The Cerknica Field

The Cerknica field (Cerkniško polje) is the largest karst polje in Slovenia. It includes the periodic Cerknica lake that belongs to one of the most exceptional natural curiosities. At high rainfall is covers 26 square kilometers and is the largest lake in Slovenia. During floods its level rises for almost six meters. The bottom and the surroundings of the lake consists of Upper Triassic dolomites, and Jurassic and Cretaceous limestones and dolomites. The polje is transected by several tectonic lines. One of these, the Idrija Fault, is one of the more important in Slovenia. In 1511 a devastating earthquake that caused heavy damages happened along this fault zone, and as a result, a part of the Idrija mercury deposit was displaced along it.

The Polje of Cerknica. Photo: Rajko Pavlovec

The Cerknica lake displays all characteristics of a karst polje: karst springs, estavellas, swallow holes, and a very complicated underground hydrological system. Waters that disappear underground in various swallow holes reappear in various, at times very distant springs. Many karst caves are found in the area. One of them is the Križ cave (Križna jama), one of the most beautiful and most extensive karst systems in Slovenia. It is famous by numerous small underground lakes and pretty ornamental dripstones.

The Cerknica lake has also a considerable historical significance. It was described by Janez Vajkard Valvasor in his famous book on Carniola »Die Ehre des Herzogthums Crain« (1689) in which he tried to explain the mechanism of the periodic lake. He submitted the description to the London Royal Society (1687), and was accepted as a member the same year. In 1747 the Cerknica lake was described in detail by Franc Anton Steinberg (»Gründliche Nachricht von dem in dem Inner-Crain gelegenen Czirknitzer See«). These works made Cerknica lake well known beyond the boundaries of Slovenia.

In addition to the geological and speleological aspects, the Cerknica lake is interesting also from a botanical and, especially, an ornithological view. Numerous water-living birds rest here, particularly during bird migration. The lake is included in the list of International Council for Bird Protection as an important region in this respect.

The Cerknica lake constantly changes its appearance and looks always different from one one visit to the next. At one time it is full of water, and at another time the peasants mow there grass. In the remaing ponds when the lake empties, numerous fishes get caught and are then collected by the natives. Several years ago the swallow holes that drain water underground were closed in order to promote local tourism. The intention was to extend the time of high waters, and to build hotels and other touristic objects. The plan failed, since water always found new pathways to escape underground. Most probably the extended high water period would also result in increased growth of water plants which would make the lake unattractive for tourism.

Rajko Pavlovec

The Polje of Cerknica in Slovenia
Photo: Rajko Pavlovec