Where is the site located?
GPS: 49° 58′ 17.93″ N, 15° 32′ 42.59″ E
The site lies in the northwestern part of the Iron Mountains National Geopark, in the area of rather gently inclined northeastern slopes of the Iron Mountains. Two sites in the close neighbourhood are worth attention.
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 Proterozoic rocks have been preserved. Rocks at this site are silent witnesses of the oldest sea in the Czech territory and of volcanoes.
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 deeper parts of the sea, periodically redeposited by huge subaquatic slumps. At the same time, submarine volcanic activity was taking place here, associated with lava effusions. The volcanoes occasionally reached above sea level with their tops, rendering emission of volcanic material in the atmosphere and its re-sedimentation in water and on dry land.
– 550 million years
The end of the Proterozoic is marked by a rearrangement of lithospheric plates, resulting in the Cadomian Orogeny. Then, several adjacent microplates were merged together. The original sedimentary and volcanic rocks were subjected to deformation and weak metamorphism by the effect of high pressures.
What does the site display today?
The near vicinity of Sovolusky is formed by rocks ranked within the Sovolusky Group of the Železné hory Proterozoic. The Group comprises rocks of sedimentary origin and rocks of volcanic origin.
Sedimentary rocks are usually grey in colour and of fine particle size (shales and foliated greywackes), as much as of coarse particle size (greywackes and conglomerates). Phyllites are locally present; these are rocks exposed to subsequent weak metamorphism.
Rocks of volcanic origin are represented by the andesite–rhyolite formation. In former times, the term spilite–keratophyre formation was used for these Proterozoic magmatites.
These are rocks which were formed by the solidification of lava, lithification of hard volcanic fragments (tuffs) and/or by mixing of volcanic products with sediments (tuffites).
Volcanic rocks have been found chiefly in the area of Sovolusky. Accordingly, this area is called the Sovolusky Volcanic Centre.
What was affected by man?
Landscape around this site is characterized by very old bedrock. It was subjected to the effect of exogenous geological forces through ages. By now, it has been levelled, eroded and covered with younger sedimentary rocks. Natural rock outcrops are very rare, being mostly represented by small boulders in forests. Similar boulders were removed from arable land a long time ago. Two abandoned quarries, which probably served as a local source of stone, revealed interesting geological information.
What was discovered?
The site of Skalka u Sovolusk features pillow lavas – distinct loaf-shaped to pillow-shaped structures, composed of notably porphyritic andesites (effusive rocks with mineral phenocrysts).
Matrix between these structures is formed by tuffitic material, also containing fragments of quartz and siltstone.
These structures evidence the presence of submarine lava effusions (cooling of lava by water from all directions). The origin of such lava can be also observed deep in present-day seas.
Thinly banded shales were exposed at the site of a former quarry near the road to Turkovice. In petrographic terms, they were classified as very fine siltstones subjected to weak plastic deformation, with a perfectly preserved thinly bedded structure with alternating light pink and grey bands.
As suggested by some geologists, these shales possibly represented lacustrine or marine sediments originated due to temperature variations (so-called varvites). If so, their mode of origin would evidence an alternation of glacial and interglacial periods, hence also the Proterozoic ice age.
“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 village of Sovolusky. A shallow quarry was operated some 600 m SW of the panels near the road to Turkovice. Later, it was turned to a field. An abandoned quarry lies some 600 m NE of the panels between fields and pasturelands (now, the Skalka u Sovolusk Nature Monument). It is accessible along a local road between Sovolusky and Urbanice. Fieldwork completed at the latter site included cleaning of the cliff face and cutting of native woody plants in its near surroundings (GPS: 49 ° 50’8.25 “N, 15 ° 39’8.01″ E).