49°54′03″N 14°03′40″E / 49.90083°N 14.06111°E / 49.90083; 14.06111

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 6 kilometres (4 mi) 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.

In respect of its being the location of the 'first global boundary stratotype section and point (GSSP)', Klonk Hill was included by the International Union of Geological Sciences (IUGS) in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as "a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history."

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.

There are various paleontologically significant objects, apart from the already mentioned graptolites it is possible to find the trilobite Warburgella rugulosa typical for the base of Devonian period, the conodont Icriodus wolschmidti and the chitinozoan Agnochitina chlupaci. Various other invertebrates can be found in the profile, such as bivalves, gastropods, crustaceans and brachiopods. Apart from animals, remains of terrestrial flora can also be found in the beds, namely the genus Cooksonia. Soils are rather undeveloped, only close above the brook in the lower part of the slope clayey screes can be found.

There is a unique monument representing the geological boundary set by international agreement. It was built in 1977 by academic sculptor Jiří Novotný.

Various plant communities can be found at Klonk. The vicinity of the brook is characterized by human planted vegetation, for example the common alder (Alnus glutinosa), European ash (Fraxinus excelsior) or Canadian poplar (Populus x canadensis), in the lower parts of the slope are the remains of scree forests with dominant sycamore. Shady limestones by the brook are a favourable location for a number of rare bryophytes, such as the craven featherwort (Pedinophyllum interruptum) or beech feather-moss (Eurhynchium crassinervum).