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 the 8th of February 1972 by the International Union of Geological Sciences in Montreal as the first formally accepted GSSP.
