General Characteristics

County (administration): South Bohemia Region
District (town): Prachatice
Cadastre: Hoslovice
Map 1 : 50 000 (sheet No): 2234, 22341
Geological region: Šumava Moldanubicum - districts ST, PT, CK, CB, PI
Accessibility: - passable footpath
Excursion locality: yes
ID : 4237

Geology

Short characteristics of the site: The location of the syenite porphyry is situated in a forest on the northern slope of the Chrasnice hill (elevation 711 m), 1.5 km south-east. from the village Hoslovice. About 200 m from the site passes a yellow marked hiking trail, which leads from the south from the village Hořejšice to the north to Hoslovice. The wider surroundings are being built by stromatitic biotite migmatite, and in addition to the porphyry dykes, small intrusions of biotite granite are approaching. The massive and bizarre rock log house is up to 5 m high, partly broken up into blocks of 1–5 m in size and forms a small rock town on. Porphyry was used, for example, on millstones, which have been preserved in a nearby Hoslov mill, which has been used as a museum. Flat exfoliation is noticeable, according to which the porphyry breaks into thinner decimeter plates. About 100 m lower along the slope, to the north there is another vein of syenite porphyry, the outcrop with a nicely developed subhorizontal exfoliation (d. B. SZ820) lies 130 m to the NE, but there are no traces of stone breaking. The porphyry is an atypically powerful body with a thickness of 30, maybe even 50 m, which is part of a vein only 300-400 m long, which, like all porphyry veins in the area, has a characteristic V-Z direction. Common porphyry veins tend to be 2–20 m thick, up to several kilometers long, and disintegrate more into isometric blocks, with disintegration into plates being an exception. Porphyry is a very tough rock with striking white growths of potassium feldspar 0.5-1.5 cm large. Black-brown, fine to medium-grained mafic xenoliths of phlogopite-rich rocks close to the mines of 1–30 cm are abundant. Some of them weather out negatively. Up to several mm large reddish grains in xenoliths resemble garnet, but these are reddish pseudomorphoses filled mainly with talc and actinolite.
Regional geologic unit: Bohemian Massif - crystalline complex and pre-Variscian Palaeozoic - Moldanubicum - Metamorphic units in Moldanubicum
Stratigraphy: paleozoikum až proterozoikum
Subject: building and decorative stone, geomorphology, petrology, history
Geological phenomenon: characteristic rock, mushroom rock, frost cliff
Genesis: magmatic, frost weathering
Rock: porphyry

Territorial conservation

Level of protection: Registered interesting geological localities
Protection of a geol. phenomenon: C - Geological phenomenon is the reason for a registration in the database of CGS
Conflicts of interests: without conflicts

References

Žáček, V. – Škoda, R. – Sulovský, P. (2009): U–Th-rich zircon, thorite and allanite-(Ce) as a main carriers of radioactivity in the highly radioactive ultrapotassic melasyenite porphyry from the Šumava Mts., Moldanubian Zone, Czech Republic. – Journal of Geosciences 54, 4, 343-354; Žáček, V. et al. (2021): Vysvětlivky k základní geologické mapě České republiky 22-341, list Vacov. - Archiv České geologické služby, Praha.

Elaboration and Updating

Elaborated by: Žáček Vladimír, 23.01.21

Citation

The database Significant geological localities of the Czech Republic: 4237 [online]. Prague: Czech Geological Survey, 1998 [cit. 2024-05-09]. Available from: http:// lokality.geology.cz/4237.

Location in map: