THE KARST LOWLANDS
Along the north-eastern edge of Snežnik and Javornik, following the Idrija tectonic rupture line, there is a string of karst fields and waters which comprise the network of the upper, karst portion of the River Ljubljanica. In the characteristic northeast to southwest Dinaric direction, Babno plain with the River Trebuhovica is followed by the Loka Valley and the River Obrh, then the Cerknica plain and Stržen and the renowned intermittent Lake Cerknica, Rakovo hollow, near which the collapsed valley of Rakov Škocjan and the River Rak are located, and the Planina plain and the River Unica. Each of these karst plains lies at a lower sea level than the previous; this is why all the listed rivers are subterraneanly connected, and for the eastern karst portion of the River Ljubljanica basin, labelled by Prof. Šušteršič as the “river of seven names”.
The Babno plain is situated in the Loka Valley. The settlement with the same name is located in a shady location and has an extremely cold climate; due to the closed nature of the basin, winters here are harsh, earning it the title of “Slovenian Siberia”. Due to a temperature inversion, the record low temperature of -35°C in Slovenia was recorded in Babno Polje in 2005. Even in summer, the temperatures remain low; for example, a temperature of -2.4°C was measured in Babno Polje in July, which is considered the warmest month.
Rakov Škocjan is a 3-kilometre long karst valley, and is the result of a collapsed cave ceiling. The River Rak springs from the Zelške cave system and is joined by tributaries flowing from below Javornik and sinks into the Tkalca Cave. Rakov Škocjan is characterized by steep overhanging rocks, several collapsed caves, and two natural bridges, which are the remains of a cave ceiling. A Natural Theme Trail winds through the valley.
The Planina plain is the last in the series of karst plains in the Ljubljanica river basin. Due to its characteristics, which are common to all karst plains, it is a typical representative of this type of formation in Slovenia. All the waters flow underground to and from the plain. The plain is surrounded by hills, and over its rather flat sediment covered dolomite bottom flows the meandering River Unica. The bottom of the Planina plain is covered in grasslands. Due to the frequency of floods, fields and settlements have been withdrawn to the elevated parts along the edge of the plain. The marshy grasslands of the Planina plain is the only site in Slovenia where the Meadow Squill Scilla litardierei) can still be found, reaching the extreme north-western part of its habitat.
The Loka Valley is a completely enclosed karst hollow with a rather flat semi-permeable bottom and is surrounded by high karst hills, from Bloke to the Račna Mountain in the north and east to the mighty Snežnik in the south and west. Snežnik is not only the highest but also the most imposing peak of the Loka plain mountain range.
The Cerknica plain is the largest karst plain in Notranjsko (70km2). Its eastern border is composed of almost non-permeable Upper Triassic and Jurassic dolomite, resulting in surface tributaries in the eastern part of the field: the River Šteberščica, River Lipsenjščica, River Žerovniščica, and to the northeast, the River Cerkniščica. The western part is composed of permeable Cretaceous limestone, while the slopes of Javornik decline steeply until they meet the fields. At their base, a large number of karst springs can be found, where the waters from the Javornik underworld, as well as Loka Valley, emerge. Together with surface tributaries, the springs yield enough water to keep the renowned intermittent Lake Cerknica filled throughout most of the year.
The Pivka Valley was already named as early as 1300. According to some theories, “Pivka” means a “water-swallowing sinkhole”. On the other hand, Melik, a renowned Slovenian geographer, is of the opinion that, since the flooded plains and lakes are dried by the porous soil, the name is not derived from a sinkhole. This peculiarity has thus given name to an entire region. The River Pivka slowly flows and meanders through the Pivka Valley, and typically floods when it rains. At the edge of the karst Postojna plain, the River Pivka disappears underneath Sovič and makes its way towards the underwater confluence withthe River Rak inside the Planina Cave. The Pivka again surfaces as the River Unica . The River Pivka’s underwater course has also created the largest Slovenian cave, the Postojna Cave.