Where is the site located?
GPS: 49° 53′ 1.97″ N, 16° 2′ 3.99″ E
The area lies in the eastern part of the Iron Mountains National Geopark, not far from its northern boundary. The crest of the Iron Mountains rises on the horizon, some 5 km to the southwest. You watch the crest from the curved margin of the East Bohemian Table. Right below, you can see the two landmarks at Luže: the pilgrimage Church of Our Lady and the Košumberk Castle. Siting of both these buildings by our ancestors was guided by natural conditions.
What is the geological position of the site?
The site is located in the Bohemian Massif, in the marginal area of the Bohemian Cretaceous Basin. The staircase of Upper Cretaceous sedimentary rocks encloses the testimony of the last volcanic activity in the Tertiary, which created the present elevations of Chlumek and Košumberk.
What happened at this site in the past?
– 95 million years
In the course of the Mesozoic, the supercontinent of Pangea was falling apart into separate tectonic plates, which drifted atop the elastic asthenosphere. In the latest Mesozoic, this area was lying on the Northern Hemisphere, becoming a part of the Eurasian Plate. The climate was very warm, controlled by the greenhouse effect which induced a progressive rise of global sea level. Flooding occurred over large portions of dry land, first by freshwaters of rivers and lakes. Later on, a large part of the Bohemian Massif became a part of a shallow sea passage. This site was located in a shallow near-shore zone, colonized by ancient vegetation both on dry land and in water. Waters were dominated by the representatives of molluscs and fish.
– 25 million years
In mid-Tertiary, this area was located almost at its present position. It was mostly built by dry land, constituted from levelled mountain ranges. It was composed of magmatic and metamorphic rocks, covered by a thick succession of Upper Cretaceous sediments. The southeastern part of todayʼs Europe was under the effect of a major orogenic cycle – the Alpine Orogeny. This process also affected the more-or-less solid basement of the Bohemian Massif. It became arched and then fragmented. Old fractures in the Earth crust became activated and induced movements of separate crustal blocks relative to one another. Fractures in the Earth crust were turned into ascent paths for lavas, which could travel over long distances from the main volcanic centre in the northwest of the Bohemian Massif.
What does the site display today?
The area of Luže lies at the southern limit of the Bohemian Cretaceous Basin. It comprises a complex of older consolidated sediments of the Peruc-Korycany Formation of Cenomanian age and younger Bílá hora Formation of Lower Turonian age. The thickness of the complex is approximately 80 m. The older formation is mostly formed by quartzose sandstone with claystone intercalations. The younger one is composed of finer sediments: sandy siltstone, marlstone and calcareous to siliceous claystone (opoka). The original horizontal-lying marine sediments were later subjected to the so-called Saxonian tectonics – a manifestation of Tertiary Alpine Orogeny – and to younger erosion, largely performed by water. As a result, the so-called Luže cuestas can be observed around Luže: slopes with asymmetrical cross sections. Their gentle slopes are parallel to bedding planes of Upper Cretaceous sediments whereas steep slopes cut across the bedding planes and show stepped morphology controlled by the contrasting resistance of the different sedimentary strata to weathering.
Stream erosion, combined with the presence of a fault in Upper Cretaceous sediments, controlled the origin of the Novohradka River valley, winding around the steep southern slope of Košumberk Hill. Meandering valley of the Novohradka River between Luže and its confluence with the Krounka Stream is particularly picturesque. Lower Turonian sediments are exposed on stoss banks, and a relatively broad alluvial plain was formed by the river in accumulation areas. The alluvial plain becomes inundated during regular floods.
The environment of Upper Cretaceous sediments is where groundwater is created and accumulated. In this area, groundwater is utilized with the use of specific objects – water-supply boreholes about 60 m deep. These objects withdraw water from sand-dominated sediments of Cenomanian age. Frome above, it is sealed by fine Lower Turonian sediments, functioning as an aquitard. These conditions provide an artesian system for groundwater: it is under pressure, discharged spontaneously onto the earth surface.
Košumberk and Chlumek hills are composed of Tertiary effusive rock ranked within the basalt group. These sites represent the easternmost occurrence of Tertiary volcanic products in Bohemia, associated with the so-called Ohře Rift, or, the Krušné hory piedmont graben in northwestern Bohemia. These basaltic elevations were no supervolcanoes but rather conduits of a small basaltic volcano, piercing through Upper Cretaceous strata. Later, they became exhumed owing to erosion which removed less resistant sedimentary rocks around. An old tectonic structure was used by the lava for its ascent to the surface: probably the deep-reaching Elbe Lineament (a system of faults transecting Bohemia in NW–SE direction, approximately following the Elbe River course). Remains after this volcanic activity can be also found in the Pardubice area (Kunětická hora Hill, Spojily Dyke, Brčekoly Dyke).
What was affected by man?
Natural elevations of Košumberk and Chlumek hills were chosen for prominent constructions by our ancestors. The Košumberk Castle was built in the late 13th century on a steep basatic hill. Its builders took advantage of geomorphic conditions which guaranteed the inaccessibility of the castle from the south and southwest. The basement rock passes fluently into masonry, which can be observed by the visitors on the courtyard or in the cellarage. The cellar also reveals six-sided disintegration of basalt, producing typical six-sided basaltic columns.
The ancient castle of the Slavata family was later extended by a fortified settlement, and finally rebuilt in a Renaissance style in the 16th century. It became destroyed after the Thirty-years War.
The two-spired church at Chlumek Hill was built in early Baroque style following the plans by Italian architect Alliprandi in years 1690–1695. Yet in the late 19th century, some scientists declared that the church was also built on an outcrop of Tertiary basalt; this statement was, however, missing any material support. Therefore, geologists conducting the major geological survey in the 1950s and 1960s inclined toward the idea that the church rested on artificial landfill. This controversy was solved by the excavation for an elevator shaft at the NE foot of the church in 2012.
What was discovered?
The excavation for an elevator shaft unveiled an outcrop of a basaltic rock, whose identity was clearly suggested by the diagnostic six-sided disintegration. The rock was determined as basanite (a variety of basalt), identical with that at Košumberk Hill. This unique exposure could not be preserved of technical reasons, but it was at least thoroughly described by the experts. The basaltic parent rock was found to contain cumulates of green mineral olivine and large amounts of inclusions of a rock called peridotite. The rocks locally formed a mosaic-like pattern, known as peridotite breccia. These rock components come from great depths and were transported with molten magma to near-surface levels from the Earth mantle. Their presence testifies to the ascent of magma along a deep-seated tectonic structure. Besides these pilgrims from the Earth interior, fragments Upper Cretaceous sediments originally lying above the lavas were alo found, altered by heat. The original clay-dominated deposits were actually baked in a natural furnace, and were therefore given the name porcelanite.
“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 placed near a cemetery at the eastern limit of the Luže village (GPS: 49°53’16.450″N, 16°2’16.801″E). This place provides a good look of the Košumberk Hill.