site 19

Where is the site located?

GPS: 49° 48′ 34.73″ N, 15° 57′ 3.35″ E

The site lies in the eastern part of the Iron Mountains National Geopark, in the easternmost extension of the Iron Mountains. Here, the mountain range gradually passes to the Českomoravská vrchovina Highland.

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 which is represented by the Hlinsko Zone at this site. Rocks formed in the Silurian sea are worth special attention: by coincidence, ancient marine organisms have been preserved in the rocks. These organisms became extinct long ago.

What happened at this site in the past?

– 440 million years

In the Silurian, the third period of the Paleozoic era, this area is located on the Southern Hemisphere, not far from the large continent of Gondwana. It lies in a basin periodically flooded by a shallow, rather cold sea. Silica is released in the weakly acidic environment and becomes consumed by marine microorganisms. Living conditions in the sea are most favoured by tiny organisms creating colonies of various shapes – graptolites.

– 350–300 million years

Carboniferous and earliest Permian (the fifth and sixth periods of the Paleozoic era) were unrestful times of major orogenic processes (Variscan Orogeny). Two continents progressively collided – Laurussia in the north and Gondwana in the south – creating the supercontinent of Pangea. This area became a part of a large mountain range of the Variscides. This mountain range extended from the territory of todayʼs Spain across southern England, France and Germany, as far as to central Europe, becoming a solid basement of the Bohemian Massif. The orogenic processes resulted in bending, fracturing and shifting of the original horizontal-lying sediments, and induced strong magmatic activity. Besides lava effusions on the earth surface, huge magmatic bodies originated deep under the surface and transmitted heat to their ambience.

What does the site display today?

Rocks in the Mrákotín area constitute a wider rock complex called the Hlinsko Zone (Hlinsko Proterozoic and Paleozoic). They are derived from Proterozoic and Paleozoic volcanosedimentary rocks whose present appearance results from high pressures and temperatures during the subsequent orogenic processes. The Mrákotín area displays the effects of the near magmatic body of the Nasavrky pluton, lying in the north. The pluton affected the ambient rocks mainly by its heat, inducing contact metamorphism. A relatively unique succession of contact metamorphosed rocks has been preserved in the near vicinity.

Cordieritic hornfels and andalusitic schist are present closest to the pluton (hornfels is a compact, baked rock; mineral andalusite requires high temperatures for its origin). These are followed by chiastolitic schist (crystals of chiastolite – mineral of the silicate group – form dark grey columns and form at lower temperatures). The most distant from the pluton is knotted schist (minerals chiastolite and cordierite form distinct grains and the parent rock looks like skin spotted with blisters). The last in the succession is shale showing almost no metamorphic alteration.

Of particular interest are rocks of the Mrákotín Formation: dark grey to black graphitic shales intercalated with very compact silicite and radiolarite beds (a metamorphic rock, originated through accumulation of siliceous tests of microorganisms). They contain well preserved fossilized colonies of Silurian graptolites, which look like symbols drawn on the stone surface. This appearance gave graptolites their name (Graptolitha = inscribed rocks).

What was affected by man?

Landscape around this site is characterized by very old bedrock. This was subjected to long-lasting erosion. By now, it has been abraded, levelled and locally covered with younger sedimentary rocks. Occurrences of natural rock outcrops are limited, being usually bound to forested places, rapidly overgrown by vegetation. Remains after old quarries are of some help for the geologists. Small quarries produced stone for local use; this was, for instance, the case of a quarry atop the hill south of the village. By now, this place has changed a lot: the quarry face is covered with talus and the quarry is overgrown by vegetation. Another possibility is to collect and study rock fragments brought to the surface by deep ploughing of the surrounding fields. Rocks can be found especially on the field along the road to the telecommunication tower southwest of the village.

What was discovered?

The first geological studies of the Mrákotín area date to the 1930s: essential types of local rocks were described and correlated with coeval rocks exposed between Prague and Pilsen (Barrandian area). It was also in this period that the fossil graptolites were discovered, their Silurian age was diagnosed, and a comparison was provided with similar graptolite fauna from Germany.

Geological survey conducted in the 1950s could still benefit from a well-exposed quarry face. As indicated by photographic documentation taken in this period, shales (thin, densely fractured intervals) alternate with harder silicites (thicker, lighter and more compact intervals). The individual strata are very steep to vertical, and minute folds can be observed in places. Steep dips of strata document the magnitude of the past orogenic pressures.
In the early 21st century, a major, detailed geological survey was conducted in this area. Graptolites got once again in the focus of attention at this time. They were thoroughly studied using modern imaging techniques, and their Silurian age was confirmed.

“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 erected on the square in the centre of the Mrákotín village. From this place, you can walk southwest, following the road towards the forest with the telecommunication transmitter. This is where you can find places yielding graptolite-bearing shales not far from the tower.

Graptolites in shales from Mrákotín (Daniel Smutek)