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The two towers of Trosky are symbol of Bohemian Paradise since immemorial. The thicker tower is called Granny and the tall and slim one Virgin.). A castle was built on tops and in between these two necks in medieval times. The necks represent coherent volcanic conduit exposed by selective erosion from surrounding pyroclastic deposits and Cretaceous marine sediments, there are still visible several landslides generated in the Cretaceous sediments.

Volcanic conduit exposed by selective erosion

The rock of the necks was classified as basanite (olivine, clinopyroxene, plagioclase, nepheline, magnetite and scarce apatite). Occasionaly, remnants of pyroclastic deposits mantling the coherent feeders can be found. The pyroclastic deposits consist of non-welded, non-compacted scoriae (fragments of strongly vesiculated basaltic rock). Such type of pyroclasts used to be ejected during strombolian-type eruptions.


This volcano sits upon Hrubá Skála sandstones. Based on comparison of altitudes of superficial volcanic deposits bases, sandstone table-rock of Příhrazské skály – Hruboskalsko – Prachovské skály appears to be formed before volcanic activity. This table-rock was already important geomorphological feature during the Miocene volcanism. Permeable Hrubá Skála sandstones did not form any aquifer and thus, solely magmatic gases drove the explosivity.

Lava tunnel

GPS position

N 50° 30.912', E 15° 13.784'



Geopark UNESCO Český ráj
Jiří Vlasák