Geological Structure in Český Krumlov and Vicinity
 

This introductory information has been prepared for those interested in sciences and for those who want to understand facts in context. It is also intended for visitors who want to plan their stay in advance using more sources than common guidebooks. The information contained herein is based on currently available scientific literature and the existing level of knowledge which is sure to be complemented and developed in the future. The content is related to the information on sciences, environmental issues, extraction of raw materials etc.

Morphology
 Geologic map of Český Krumlov and surroundings
The look of the landscape is created not only by a particular geologic structure but also by processes and events which affected the region's geologic history, mainly in Quaternary (Tertiary). The processes include tectonics, as well as exogenous phenomena - erosion and denudation.

One major element is a valley of the Vltava River and its tributaries, often an irregular network of minor streams. The deeply cut Vltava valley with sheer, often rocky slopes, has numerous pronounced meanders. In contrast with this low formation is the dominant elevation, the highest point of Blanský Forest, Kleť Mountain (1084 m). Its top is round with a number of surface outcrops and debris avalanches (cableway, mountain base, lookout tower, observatory).

The prevailing landscape character is polymorphous and rolling as created by block interactions, with hills, short combs, and both cut and flat valleys. In terms of geomorphology the Český Krumlov region is part of the Šumava system, a sub-province of Šumava mountains, with an average height of 634.4 m. In the east it borders on the foothills of the Novohradské mountains.

Geological Structure
The regional geology of Český Krumlov is included into the pre-platform base of the Bohemian Massif, in the Moldau-Danubian area. Moldanubicum is a historical term use for the area created mainly of extensively metamorphosed rocks with granitoid massifs of Varisan age. Moldanubicum is considered a pre-Paleozoic element.

Geologic map of Český Krumlov and surroundings, explanations

More specifically, it is a part of Šumava Moldanubicum created by various rock complexes catamorphosed, mostly into amphibolite facies, exceptionally they are also sets with mesozonal or epizonal metamorphozes. Typical for Moldanubicum is the occurrence of highly metamorphosed rocks, e.g. granulites, eclogites and other. Also common is intense migmatisation and penetration through granite rock massifs.

Two different series of rocks can be identified in Moldanubicum, with different content. One group is uniform, the other diverse. The uniform group is in terms of petrography represented mainly by various types of paragneiss (biotite, silimanite, muscovite), migmitised to a various extent. They are probably pelite and psammite sediments from a deeper sea. The diverse group also contains paragneiss, as in the uniform group, but there are other rocks, such as quartzites, quartzite gneiss, erlans, graphite-based rocks, marbles, amphibolites, amphibolite gneiss, granulites, eclogites, ortogneiss.

This diverse series appears in several stripes. The central stripe runs across Český Krumlov. The diverse series may be considered without major speculation to be a shallow sedimentation space, where marbles and graphite-based rocks represent material of organic origin.

In terms of stratigraphy, the diverse group is considered to be younger of the two. However, its age is being disputed. Not enough data are available for a convincing conclusion. Findings of microphosilia in marble from Český Krumlov might contribute to more accurate dating. Of substantial interest is the fact that no sediment cover is deposited on Moldanubicum. Radiometric dating has shown at least medium Proterozoic age (523 plus minus 38 million years, for a rock from Rožmberk-Kaplice unit). The rocks are at least twice deformed and sometimes metamorphosed (deformation phases „Moldambic", Cadomian, and partly also Hercynian).

Český Krumlov
The Krumlov stripe includes a substantial representation of granulites and with them related ultra-basic rocks and eclogites; very ample are also graphite-based rocks and graphites (stripe Černá - Český Krumlov - Přísečná). Common are ortogneiss bodies. The massif of Kleť Mountain (Blanský Forest) is formed of granulites. More rocks appear in layers of various thickness. They are again amphibolites, erlans, marbles, quartzites. Impact of metamorphose has been proved in at least two regional events. Magnetic materials are represented by leucocratic vein granites, granodiorites and various porphyrs.

Hydrogeology
The Český Krumlov region ranks among the typical areas where water is connected with fracture systems of crystalline complex rocks and to a minor extent with covering formations and weathered jacket. Collectors are drained into basins of the Vltava (Moldau) and Polešnice rivers. In terms of chemical composition the waters have low mineral content up to 100 mg.l-1 (calcium sulfate, calcium bicarbonate, calcium sulfate-bicarbonate). The capacity of springs varies depending on fracture permeability in rock complexes. Circulation of underground water is relatively fast and significantly depends on precipitation supply. An exception in the range of crystalline complex collectors are underground waters linked with the position of crystalline limestones. No mineral waters occur here.

Mineral deposits
The region of Český Krumlov is not considered to be of major importance in terms of raw materials extraction. We know that building stone hauling was extensive here in the Middle Ages. The evidence can be found e.g. in the St. Wenceslas cellars at the Český Krumlov castle, relics of old paving and parts of some secular and ecclesiastical buildings. The stone was quarried directly on the site or in its immediate vicinity. Later, with development of the building industry and transportation, stone was brought to construction sites from more distant locations. In terms of petrography, the stone included granitoides, granulites, crystalline limestones and, to a lesser extent, paragneiss rocks, now mostly used for aggregates. No decorative stone was hauled in the region. The greatest existing quarry is now in the east slope of Kleť and in Chvalšiny - Zrcadlová huť. The quarry in crystalline limestones in Vyšná has been closed.

The traditional raw material of the Český Krumlov region (stripe from Černá v Pošumaví via Český Krumlov) are graphite ores. We can find here deposits historical, exhausted, currently preserved in the phase of mining or surveying. The deposits have evolved at borders of paragneiss and crystalline limestones. Graphite had been already used by the historical Celtic population in the region.

In the past Český Krumlov was a mining town. Even now we can find traces of the historical mining activities, e.g. minor ore dumps, grade level deformations, outlets of sunk galleries, and even some buildings. The relics are mostly reminiscent of fine silver mining from vein structures in sub-surface cementation zones.

Investigating rocks employed for construction in Český Krumlov (from pavings to churches and the castle), we will find their origin to be mostly local or from the near vicinity. E.g. a part of castle park wall might serve as a petrographic collection of regional geologic structure. This naturally does not apply for some precious and decorative details of local buildings or for architectural excesses from the recent past.

Mineralogical issues would require a discussion too extensive for this material and we refer those who are interested in it to scientific literature where both experts and collectors may find sufficient information for their work or hobby.

Additional information on maps can be found at http://www.cgu.cz

(jsch)

Further information :
History of Mining in Český Krumlov
Geological and Soil Conditions in the Český Krumlov Region

 



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