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From the Valley to the Plateau

The World of Slope Processes and Karst Phenomena

Rising on the north and east edge of the Vipava Valley are the picturesque, mostly limestone walls of the south edge of the Trnovo Forest Plateau and the south and west edge of the Nanos Plateau. Due to their high level of conservation, a myriad of geomorphologic and geologic features and botanical peculiarities, both areas were labelled as national natural heritage and protected as a natural park.

There are many alpine and other trails starting in the villages down in the valley, which run over rubble-covered slopes and steep walls to the edge of the valley and to the edge of the Trnovo Forest Plateau and Nanos Plateau, which boast impressive summits such as Čaven (1186 m), Kucelj (1237 m), Otliški Maj (847 m), Kovk (963 m), Pleša (1262 m) and others. Overcoming an altitude difference of almost a thousand metres is quite a feat but worth it because of the beautiful vistas of the Vipava Valley opening up as one makes the trek. Springtime in particular is especially generous with colourful scenes of the local flora and fauna.

The slopes and rocky terrain provide an outdoor classroom teaching us about slope processes and karst phenomena up close. The slope processes which are the direct result of the geologic composition of this area, of the shape of the surface and strong mechanical and chemical weathering of the rock in different climate conditions, have always been very intense and continue to be so. Mechanical weathering and collapsing of carbonate rocks at the end of the Ice Age were by far the most prominent processes. The disintegrated rocks slid down into the valley and created massive amounts of slope rubble that now covers the slopes below steep walls. In many places the rubble coagulated to form breccia. A prominent feature are individual large rocks and blocks (measuring up to several hundred metres), which broke off, rolled off or slid down from the rocky precipice into the valley. Examples of this are: Mala Gora, the very prominent breakdown called Vitovski podor, nestled between the Veliki rob and Vitovski hrib, Gradišče, Visoko, and other blocks.

The surveys conducted during the motorway construction on the slopes of Nanos and in the valley revealed that this area teems with fossil landslides. A remarkable example is the Selo fossil landslide which was activated more than 42,000 years ago, covering the wider area that is today occupied by the villages Selo, Črniče and Batuje with over 150 million cubic metres of material, the landslide brought down a 50 metre thick rubble layer. Being a fixture in this area, landslides make this area one of the most landslide prone areas in the country, which is highlighted by the many still active sliding processes and landslides, for instance the landslide Slano blato, the landslide in Šmihel, the landslide in the village of Stogovce over Lokavec and the Znosence landslide under the settlement of Col.

The rocky, mostly forested terrain of the Trnovo Forest Plateau and Nanos Plateau, which rises over the valley, features no surface water and is karstified. The area is noted for its karst features which were created on account of the specific rock type. The area is mostly built from carbonate rocks (limestones, dolomites) of Mesozoic age, i.e. 100 to 220 million years old, which were created in the erstwhile sea. This is demonstrated by the wealth of fossilized remains of shells, snails, corals, sponges, echinoids and other organisms preserved in the rocks. There are many fossil sites on Gora and the Trnovo Forest Plateau and Nanos Plateau. The rich sites of well-preserved fossilized corals and sponges in the Jurassic reef limestone that makes up a large part of the Trnovo Forest Plateau are especially well-known and valued among the expert community. Not many people know that 150 million years ago a massive coral reef similar to the modern Great Barrier Reef in Australia (Goričan, 2010) took up the 10–20 kilometre wide section from the Soča Valley via the Trnovo Forest to Bela krajina (White Carniola) and further toward Croatia, Bosnia and Montenegro. Throughout the geological past, seas had withdrawn, the invisible forces within the depths of the Earth caused the terrain to rise, fall, thrust, break and fault, creating mountains, hills and valleys of today. This might sound straightforward but in reality it was far from that – it was an interplay of many factors and processes over a very long geological period. The walls of the south edge of the Trnovo Forest Plateau and Nanos Plateau as they are today constitute the thrust edge of the so-called Trnovo and Hrušica footwall, in which Nanos is a thrusted and overturned fold.

Up on the surface, the limestones were exposed to various weather conditions. The most distinctive property of limestones is solubility in acidic water. The precipitation water dissolves the rock and carves out intriguing shapes on the surface and underneath it. Such karst phenomena can be observed and examined on the Trnovo Forest Plateau and Nanos Plateau, as well as at other locations where there is limestone. Those karst phenomena that particularly stand out due to their distinctive properties were declared natural heritage or as having an outstanding natural value, while some were even protected as natural monuments.

A common group of karst landforms are solution pans, which are oval-shaped hollows carved into the rock, which can reach lengths from a few centimetres to around one metre. They emerge due to rock erosion caused by the standing water that gathers after rainfall. The hill of Križna Gora uphill from Ajdovščina boasts the biggest solution pan in Slovenia (Orjaška škavnica), which stretches over ten metres in length and around five metres in width. Among the smallest surface karst phenomena are channels which are elongated, parallel-running hollows carved out by water running over the rock in the direction of the steepest slope. A similar phenomenon, but on a larger scale, is karren (škraplje in Slovenian) which are created when the rock dissolves faster along the fissures and other less resistant surfaces on the main rock. Extraordinarily well-developed karren can be observed off the main road between the village of Gozd and Kovk below the hill of Sinji vrh. On the surface there are very visible thick strata of light limestone furrowed with deep karren and channels, creating an outstanding microkarst relief.

Among the most remarkable karst landforms is surely the Otlica Window. From the valley it appears as a hole in the rock wall, but looking at it from the north side, it offers an extraordinary view of the Vipava Valley. Called the “Hole” or “Hollow” (Otlica or Votlica in the local dialect), it lent its name to the village of Otlica from where a nice walk is to be had to this astounding site. Another way to get to the Otlica Window is via the trail Pot po Robu (from the village of Predmeja to Col), running between lovely meadows and offering scenic views of the Vipava Valley, the Karst and the Adriatic Sea. The Otlica Window has an upright lens shape and is circa ten metres high and up to six metres wide. According to legend it was formed by the Devil’s horn. Scientists account for the genesis of this orifice, i.e. natural bridge, with a powerful tectonic fault which is also very noticeable in the wall over the orifice. The rock cracked because of the force generated by the fault and became weaker. This made it more susceptible to weathering, washing away and erosion, which created the natural window/bridge. Some 20 metres downhill of the big Otlica Window is another similar but smaller window. It is around four metres tall and two metres wide, but it is out of direct sight and therefore less known. The light grey Jurassic limestone in which the windows were created also features fossilized remains of corals and sponges.

A very distinct and common karst form on the plateaus of the Trnovo Forest and Nanos are karst hollows of various shapes and sizes, sinkholes reigning supreme. Larger, closed hollows of irregular shapes with a dissected, partly levelled bottom are referred to as uvala by the experts. Locals from the central part of the Trnovo Forest call them lazna (e.g. Mala and Velika Lazna, Avška Lazna, Krnica and Smrečje). The deepest karst hollows with steep slopes and irregular rims are called draga by the locals. During the Ice Age they were significantly shaped by glaciers. In the extreme northwest part of the nature reserve called Golaki lies the hollow of Mrzla draga. Mojska, Smrekova draga and Črna draga are lined up in the southeast direction. Due to their large size cold air lingers at the bottom, creating a temperature and vegetation inversion, giving rise to frost hollows. The biggest and most well-known frost hollow is Smrekova draga. Other large-scale karst depressions, referred to as konta, are typical for the Golaki ridge and higher areas of Čaven (Kodelja, 2013). Their slopes are steep and rocky, the bedrock is dotted with cup formations and cup-shaped sinkholes.

The water percolating through the permeable rocks has expanded the fissures and created a captivating underground karst world of caves and shafts. The predominant form on the Trnovo Forest Plateau and Nanos Plateau are shafts, with a handful of horizontal caves thrown into the mix. The deepest cave in the country, outside of the Alps, was discovered northeast of the village of Predmeja, going 884 metres deep. A peculiar phenomenon among the caves on the Trnovo Forest Plateau and Nanos Plateau is the occurrence of ice caves where snow and ice is present year round. The most famous is the Big Ice Cave at Paradana. It is also noted for having been used to harvest ice in the late 19th century, the blocks of ice loaded on to carts to be sold in Gorica or Trieste.

Tanja Lukežič, B.Sc. (Geology)
Institute of the Republic of Slovenia for Nature Conservation, Nova Gorica Regional Unit

Sources and literature available with the author.