Where is the site located?
GPS: 49° 56′ 14.16″ N, 15° 32′ 18.67″ E
The site lies in the northwestern part of the Iron Mountains National Geopark, right on the steep slope of the Iron Mountains. It provides a good view towards the west, down to the Čáslavská kotlina Basin.
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.
The rocks beneath your feet were formed in an ancient ocean and later subjected to orogenic processes. The present character of the landscape was strongly affected by the presence of an old fracture in the Earth crust (a lineament).
What happened at this site in the past?
- 750 million years
In the Proterozoic, this area was lying on the Southern Hemisphere. It was a part of a seafloor not far from the continent of Gondwana. Here, thick deposits of fine sediments were formed in the sea. At the same time, submarine volcanic activity was taking place, associated with lava effusions.
– 554 million years
The end of the Proterozoic is marked by a rearrangement of lithospheric plates, resulting in the Cadomian Orogeny. The deposited rocks were metamorphosed, intermingled and hardened under orogenic pressures and temperatures, forming a mountain range. Hard rocks were crushed at the origin of deep fractures – tectonic lineaments. Occasionally, these fractures functioned as paths for molten magma in its ascent to the surface.
– 25 million years
In the course of the Tertiary, this area was composed of hard metamorphic rocks, locally covered by deposits of the Late Cretaceous sea. It became incorporated in the Eurasian plate and drifted to the Northern Hemisphere. The rocks lie in an unstable environment close to an old deep-reaching fault; as a result, they are densely fractured and jointed.
The Alpine Orogeny in the SE part of Central Europe built the Carpathians and the Alps and reactivated the movement of crustal blocks by as much as several hundred metres. The sedimentary cover became soon eroded.
What does the site display today?
Southwestern slopes in the NW tip of the Iron Mountains, approximately between Týnec n. L. and Třemošnice, are composed of metamorphic rocks of the Podhořany Crystalline Complex. These rocks include paragneisses and mica-schists, locally intercalated with amphibolites. They are mostly dark in colour, usually greyish black. They contain lustrous mineral biotite (dark mica) and another mineral – garnet – which has the appearance of red grains and aggregates. Under strong magnification, you can see that some of the minerals are arranged parallel to one another – aligned. These minerals evidence the origin of the rock under high temperatures and pressures, during a process called regional metamorphism affecting wide areas. The exact site where the rock originated has not been fully explained yet, much like its relationships to other rock complexes. Most probably it represents old slices dismembered from a deep storey of the Bohemian Massif and gradually folded into younger rocks.
The present relief morphology was controlled by a tectonic structure of regional importance, which appears in eastern Bohemia on the earth surface between Týnec nad Labem and Krucemburk. It is called the Železné hory Fault (Iron Mountains Fault), representing one of the structures of the so-called Elbe Line. This line is an old, deep-reaching fracture elongated NW–SE (so-called Sudetic strike). It is hidden beneath younger sediments, stretching as far as to Dresden.
The steep southwestern slope of the Iron Mountains is a product of the Saxonian tectonics, which was a response to the Alpine Orogeny. Blocks along the Železné hory Fault moved relative to each other. Hereat, the block of the Iron Mountains was uplifted by several hundred metres and simultaneously segmented by a number of transverse faults. Older Proterozoic rocks were thrust over younger rocks.
What was affected by man?
The rocks at Podhořany are exposed in an abandoned shelf quarry on the northern edge of the built-up area. The extracted rock was used as aggregate for local purposes (e.g. road construction). At present, the quarry face is about 10 m high.
What was discovered?
The host rock is fine-grained biotite gneiss and amphibolite containing porphyroblasts (mostly oval grains) of garnet several millimetres in size.
The quarry face perfectly displays the strong tectonic deformation of the rock. It shows dense tabular jointing with the tables being dragged to a steep upright position.
“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 erected in the eastern part of the Podhořany village, on the left-hand side of road 17 from Chrudim to Čáslav. The above described quarry lies about 100 m to the east of the last house. It is accessible along a local path.