Chýnov Cave - Natural conditions
GEOLOGY OF THE AREA
A major part of Southern Bohemia belongs to one of the most ancient geological areas of the Bohemian massif core, eferred to as the moldanubic according to the Latin names of the Vltava and the Danube rivers. It consists of especially metamorphosed rocks – gneiss, mica schist and migmatite, penetrated with volcanic rocks. However, the stratigraphic layout of the moldanubic rocks is not totally clear so far; usually, there are two basic groups of layer succession - the older being referred to as featureless, the younger as varied. The varied group is characterized by copious inserts of divergent rocks such as calcites, amphibolites and erlanes. This group includes hard-grain calcites (marbles), where the Chýnov Cave was formed.
The base of the featureless group rocks is believed to have originated as early as the Proterozoic, i.e. more than a billion years ago; the formation took place in a vast sea basin. From that time onwards, active volcanic activity took place in several areas. Besides underwater eruptions of lava, the active volcanoes also emitted large quantities of ash and fractions of rocks. Ample remainders of simple organisms with calcite shells settled, together with volcanic materials in a sea that had by then become shallower and thus formed layers of original sedimentary calcites. The orogenetic processes that took place in the following geological eras metamorphosed the sea bed sediments under high pressure and temperature. In this way the crystalline calcites (marbles) were formed; the volcanic products turned into the present amphibolites. The rocks were broken in repeated tectonic cycles and corrugated in mountain ridges that ceased to exist a long time ago. The older orogenetic stages that contributed the most to the present geological structure of the area include the Variscan orogeny in the Palaeozoic, approximately 380–250 million years ago.
A 100 – 150 m thick and tectonically considerably eroded horizon of crystalline calcites embedded together with the amphibolites in surrounding paralic formations ranges from Velmovice, via Pacova and KladrubskáMountain, towards Lejčkov in the total length of 4–5 km. The entire formation inclines to the north under a gradient of 40-50 degrees. The karst system of the Chýnov Cave was formed in a bed of hard-grain marble, called “řeďák” in colloquial Czech, which forms only a 10 m thick layer surrounded by the amphibolites.
HYDROLOGY
In spite of the long-term research the nature of the underground flow and its relation to the surface hydrographology of the area remains unsettled. The courses of the underground flows east of the cave, i.e. the inflow and its sources, are still unknown.
The outflow has been examined somewhat more thoroughly. Even the first expert report of 1863 executed by Dr. Frič and Prof. Krejčí contains the hypothesis that both of the then known lakes (Devil’s Lake and Purkyně’s Lake) are connected. However, not until the 1940s did Vl. Homola and C. M. Schüller prove that the water in the Chýnov Cave was only a small section of another subterranean stream, which had contributed significantly to the formation of the entire system. Although several tracking tests were carried out at that time, the place where the water rose was not discovered. This problem was not solved until 1962 when F. Skřivánek performed tracking tests using a fluorescent pigment and proved that the water from the cave rose in the Rutice Spring, approximately 1.5 km from the cave. At the same time the explorers managed to get into the system of corridors through which water flows from Purkyně’s Lake towards this karst spring. However, the progress was brought to a dead stop at a shallow siphon filled with detritus from a collapsed corridor. The complexity of the watercourse between the cave and the karst spring was also proved in the following years by research conducted by V. Mach.
The discovery of permanently flooded areas east of Homola’s Lake, i.e. opposite the inflow of the underground water, was another great surprise. Divers from the Czech Speleological Society managed to get 140 m upstream and reached a depth of 45 m below the level.
The last section of the watercourse connecting the already known areas of the cave was not discovered until the water in the siphon located between the Devil’s and Purkyně’s Lakes was pumped out in 1993.
The research carried out so far shows that the water of the subterranean stream flowing through the Chýnov Cave permeates from the surrounding non-karst rocks. It flows through a calcite zone that forms drainage, at the edge of which the water rises to the surface at Rutice. Its course even flows under several surface streams, whose water, however does not permeate underground.
The temperature in the cave is very stable: 8.7 °C, the flow rate is between 6-9 l/s.
In recent years the underground flow rates have been observed systematically, and so has their dependency on the waterfall, which may result in new facts about the sources of ground water. The Rutice Spring is now a source of drinking water, whose high quality was provided by pronouncing the area a hygienically protected area in 1992.
MINERALOGY
The mineralogical exploration of the area began in the 19th century, mainly due to the Schwazenbergs' quarry set up for the purpose of exploiting calcite on the south eastern slope of Pacova Mountain. After that, extraordinarily varied mineral associations were proved and nowadays there is no doubt that the Pacova Mountain is remarkable mainly due to the quantity of minerals described within a single locality. Nowadays, more than sixty are known and the importance of the location exceeds the regional scale.
The National Museum’s expositions include a 47 cm, 20 kg smoke-stone crystal found near Chýnov in the 1950s. A pargasite variety with an extraordinarily high content of aluminium has been described in the mine. The Pacova Mountain is the world’s second excavation of this stone, after Cashmere. In the amphibolites forming the roof of the calcite formation, the effects of the mineralization of the alpine veins have been discovered over the past few years. Interesting samples of garnets, diopside, arsenopyrite, dravite and titanite found in Pacova Mountain are also worth mentioning.
The mineralogical richness was caused by the repeated metamorphoses of the group of rocks of different petrography and chemical compositions that resulted in several different mineralization processes.
More than one third of all minerals described in the locality have been discovered in the Chýnov Cave system as well. Some of them were unknown from Pacova Mountain until then. Minerals that are contained directly in the calcite also influence the colouring of the walls of the cave. After the less resistant calcite dissolves, most of them remain in the cave sediments, the exploration of which brings new facts in the field of mineralogy. The violet variety of tremolite – hexagonite – that was discovered recently is truly unique. Until then this mineral was known in only very few places in the USA and Canada. The Chýnov Cave thus represents the only locality where hexagonite can be found outside North America.
The minerals frequently fill tectonic cavities in rocks. Palygorskite, popularly called “rock rose”, is one example of such a mineral. Although it is a mineral, it is very flexible and soft in moist environments.
As the corrosion of calcites continues under the surface of the subterranean stream, the forms of more resistant minerals remain on the walls. This phenomenon is referred to as selective corrosion. Formations of quartz, chalcedony or palygorskite hang down the ceilings and walls of the corridors and resemble curtains. The exploration of these mineral forms underground was impossible until explorers managed to reach the permanently flooded areas of the cave. These formations are subjected to quick destruction outside water and are hardly ever preserved.
Some layers of crystalline calcites contain opal, which is however only apparent in ultraviolet illumination, in which it is marked in bright green. Opal is also contained in the sinter fillings of the cave.
BIO-SPELEOLOGY
Unlike many other caves, ChýnovCave is not connected to any palaeontology findings. This underground cavity is likely to have been inaccessible to larger animal species.
These days the cave is inhabited by several animal groups. Invertebrates are represented by certain species of snails, crustaceans and insects. Over the past decades the Meta menardi (European cave spider) has successfully inhabited the corridors near the surface.
However, bats are certainly the most attractive of the inhabitants of this underground world. So far 10 species of these representatives of the Chiroptera order have been registered in the cave. Given the numbers of hibernating individuals the cave can be considered to be the largest known winter quarters of Natterer’s bat (Myotis nattereri) in Europe. Regular inhabitants include the Greater mouse-eared bat, Daubenton’s bat, Common long-eared bat and recently also the Barbastelle. The Serotine and Whiskered bats represent irregular visitors to the cave. The presence of the Geoffroy’s bat and Brandt’s bat in the location is rather exceptional. When catching bats in nets outside the hibernation period we rarely catch the Bechstein’s bat.