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Stone and man

Střeleč

The quarry Střeleč is one of the most significant deposits of glass sands in Bohemia. The raw material of Střeleč has world's parameters, and after treatment is used mainly for production of crystal. Major layer of white-grey, fine- to medium-grained quartz sandstones reaches up to 88 m in thickness. The sandstones, from which glass sands are obtained, have high degree of granulometric sorting and of chemical purity. They formed during the Cretaceous epoch roughly 90 millions years ago in a shallow sea. Sandstones are dated by findings of fossils, like e.g. Inoceramus cf. lusatiae And. or Cardiaster ananchystis (Leske), to lower Coniacian. Sandstones are fine- to medium grained. The sandstone mined consists from more than 95 % of quartz, grains of feldspars and micas are solitary. In cement is nearly exclusively kaolinite. Content of the useful fraction (0.1 to 0.6 mm) varies about 85 %. Content of harmful substances, determined by the occurrence of heavy minerals, varies in the range 0.001 – 0.004 % Fe2O3. Heavy minerals, among which tourmalines prevail, are concentrated in fraction under 0.1 mm. About 40 % of heavy fraction is formed by ilmenite, anatase, rutile, sphene, brookite, and product of their transformation – leucoxene. Zircon, staurolite and garnet are common. The formation of glass sands is terminated by so-called red layer of fine-grained, strongly clayey sandstones with large amount of ferric pigment. Above it lie yet 5 to 40 m of yellowish to yellow-gray sands, usually more consolidated, which are used mainly as foundry sands due to their lower chemical purity. The original classic mining by blasting and dredging has been until the year 1980 substituted by hydraulic mining, at which high-pressure water is used to dislodge the sandstones. Thus the present extraction method minimizes blasting operations. Mining operation exposed tertiary vein basalts at western rim of the quarry. The tertiary volcanic activity was accompanied by horizontal and vertical movements, and so many subsidences and upraises took place. Geometry of the deposit is determined by fault lines. The upper-Cretaceous sediments, inclined to southwest, gradually sink along faults, like Eximos, the Hrdoňovice fault, Skaříšov faults or Střeleč fault. The course of fault lines influences groundwater regime. The faults sometimes work as hydraulic barriers, another time they are featured by drainage effect. Integral part of the mining activity is also the concurrent recultivation process. In the quarry and surroundings around 180 species of organisms have been registered, including 16 especially protected vertebrate species. A tighter linkage to the artificially made biotope show sand martins.

GPS position

N 50° 29.918', E 15° 15.581'
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Contact

Geopark UNESCO Český ráj
Kontaktní osoba: Jiří Vlasák
Tel.:+420 731 413 755
e-mail:vlasak@geopark-ceskyraj.cz
http://www.geopark-ceskyraj.cz