IDRIJA UNESCO GLOBAL GEOPARK
The Idrija Geopark is characterized by complex geological structure, but is also one of the most well-researched areas of Slovenia. A wide range of geomorphological, karst and hydrological phenomena tells a story of the origins of the Idrija region, 340 million years ago.
The Idrija mercury ore deposit originated 238 million years ago when hot solutions (160–210 ˚C) bubbled from the depths to the surface and brought mercury along with it. On their way to the surface, they converted older rocks into ore. Cinnabar ores originated at the same time as the rocks in the ore generation period. The cinnabar mineral is one of the forms of mercury, the second being elementary mercury. Both forms can be found in the Idrija mercury ore deposit – in terms of the quantity of mercury, it is the second largest in the world. Mercury is extremely pure and frequently occurs as drops of elementary Hg. In terms of its origin and geological structure, the deposit is extremely complex, and as such, it represents a globally relevant natural monument. Its special status is warranted by the diversity of cinnabar ores that can be found in the area.
Anthony’s Mine Road is the oldest preserved part of the deposit still accessible after the closure of the mine. Today, it is a tourist attraction that features an impressive range of geological phenomena with elementary mercury and presents the methods of ore extraction used throughout the history of the mine.
Tectonic windows can be seen in areas where several nappes and thrust slices are sequenced on top of each other. A tectonic window is part of a younger base and is an exposed area in a thrust block of rocks. It is generated through erosion, which cuts deep into the nappe (the block of rocks) to expose the underlying material. Tectonic windows are characterized by younger rocks within and older rocks without. In the contact between rocks, the younger rocks are positioned under the older rocks. Windows can vary greatly in size, ranging from several meters to several hundreds of kilometers.
At the bottom, the Strug tectonic window exposes Upper Cretacious limestones and Paleocene-Eocene flysch rocks of the Hrušica nappe, with an overlying plate of Lower and Upper Cretacious limestones of the Koševnik thrust slice. The Idrijca River has eroded so deep into the overlying Koševnik thrust slice to expose the youngest Paleocene-Eocene and Upper Cretacious rocks of the underlying Hrušica nappe, which extends across several kilometers into the Idrijca River Valley in Strug, where typical contact between rocks can be observed.
Tomaž’s table is located near the village of Ravne pri Žireh and is a large, natural stone plate lying atop of a narrower stone pillar. The stone plate is completely separate and is only in contact with the pillar in a single point. It originated as a series of geological coincidences that took place through a period of 240 million years and created the conditions for its formation during a period of limestone mud sedimentation with intermittent layers of clay from dry land brought on by storms. Two layers of limestone were thus formed and later converted into dolomite. Nestled between the two layers was a layer of shale. The dolomitization process (i.e. the conversion of limestone to dolomite) caused the lower layer to be less resistant to the elements and the upper layer to be more resistant. In later geological periods, the upper layer was shaped through tectonic shifts with faults in different directions. Covered in other layers, it waited for tectonic forces to lift the Alps and the other layers towards the surface. Its final form was shaped by atmospheric influences through erosion and corrosion.