17 B GEOLOGICAL HERITAGE B.1 Geological conditions of the area Regional geological characteristics of the Iron Mountains National Geopark The Iron Mountains National area an exceptional phenomenon within Geopark (IMNG) lies around the contact the whole of Europe (see the practical of several regional geological units. sample of the geological excursion ˮ, freely The Bohemian Massif is the most “History of the Earth in two days available also at website extensive relict of the Variscan Orogen http://www.geovedy.cz/cs/files/Geovedy_ in Europe. As such, it often attracts visitors Brozura_ZS_A5_web.pdf). Scientists and for its varied geological composition. renowned geologists of the past were Geological variety of the Iron Mountains aware of this fact and studied the Iron over a relatively small area can be Mountains region. Historical line of their presented within one or two days, which activities is shown in Fig. 9. makes this Fig. 9. Early times of geology and history of research in the Iron Mountains. Source: IMNG Archive. General geological characteristics Based on current knowledge, the western of the IMNG area are based on the regional part of the IMNG area is ranked within geological pertinence of its separate parts the Teplá-Barrandian Zone – the so-called (Fig. 10). Much like the whole Bohemian Bohemicum. This includes the so-called Massif, the Iron Mountains represent Iron Mountains Proterozoic and the Iron a component of the Variscan Orogen. Mountains (or Chrudim) Paleozoic. Central 17 18 part of the IMNG area is formed of the Permo-Carboniferous Jihlava by plutonic rocks of the Iron Mountains Graben near Kraskov. Cretaceous plutonic complex (Iron Mountains sediments are of wide areal distribution, or Nasavrky pluton) and the Oheb located along the western, northern and Crystalline Complex. This complex eastern limits of the IMNG. They belongs, in terms of the regional geological concentrate to the „Long Furrow“ area and subdivision, to the Kutná Hora–Svratka the Chrudim area. Tertiary rocks are region which is, in turn, ranked dominated by volcanics at the Košumberk to the Moldanubicum s. l . Castle near Luže (Fig. 11). The eastern part of the area is formed Quaternary rocks have a wider by rocks of the so-called Hlinsko Zone distribution in the area of Rosice and (Rhenohercynicum) and rocks Chrtníky. They have the character of loess of the Poli čka Crystalline Complex rich in molluscan fauna. Quaternary (Moldanubicum). The area is also reached processes (mostly weathering and erosion) by rocks of the Kutná Hora Crystalline are prominent at a number of sites, taking Complex (Chot ěbo ř area) and the Svratka their share in the geomorphic character Crystalline Complex (area of Otradov and of the whole area. As such, they pose Prose č). a vital aspect of the Geopark geology. These pre-Variscan and Variscan Below, you can find the descriptions units are overlain by post-orogenic rocks of the individual regional geological units of the “platform ˮ units. The oldest of these lying in the Geopark territory. rocks are represented by a relict Fig. 10. Geological subdivision of the territory of the Iron Mountains National Geopark with designated regional units. 18 19 Fig. 11. Ruins of the Košumberk Castle built on Tertiary basalt (a painting from the 19th century). Iron Mountains in different periods these days. In spite of this, there are many of geological history sites displaying clear evidence – often very peculiar – of volcanic activity. Proterozoic Another phenomenon is linked with Proterozoic is the oldest geological the presence of the Iron Mountains Fault. unit represented in the Geopark territory. This major structure is now well visible It is generally constrained by ages 2.5 because it was associated with the uplift billion years to 542 million years. of the Iron Mountains by 600 m or more The oldest rocks within the Geopark have (Fig. 12). This uplift did not take place been dated to less than 700 million years. within a single day, of course: it lasted During their long history in the Iron several million years. The presence Mountains, the Proterozoic rocks were of major deep-reaching ruptures allows subjected to several major pressure and the ascent of hot fluids. Such process along temperature changes. As a result, most the Iron Mountains Fault resulted rocks of this age have been markedly in the origin of ores and other minerals. metamorphosed. Large accumulations of minerals represent The Proterozoic era in the Iron deposits which can be – in ideal case – Mountains is connected with two important exploited. The high number of mineral phenomena. One of them is volcanic deposits along the Iron Mountains Fault activity, both subaquatic and subaerial. also gave name to the adjacent mountain Prominent superficial products like range. volcanic cones have not been preserved till Fig. 12. The range of the Iron Mountains from the southeast (12a) and northwest (12b). 19 20 Moldanubicum of individuals collected during the whole history of paleontological research in this Moldanubicum is represented area (Fig. 13). Much more abundant fossil by several lower-order units in the territory finds in the Geopark territory include of the IMNG. The largest area is covered cephalopods with straight, conical shells by the Oheb Crystalline Complex (genus Orthoceras or similar), brachiopods in the SW and W parts of the area, a unit (a group similar to bivalves), and crinoids ranked to the Kutná Hora–Svratka Region which belong to the group of echinoderms. (Moldanubicum s. l. ). Lithologically most The Mrákotín area yielded finds of yet abundant types are red orthogneisses another interesting group: graptolites. intercalated with fine-grained biotite At a first glance, graptolites resemble shiny paragneisses and amphibolites. Quartzitic lines on dark stone; in fact, these are paragneisses and serpentinites are also colonies of tiny organisms ranked among present. Stratigraphic rank of the meta- the group of hemichordates. sedimentary units and the age of pre- Variscan orthogneisses are unknown. Paleozoic rocks in the Geopark territory posed an important source Rocks pertaining to the Kutná Hora of building material, especially in the past. Crystalline Complex are exposed Very hard Ordovician quartzites were used to the south of the Oheb Crystalline for the construction of buildings. Today, Complex, in the E and NE surroundings one of the largest and most attractive of Chot ěbo ř. They are represented quarries in the Geopark – the Prachovice by reddish orthogneisses, banded Quarry – is opened in the Paleozoic rocks: migmatites and biotitic greywacke Silurian and Devonian limestones (for paragneisses. Two-mica migmatites and Silurian geology in the IMNG. biotitic migmatites are present The importance of this area in the past and in the immediate surroundings present is stressed by the educational of Chot ěbo ř. Rocks in the E part trail “Around the Prachovice Quarry ˮ. of the Geopark area (around Otradov and The trail also provides views of otherwise Prose č) are ranked to the Svratka inaccessible mining area. Occurrences Crystalline Complex. They are dominated of Late Paleozoic (Permo-Carboniferous) by porphyroclastic orthogneisses with rocks in the IMNG area are rare, being frequent intercalated bodies concentrated solely to the area of Kraskov of amphibolites, limestones and skarns. near Se č. Reddish fluvial sandstones to conglomerates locally contain finer- Paleozoic grained tuffitic beds. Paleozoic is the general term for These fluvial sediments were the era constrained by the ages of 542–251 reported to yield rare finds of fossilized million years. This long stage in the Earth wood (araucarites). The relict near history is subdivided into several periods. Kraskov documents the southern end Each of the periods is represented – at least of the so-called Jihlava Graben, which to a limited degree – in the Geopark developed on the deep-seated P řibyslav territory. The Paleozoic era is linked with Fault. Besides this relict, isolated one of the most typical and most famous occurrences of Permo-Carboniferous rocks fossil representatives: a trilobite. This of the Jihlava Graben have been also ancient arthropod can be found at several described from the SW surroundings sites in the IMNG. Its finds are sporadic of Hradec Králové. and rare, with only several dozens 20 21 Fig. 13. Brloh. A quarry face in quartzite (Paleozoic – Ordovician) and boulders of the surf facies (Mesozoic – Cretaceous). The quartzites contain trilobite fauna. Mesozoic display effects of ancient sea surf – large, abraded boulders (Fig. 13). In contrast to the Paleozoic record, Mesozoic rocks preserved in the Geopark After moving only a few hundred represent only a relatively short time span metres basinward, the visitor finds himself (ca. 100–85 million years). In spite of this, in an “open ˮ sea full of sharks, fish, the Geopark features sites with unique bivalves and cephalopods. Abundant geology, which are worth our attention. remains of these organisms can be also found in the sedimentary record today The period represented (Fig. 14.). by the preserved Mesozoic sediments was marked by the biggest known flooding The wide variability of land surface in the whole Earth history. in environmental conditions over a small This flooding resulted in the origin area is also documented by the occurrences of a number of shallow, relatively warm of freshwater and swamp environments. seas with flourishing life. Then, most Such sites contain fragments of Cretaceous of the Geopark area was lying under coal together with remains of leaves and the water. Some places, however, were stems of plants inhabiting the Cretaceous lying near the shoreline. Such places now swamps. Fig. 14. Chrtníky. A quarry in basaltic rocks (Ordovician) and marlstones (Cretaceous) containing marine fauna. Excursion of the secondary grammar school from Chrudim. 21 22 Tertiary formerly subjected to exploitation for brick production. Sporadically developed eolian Tertiary rocks are not too abundant sediments form rather significant in the IMNG area.