Geological History of the Geopark

Very long and rich geological history of the Geopark is evidenced by the presence of a wide variety of rocks from different geological periods.

The oldest rocks encountered in the Geopark territory come from the latest Precambrian, being about 550 million years in age. Granites and diorites which penetrated from the Earth interior to this level during the Cadomian orogeny were, however, later turned into muscovite metagranites and metadiorites during the Variscan orogeny in the Late Paleozoic. As such, they can be found in the Geopark now, especially in the channel of the Jizera River near Semily.

Rocks exposed to the surface around the village of Jílové near Držkov can be dated to the Early Paleozoic, probably to the Cambrian to Ordovician. These weakly metamorphosed chlorite-sericite phyllites (“Železný Brod schists”) are unique in their contents of ichnofossils (trace fossils) as well as body fossils. Bodies of these rocks were folded together with lenses of older crystalline dolomites. The presumably biggest dolomite lens crops out near the village of Bozkov and is probably Cambrian in age. The dissolution of these carbonate rocks led to the formation of a relatively large cave system. Rich karst decorations in the caves result from stream erosion and the presence of zones of contrasting resistance in the rock massif.

The crystalline rocks are separated from younger units further southwest by a prominent geological structure: the so-called Lusatian Fault. This structure experienced repeated tectonic movements of blocks on its either side in the past and remains active till today. The eastern and northeastern parts of the Geopark are covered by sedimentary rocks of the Krkonoše Piedmont Basin. These include mostly freshwater sediments, fine lacustrine claystones and siltstones, or fluvial sandstones and conglomerates, often arkosic (with an admixture of feldspar). Thin coal seams and horizons of bituminous shale (combustible carbonaceous shale) are locally present, occasionally with low-grade copper mineralization. Sediment deposition was taking place from the Late Carboniferous to the Early Triassic, and especially the shales contain abundant fossils from these periods. Flora is represented by several species of ferns, sphenopsids and conifers, while faunal remains include mostly clam shrimps, fishes, sharks, amphibians and insects. Numerous ichnofossils and coprolites (fossil droppings) are also present. Arkoses in the Nová Paka area well known for their rich occurrences of petrified (silicified) stems and other parts of plant bodies. Stems of arborescent sphenopsids, ferns, pteridosperms and conifers have been reported from this area. Deposition of material in the basin was accompanied by several stages of volcanic activity, producing basalts (so-called melaphyres), pyroclastics and also siliceous solutions. The latter encrusted parts of plant bodies, gave rise to carnelian concretions in the sediments, and filled vugs in the melaphyres in the form of chalcedony, agate and other forms of silica.

The central and the southern parts of the Geopark are covered by rocks of Cretaceous age. After the Late Cretaceous (Cenomanian) marine transgression some 100 million years ago, quartzose sandstones were deposited in this area in a thickness of several tens of metres. The deposition continued for the next ca. 15 million years (in the Turonian and Coniacian). This resulted in a ca. 100 m thick succession of calcareous sandstones and sandy limestones, followed by another package of quartzose sandstones of about the same thickness. The deposition was terminated by the retreat of the sea at the end of the Cretaceous. Later on, extensive rock cities were developed in these sediments by erosion.

In the Neogene, about 17 million years ago, volcanic activity occurred again, being linked with the Lusatian Fault. It peaked in several stages and lasted until the latest Tertiary 4.5 million years ago. Mostly basic volcanism produced some of the well-known landmarks of the Geopark, such as the hills of Trosky, Zebín, Kozákov, Čertí kopeček, Kumburk and others.