Where is the site located?
GPS: 49°50’15.60″ N, 15°39’30.53″ E
The site lies in the western part of the Iron Mountains National Geopark, in the central part of the main ridge of the Iron Mountains. We find ourselves on the eastern slope of the rocky spur of the Oheb Cliff (Ohebská skála).
What is the geological position of the site?
The site is located in the Bohemian Massif, in the marginal area of the Central Bohemian Region, where metamorphosed Proterozoic rocks are exposed, mixed with somewhat younger magmatic rocks. Rocks at this site are silent witnesses of the past orogenic processes. The appearance of the present landscape was considerably affected by movements along a deep fracture in the Earth crust.
What happened at this site in the past?
– 550 million years
The end of the Proterozoic is marked by a rearrangement of lithospheric plates, resulting in the Cadomian Orogeny. The original rocks, produced by marine sedimentation and submarine lava effusions, were subjected to metamorphism by high pressures and temperatures. A deep-reaching fracture originated in the Earth crust.
– 350–300 million years
At the end of the Paleozoic, the rocks were subjected to another orogenic process – Variscan Orogeny. Again, this process changed their material composition and induced compressional deformation. Hot magma was released from the depth of the Earth by the orogenic processes. Bodies of magmatic rocks were formed and folded together with their host rocks.
– 25 million years
In the course of the Tertiary, this area was composed of hard metamorphic and magmatic rocks. These rocks were forming a flat, levelled elevation, covered with younger sediments. An unstable environment close to an old deep-seated fault resulted in dense jointing of the rocks and their fragmentation into separate blocks. The Alpine Orogeny in the SE part of Central Europe formed the Carpathians and the Alps. Related compressive stresses reactivated the movement of the crustal blocks by as much as several hundred metres. The sedimentary cover became soon eroded.
– 2 million years
In the course of the Quaternary, the exposed rocks were eroded by water, wind, and fractured by frost. They were subjected to weathering, forming characteristic rock monuments. Channels were shaped by their streams.
What does the site display today?
The area around the Seč Reservoir is formed by metamorphic rocks pertaining to the Kutná Hora (Oheb) Crystalline Complex. A typical representative of this group of rocks is the Oheb orthogneiss. Its grey to pinkish colour is due to the presence of mineral orthoclase (feldspar group) and abundant mica flakes (light mica – muscovite, dark mica – biotite). A closer view of the cliff face reveals evidence of how the original magmatic rock was changed by renewed orogenic processes deep in the Earth. Partial melting of the rock occurred under high temperatures, and changes in the arrangement of the individual rock components occurred under high pressures. Minerals distinctly show parallel alignment, forming banded or leaflike structures, and locally displaying folding of mica-rich bands.
When viewed from a distance, the cliff documents the action of exogenous geological forces (water, frost). It is dissected by a dense system of joints along which the cliff becomes progressively disintegrated into separate blocks. In winter time, water infiltrated in the top parts of the Oheb rocky spur can be observed to flow out of the joint system, creating remarkable icicle walls. The whole rocky spur exhibits a large number of geomorphological phenomena (frost-riven cliffs in top portions, boulder fields and talus on slopes).
The Oheb rocky spur received its name after the stunning change in the course of the Chrudimka River (oheb is the Czech word for a bend): it really bends at this site in such a manner that it resembles a bent elbow. This fact is caused by the so-called stream piracy – an effect resulting from the movement of the individual blocks in the Tertiary. The block of the Iron Mountains was uplifted along the Železné hory Fault. The stream flowing across the uplifted block thus gained a higher erosion potential and gradually propagated headward until it tapped the paleo-Chrudimka River flowing towards the northwest (much like the present Doubrava River).
What was affected by man?
The original valley of the Chrudimka River was deep and rocky. The cliffs were rising as high as 100 m above the Chrudimka River meander.
A decision on the construction of a dam was made in 1922. The reservoir was designed to eliminate big floods and spring floodings, to regulate the flow in the middle reach of the Elbe River and to be used for electricity production.
Construction works took place between 1923 and 1935, considerably affecting the rock environment in the area. First, new access roads were constructed and a tunnel leading to the dam was driven in the cliff topped by the Vildštejn Castle. The dam was placed by our ancestors between the Oheb rocky spur and the Vildšten Castle cliff.
What was discovered?
The construction was preceded by an engineering-geological exploration, the report of which was elaborated by Professor J. Woldřich, the Director of the State Geological Institute. Rigs for core drilling in hard rocks were not available at that time. Therefore, the exploration made use of excavated test-pits, cuts and slope ditches. Strikes and dips of joint surfaces were measured on cliff faces. These works were necessary to test the vulnerability of the construction against mass movements and collapses of unstable cliffs. Local rocks were found to be too fractured to be used for the construction of a stone dam. Several other sites were proposed as sources of appropriate building material. Professor J. Woldřich decided to use the “graniteˮ quarry at Libkov, which yielded quartz diorite. This rock met the requirements for compactness and hardness, and was dressed to the shape of Cyclopean masonry. A newly constructed cableway 7.5 km long was used to transport rock blocks from the quarry to the dam construction site. It could be operated also in winter, when roads became blocked by snow. This way, about 200,000 tonnes of stone were transported to the construction site in total.
“The Iron Mountains – a geologically significant region” project of 2014
An information panel was manufactured within the project of “The Iron Mountains – a geologically significant region”. It was placed near Road 343 from Seč to Proseč, at the site of rock outcrops behind the dam. Rock surface of small area has been polished on the outcrop near the panel to display the rock in unweathered state. A metallic plate has been mounted next to this surface, bearing the name OHEB GNEISS (GPS: 49°50’8.25″ N, 15°39’8.01″ E).