Close to the highway Ljubljana-Koper you find a beautiful small valley, which used to be a cave before the ceiling collapsed.
We are in Karst area: the underground consists largely of limestone. Rainwater slowly but surely dissolves the limestone and hollows it out. Under our feet, the rock is like an Emmentaler cheese, full of large and small holes. There is little water on the surface: rivers and streams flow deep underground. Only in some of the deepest valleys, the water reaches the earth’ surface. Rakov Škocjan is one of the smallest of these valleys but by far the most spectacular.
In a distant past, there was no valley here. Instead, there was a large underground river. Back then, this river was flowing through a spectacularly large cave and because of the eroding powers of the water. Over time, the cave became bigger and bigger. Because of this, the ceiling of the cave became ever thinner. Then, in the ice-ages cracks appeared in the cave-ceiling. In summer, water seeped through these cracks, and in winter, this water froze. At low temperatures, ice expands, and it made the cracks ever deeper and wider. Until, one day, the ceiling of the cave started to collapse!
Underneath the Large Natural Bridge, the water flows through the limestone rock wall!
The limestone rocks which fell in the river have been dissolved by the water. Nowadays, all that remains of the former cave is a lovely small valley in the middle of the forest. In most of the valley, little reminds of its past, but we can still see what has happened: upstream, the process continues! Here, the river still emerges from a large cave-system in a maze of holes and chambers with spectacular limestone cliffs. For any naturalist who visits Slovenia this place is a must-see. It is called the Small Natural Bridge, but it is small only in comparison with another rock formation, which we find at the other end of the valley.
Only 2.5 km downstream from the Small Natural Bridge, the water disappears again underground, in an equally impressive cave. Just in front of the cave, we find the Large Natural Bridge, which stands as a natural Arc de Triomphe above the river. This geological structure is so large, that there is an unpaved road over it. From this road, you have an impressive view over the valley. This all is so overwhelming that you have to experience the place yourself. Photographs simply never capture the grandeur of the place …
On the way to the Small Natural Bridge, we pass this fairytale man-made bridge …
The air is moist and cool due to the proximity of the river and the caves: ideal for Ferns and Mosses which we find in abundance. Remarkable is a stand of Peat-moss, which grows straight on limestone rock! In the coldest places, we find moss covered Maple trees, with Polypody ferns growing high up against the stems. Together with the abundant Hart’s-tongue Ferns, they give a distinct tropical impression besides the cool water.
The river itself is home to many fish. In the upstream part, we may see Grayling and also the nocturnal Burbot is present. Lower down, Chub, tench and Pike are abundant, while large shoals of Minnows roam the entire river. At some springs along the river, we can get a glimpse of the underground world of the Slovenian Karst: tiny specks of white in the sand turn out to be the remains of the shells of cave-snails, which are found nowhere else in the world! Recently, Crayfish have been observed near one of the springs. They used to be abundant here but were wiped out because of water pollution in the past. It is unclear if their re-appearance is a natural recolonisation from Lake Cerknica (through the cave-system!) or the result of a recent release.
The water of river Rak is crystal clear and, in many places, we can see fish swimming, including these Minnows.
The fast-flowing river attracts birds as well. Dippers can often be found near the Large Natural Bridge while several pairs of Grey Wagtails breed at both ends of the valley. On the limestone cliffs, we discovered a population of Horvath’s Rock Lizards, but these are not easy to find. In contrast, it is hard to overlook the numerous Edible Frogs which live along the middle part of the river. They sometimes chase the dragonflies: both Beautiful Demoiselle and the, no less beautiful, Banded Demoiselle are abundant.
The forest along Rakov Škocjan was devastated in a series of recent events and is currently starting to regrow. It started with a heavy ice-rain in February 2014 when nearly all trees lost branches and of some trees, only the main stem remained. In 2015 and 2016, Bark-Beetles increased in numbers and killed many of the Spruce, an alien tree-species which is planted for wood production. Subsequently, heavy machinery which was used to remove the dead Spruce, which destroyed much of the undergrowth, including young trees which were already present. This destruction of young trees has set back forest regeneration for several decades and opened up the undergrowth for the colonisation of invasive plants.
The air is cool and moist due to the proximity of the river and the caves: here Ferns find ideal growing circumstances.