Where is the site located?
GPS: 49° 54′ 57.69″ N, 15° 46′ 18.96″ E
The site lies in the central part of the Iron Mountains National Geopark, not far from its northern limit. Here, the range of the Iron Mountains is terminated, passing into the flat region of the East Bohemian Table towards the town of Chrudim.
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 Paleozoic and Mesozoic ages are exposed here. Rocks at this site witnessed the presence of two marine floodings of different ages.
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, brachiopods, bivalves and cephalopods . The kingdom of plants was represented chiefly by primitive organisms, such as algae.
– 95 million years
In the latest Mesozoic, this area drifted to the Northern Hemisphere, becoming a part of the Eurasian Plate. It became progressively flooded by Late Cretaceous sea, which was also shallow but – unlike the previous sea flooding – warm. This site was located on a seashore, where shoreline cliffs protruded from sandy beaches, breaking the waves. The sea is inhabited by various bottom-dwelling organisms, such as oysters or brachiopods.
What does the site display today?
Southeastern vicinity of Rabštejnská Lhota is formed by Paleozoic rocks, pertaining to the Míčov Formation. These rocks date to the Mid Ordovician. They enter a wider rock complex preserved since Paleozoic times – the so-called Chrudim Paleozoic. In this area, the Míčov Formation is represented by the grey Skalka quartzites. These are resistant rocks derived from sand-dominated sediments with a high proportion of silica. Their present appearance results from subsequent orogenic processes. They commonly contain light to white quartz veins, which healed fractures in the rock. In the area between Mladoňovice and Slatiňany, quartzites form several elevations elongated in the direction SW–NE.
Northwestern vicinity of Rabštejnská Lhota lies in the marginal part of the Bohemian Cretaceous Basin the fill of which is represented by Upper Cretaceous sediments of Cenomanian to Lower Turonian age. Sediments at this site are coarse- to medium-grained sandstones, mostly yellowish to greyish-green in colour, containing discernible grains of green mineral – glauconite. They are ranked within the Korycany Member.
What was affected by man?
Quartzite outcrops were subjected to quarrying by our ancestors. Quartzite was primarily used as stone for the production of aggregate to be used in road construction. The overlying sandstone outcrops served as a source of sand for local construction purposes.
Quartzite extraction soon exposed interesting geological phenomena. The quarry at the limits of Rabštejnská Lhota was fortunately abandoned, and protective measures could be adopted in 1950s. Extraction in the Na Podhůře Quarry lasted until 1980s; since then, the quarry has been used for the disposal of inert wastes, becoming gradually filled. A major remediation of this mining-affected area occurred in the last few years because the quarry became a part of the restricted area of the Podhůra Recreation Forests.
What was discovered?
The “Na Skaláchˮ site provides an excellent example of the first flooding of Late Cretaceous sea – the first transgression (of Cenomanian age). The outcrop clearly shows the underlying grey Paleozoic quartzite transgressed by yellow to yellowish-grey porous sandstone of Upper Cretaceous sandstone, whose strata dip at an angle of 5–10°. Pebbles and blocks, documenting the proximity of a surf zone, are present in depressions on the surface of the quartzite cliff at the base of the sandstone. The pebbles are composed mainly of the underlying quartzite. They are bond together by sand-sized material.
In fact, this outcrop poses a fossilized shore of Late Cretaceous sea, with protruding quartzite cliffs. The sandstone also contains poorly preserved fossil remains, such as brachiopod shells and shark teeth.
Another outcrop within the “Na Skaláchˮ site shows a bedding plane in quartzite bearing traces after the activity of Ordovician organisms – inchnofossils of the Skolithos type. These elongated tube-shaped structures were built as dwelling burrows by organisms on the sea bottom.
The second flooding by Late Cretaceous sea, Lower Turonian in age, was documented in the Na Podhůře Quarry. Unfortunately, this outcrop was destructed in the progress of quarrying.
“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 installed at the southern limit of the “Na Skaláchˮ site. The actual geosite lies some 50 m to the north, in a forest. Relics of the Na Podhůře shelf quarry are visible about 700 m to the northeast, under the Bára lookout tower.