Where is the site located?
GPS: 49° 59′ 54.74″ N, 15° 33′ 10.35″ E
The site lies in the northern part of the Iron Mountains National Geopark, in an area where the mountain range passes to the flat region of the East Bohemian Table. To the north of here, spectacular rock outcrops were unveiled by an abandoned quarry.
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 Paleozoic and Mesozic sedimentary rocks have been preserved. These rocks were deposited in two seas of different ages.
What happened at this site in the past?
– 490 million years
In the Ordovician, early in 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 probably temperate to cold, with marked fluctuations, which affected the sea level. The sea was inhabited by invertebrates, the best known representatives of which were trilobites, brachiopods, bivalves and cephalopods.
– 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 shallow but – unlike the previous sea flooding – warm. This site was located on a seashore, where shoreline cliffs protruded from sandy beaches and were breaking the waves.
What does the site display today?
Rocks referred to as the Lipoltice Formation have been preserved in a narrow strip between the villages of Jankovice and Horní Raškovice. They date to the earliest Ordovician and enter a wider rock complex preserved since Paleozoic times – the so-called Chrudim Paleozoic. In fact, the original rocks were unconsolidated marine sediments: various sands and muds. These became affected by later geological processes, which turned them into solid rocks mostly grey in colour but locally reddish- to rusty-coloured by iron compounds. Sands were changed for sandstones and quartzites, muds were changed for siltstones and shales. The dips of sedimentary strata were altered by orogenic processes. The original flat-lying sediments were bent under high pressures, now displaying a concave-upward structure of the Přelouč syncline in the near vicinity of Přelouč. Older sediments (Cambrian) have been preserved on the margins; they pass to progressively younger sediments in the centre.
Remains after the Late Cretaceous sea have the character of the so-called surf zone. Local depressions in the underlying Ordovician sediments are filled with light greyish-yellow sediments. These actually document the marginal parts of the Late Cretaceous sea, now preserved in the sedimentary fill of the Bohemian Cretaceous Basin. Sediments in the Brloh area are represented by sandstones. In places, they contain coarse pebbles of various rocks – then, the sediments are called conglomerates.
In the near vicinity of Brloh, rocks of the Lipoltice Formation as well as Upper Cretaceous rocks can be encountered in the valley of the Brložský potok Stream. Over ages, the stream cut down through almost horizontal-lying Upper Cretaceous strata into steeply dipping beds of Ordovician quartzose siltstones, shales, quartzose greywackes and sandstones.
What was affected by man?
Resistant rocks of the Lipoltice Formation were extracted in a number of small quarries in the southern environs of the village and utilized as building stone for local purposes. Stone from the largest quarry has been used for the construction of the Wonka Bridge across the Elbe River in Pardubice. Stone extraction revealed interesting geological phenomena, and the near vicinity yielded significant fossils.
What was discovered?
The studies of Ordovician sediments around Brloh date back to 1930s, when representatives of Ordovician fossil fauna were first discovered here: trilobites and gastropods. The fossils were somewhat compressed but some of them could be reliably determined and compared with similar fossils of the same age from the Barrandian area (i.e., the principal region of exposed Paleozoic rocks in Bohemia, lying between Plzeň and Prague).
The finds around Brloh thus allowed to extend the occurrences of the same Paleozoic rocks as far as to eastern Bohemia. It is worth mentioning that fragments of the same rock, also containing fossil remains, were found at Semtín near Pardubice. They are represented by the so-called Semtín breccia – a rock transported from the depth to the earth surface by a Tertiary basaltic dyke.
Studies in the Brloh area continued in the 1940s and 1950s. Fossil finds were deposited in the collections of the National Museum in Prague, where they are still kept at present.
“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 at the edge of a quarry lying behind the last house on the southwestern limit of Brloh. Fieldworks entailed cleaning of cliff faces from vegetation and their washing by jet water. Revitalization of the quarry surroundings was contributed by the community of Brloh. A metallic plate has been mounted on a rock outcrop, bearing the name QUARTZITE.