Klonk

Czech national nature monument

Klonk is a national nature monument in the Bohemian Karst Protected Landscape Area in the Czech Republic. It lies in the northern part of the municipality of Suchomasty, approximately south of Beroun. It is globally recognized as a geologically important location due to the presence of the Global Boundary Stratotype Section and Point (GSSP) marking the boundary between Silurian and Devonian periods. The boundary was set based on the first appearance of the graptolite Monograptus uniformis in the bed number 20. The stratotype was ratified on 8 February 1972 by the International Union of Geological Sciences in Montreal as the first formally accepted GSSP.

The positioning of the boundary between Silurian and Devonian has been a matter of discussion since the 19th century. The so called hercynian question was resolved only after the ratification of the GSSP in 1972. The monument was declared as a protected area in 1977.

Klonk is a rocky slope by the right bank of Suchomastský brook. It contains two formations of upper Silurian (Kopanina formation, Požáry formation) and one formation of lower Devonian (Lochkov formation named for the nearby 'Lochkov profile'). Kopanina formation is represented by calcareous shales, Požáry formation by gray tabular limestones and bioclastic limestones can be found in Lochkov formation. The boundary beds are intact, without disturbance by tectonic processes, there are no facial changes and the profile is made of one continuous sequence. Apart from animals, remains of terrestrial flora can also be found in the beds, namely the genus Cooksonia. Most of the slope is covered by vegetation which is not native to the location – black pine (Pinus nigra), European larch (Larix decidua), black locust (Robinia pseudoaccacia) and Norway spruce (Picea abies). The original community of oak hornbeam forests (sessile oak (Quercus petraea), common hornbeam (Carpinus betulus), field maple (Acer campestre)) can only be found in the upper parts of the profile. The formerly woodless areas tend to be covered by bushes of dog rose (Rosa canina), blackthorn (Prunus spinosa) and common dogwood (Cornus sanguinea), atop the hill we may find an area of xerothermic grasslands such as spotted knapweed (Centaurea stoebe) and hairy oxytropis (Oxytropis pilosa).

The location is a part of long geological educational trail, which also leads through adjacent areas of importance such as Lom na Kobyle, Zlatý kůň, Koda and others. Individual localities aren't necessarily related and it is thus possible to travel only through certain segments of the trail. The trail is suitable even for cyclists and the information boards are in Czech with English abstract.