate and developments outside and inside the caves.  We use various methods to acquire as much data as possible from such sediments. 

However, in spite of all the research done in relation to caves and the studying of their content in Slovenia, there is still a lot that we do not know.  There are still a number of unexplained questions in relation to their origin and development; if we add the number of 10,000 caves that we know, we can conclude that much work has been left for our descendants. 

Text by Nadja Zupan Hajna; full text in Sinfo, July 2011 

Postojna Cave - World renowned beauty

Its 21 km of galleries, tunnels and magnificent halls make the Postojna Cave one of the world’s best known tourist caves. Its many unique, picturesque features have been enchanting visitors from all over the world for almost 200 years. The Postojna Cave is the only cave where you can experience an exciting train ride through a breathtaking underground landscape. Riding under magnificent arches is an experience you will never forget.

The Postojna Cave is the home of almost 100 species of animals living in complete darkness. The most fascinating among them, the Olms, have been exciting peoples imaginations for centuries, and it was one believed that they were the offspring of a dragon. They are blind and can live up to 100 years.  
A tour of the Postojna cave takes around 90 minutes and is available in 15 languages. No special equipment is required, but a warm sweater and comfortable shoes are recommended, as the temperature in the cave is 10 °C.

Postojna is situated in the south-west of Slovenia, almost exactly on the arithmetic intersection of the route between Ljubljana, Portorož, Rijeka and Trieste. Many railway and bus connections lead to Postojna from the mentioned locations. By car Postojna can be reached on the A1 motorway from the directions of Ljubljana, Koper or Triest, choosing exit »Postojna«.