The Klonk NNM is of worldwide stratigraphical importance, as it lies on the border between two older Palaeozoic formations – the Silurian and the Devonian.
The national nature monument lies in the parish of Suchomasty in the Český kras – Bohemian Karst and in the Středočeský kraj – Central Bohemian region. The monument was declared in 1977 and covers an area of 8.9ha and lies at elevations of 322-418 metres above sea level.
The steep slope known as Klonk lies to the north of the village of Suchomasty and above the Suchomastský potok Stream and is built of layers of limestones from the Upper Silurian and Lower Devonian periods. The right-hand (southern) part of the outcrop is built of tuffite shales and limestones of the Kopanina formations. However, the left-hand (northern) part of the outcrop is of stratigraphic importance. Here the Požáry Formation from the uppermost Silurian and the lower positions of the Lochkov Formation from the lowest Devonian are exposed. The highest Silurian and lowest Devonian are both developed lithologically here as dark-grey and mostly fine-grained block limestones, which are interbedded rhythmically with bodies of calcareous shales. There is no change in appearance and the profile forms a continual sequence.
The border between the Silurian and the Devonian is defined by the increasing numbers of the dominant graptolite Monograptus uniformis, which is regarded as the lowest Devonian graptolite zone on a world-wide scale. Here the graptolites are accompanied by other stratigraphically significant animal groups. When the graptolites and other fossilized animals occur together, this enables mutual controls of the stratigraphic data and wider international comparisons to be carried out. These fossils include trilobites such as Warburgella rugulosa, which is a characteristic species for the base of the Devonian. The conodonts Icriodus woschmidti and Angochitina chlupca are also dominant fossils which can be found at Klonk. These dominant species are accompanied by numerous other fauna types, such as nautilids, graptolites, trilobites, molluscs, gastropods, brachiopods, phyllocarid crustaceans etc. Remnants of terrestrial flora have also been found here. The sequence of border strata is exposed on the Klonk slope without any negative tectonic disturbance. The individual layers in the exposure profile are numbered. The actual border between the Silurian and the Devonian takes place inside layer # 20.
Klonk was chosen as a stratotype of the Silurian–Devonian border after wide international discussion and many assessment trips to Silurian-Devonian regions around the world. The final decision to recognize Klonk as a stratotype was made by the World Stratigraphic Commission at the 24th International Geological Congress in Montreal in 1972. Klonk was the first stratotype of a border between geological formations, which was accepted according to the stricter new rules of stratigraphic classification.
The steep slope of Klonk is protected as a geological site of world importance. The locality was declared a protected natural formation in 1977, and is now a national nature monument. A monument which symbolizes two geological strata, was erected at Klonk in 1977 to commemorate the international declaration. The memorial is by the sculptor Jiří Novotný, is made of Zbuzany marble (Dvorecko-Prokopský limestone), weighs 6 tonnes and was the first geological monument in the world.