Where is the site located?
GPS: 49°53’24.480″N, 15°38’28.240″E
The site lies in the central part of the Iron Mountains National Geopark, on the northeastern slope of the main crest of the Iron Mountains. One of the highest elevations is the Bučiny Crest, reaching the altitude of 605 m. The quarry at your feet is the largest quarry in the Iron Mountains, providing an insight into three Paleozoic seas of different ages.
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. Sedimentary rocks of Ordovician, Silurian and Devonian age and Paleozoic volcanic dykes are exposed here.
What happened at this site in the past?
– 490 million years
In the Ordovician, the second period of the Paleozoic era, this area was lying on the Southern Hemisphere. It was a part of a shallow sea not far from the supercontinent of Gondwana. The climate was temperate to cold, with marked fluctuations, which controlled the changes in sea level. The sea was inhabited by invertebrate organisms, the best known representatives of which were trilobites, graptolites, brachiopods, bivalves and cephalopods. The kingdom of plants was represented chiefly by primitive organisms, such as algae.
– 440 million years
In the Silurian, the third period of the Paleozoic era, this area shifted to the north, still united with Gondwana. The shift to a markedly warmer zone of the Southern Hemisphere induced a rise in sea level. Colder seas host the rapidly developing group of graptolites. Particularly abundant are bivalves, gastropods and cephalopods, especially those with elongate conical shells. Corals, sea sponges and bryozoans are on the rise. Very common are crinoids and, of course, trilobites. Plant kingdom experiences the emergence of vascular plants and their colonization of dry land.
– 417 million years
In the Devonian, the fourth period of the Paleozoic era, this area together with Gondwana shifted to the tropical zone of the Southern Hemisphere. This shift induced a considerable reduction of the width of oceanic realms between Gondwana in the south and Laurussia in the north. The Devonian sea was a shallow and warm one, characterized by a boom of coral reefs. Neighbourhood of the coral reefs was inhabited by bivalves, brachiopods and crinoids. Cephalopods of the order of ammonites appeared for the first time. Trilobites were on retreat, with progressively lowering species diversity. Graptolites became totally extinct.
The Mid Devonian marked the onset of unrestful times of major orogenic processes (Hercynian Orogeny), which culminated in the next period – in the Carboniferous. Gondwana collided with Laurussia, giving rise to the supercontinent of Pangea. This area became a part of a giant mountain range of the Hercynides. This mountain range arched from the territory of Spain across southern England, France, Germany, as far as to central Europe, becoming a solid basement of the Bohemian Massif.
What does the site display today?
The area of Prachovice is formed by rocks of the Chrudim Paleozoic, which are Ordovician, Silurian and Devonian in age. They are widely distributed in the northwestern part of the Iron Mountains, being preserved in the so-called Vápenný Podol syncline.
Ordovician sediments comprise intervals of grey shales and quartzites. The Silurian period is characterized by dark graphitic shales and dark bituminous limestones. The Devonian period is represented by light smeared limestones.
The original sedimentary rocks of Ordovician, Silurian and Devonian seas were subjected to later orogenic movements and weak metamorphism. Their strata were fractured and bent, forming a set of tectonic blocks having the character of thin slices. These blocks were thrust upon one another. The resulting structure of the Vápenný Podol syncline is a complex one, including numerous thrust faults and post-tectonic intrusions of magmatic dyke rocks.
What was affected by man?
The early mining was associated with the use of Devonian limestones approximately from the latest 19th century onwards. Extracted material was a source of lime to be used in building industry, sugar industry and chemical industry. The smeared Vápenný Podol limestone was also utilized in stonemasonry (main altar of the Church of the Assumption of Virgin Mary in Chrudim).
Besides limestone, other rocks started to be used later. They were processed directly at the site of their extraction in the cement works. Plans for the construction of cement works were interrupted by World War II but realized in 1954. Since then, the Prachovice Cement Works have been in operation and the mining area has been repeatedly expanded. Now, it features a multi-level open-pit mine. As mining progressed, spectacular geological phenomena were unveiled and thoroughly documented by scientists, owing to the understanding of the management of the mining company. These discoveries are now to the great benefit of further scientific studies and promotion of geosciences in the public.
What was discovered?
Geological studies helped to precisely determine the structure of the Vápenný Podol syncline. The distribution of the individual formations was studied and their tectonic deformation was mapped. At this occasion, fossil remains of marine organisms were found in weakly deformed rocks. These included both macrofossils (visible by naked eye) and microfossils (visible under a microscope). Based on the finds of these minute fossils, Silurian and Devonian strata could be distinguished from each other, and a type section of the Prachovice Formation was defined. Large fossils permitted correlation with coeval Silurian fossils coming from the vicinity of Prague (the so-called Barrandian area). The most frequent fossils are floating organs of crinoids (loboliths), resembling bulbous particles. Conical shells of cephalopods of the species Orthocycloceras fluminese and crinoid plates are also common.
Limestone mining revealed karst cavities associated with the tectonic contact between dark shales and limestones of the Prachovice Formation. It also disclosed dykes of magmatic rocks of the lamprophyre group, such as minette – a mafic (dark-coloured) rock having a composition similar to granite, with a high proportion of dark mica flakes. Lamprophyre dykes are of various thicknesses.
The area of the Prachovice deposit features not only various types of rocks but also spectacular minerals. More than 20 mineral species have been reported. Rhombohedral and scalenohedral crystals of calcite of various colours, retrieved from karst cavities at quarry levels 5 and 6, are among the most beautiful specimens. The crystals are 10–20 cm in size; they became a showpiece of many mineral collections. Rare minerals are exemplified by millerite from the graphitic shales and by lead and zinc sulphides (galena, sphaletite) associated with quartz lenses.
Limestone portions showing incipient karstification were discovered by deep drilling within a hydrogeological survey. Old, buried karstic soil called terra rossa was found in cavities within these portions. This soil contains accumulations of radionuclides (notably uranium isotopes) produced by weathering of minerals from the adjacent magmatic bodies of the Železné hory pluton.
“The Iron Mountains – a geologically significant region” project of 2014
Two information panels were manufactured within the project of “The Iron Mountains – a geologically significant region”. They were installed at the southwestern limit of the Prachovice Quarry next to the concrete manufacture unit. The quarry is an active one, and any access is possible only after a prior consent by the company who operates the quarry.