Geological Conditions

Only very few capitals have such diverse bedrock conditions as Prague, consisting mostly of the sediments of three seas which had flooded the Prague area in the course of the geological history of the Earth. The oldest rocks, dark shales and greenish graywackes, originated in the Proterozoic,i.e. more than 570 millions of years ago. The older Kralupy-Zbraslav unit outcropping NW of the city in the valleys of the Vltava, Šárka Brook and Únětice Brook, is characterized by numerous lenticular intercalations of silicites - lydites which are highly erosion resistand and form the well-known Divoká Šárka gorge as well as such rocky hills as the Ládví. The highest strata of this unit, the Zbraslav group, is characterized by a high quota of acid volcanites, rhyolites, dacites and their tuffs, forming the rocks of the Vltava valley above Zbraslav and in Břežanský Důl, with a subsidiary representation of basic spilites. The more recent Štěchovice unit consists of a monotonous flysh complex of shales and graywackes with infrequent intercalations of conglomerates with compressed pebbles. It forms a major, but generally flat area on the SE border of Prague and appears in the Modřany clough.
In the broad interval at the end of the Proterozoic and the beginning of the Paleozoic these rocks were affected by the Cadom folding phase. The folded complexes were successively flattened by degradation processes and the so formed surface was flooded again by the sea at the beginning of the Ordovician.
The complex of Paleozoic rocks overlying discordantly the folded Proterozoic strata originated by continuous sedimentation proceeding from the beginning of the Ordovician to the Middle Devonian in an elongated depression of SW - NE direction which we call the Prague Basin. In the area of Úvaly and Brandýs this formation dives below the Cretaceous formations. Due to these stratigraphic conditions the youngest rocks crop out in the proximity of the basin axis, the older strata appearing towards its edges, i.e. towards NW and SE.

Geological map of Prague (according to the data of the Czech Geological Institute, Prague).

The largest area of the basin is occupied by the Ordovician rocks, forming the bedrock of the city centre, vast areas in the East and broad zones in the NW and SE. The prevailing clayey shales and siltstones are of low endurance and appear in the depressions and flat areas of lower altitude. In the area consisting of these rocks the Vltava valley broadens to form a basin. In the Ordovician also the strata of light quartzites crop out, forming two more or less parallel stripes - the older quartzites of the Skalka on the Dobrotivá base, and the younger Řevnice quartzites on the base of the Libeň formation. They form inconspicuous elevations and rocky ridges, such as the Skalka in Košíře, the Vítkov, the Bílá Skála (White Rock) in Libeň or the Rohožník near Dubeč. Also the flysh formations, in which hard sandstones to quartzites alternate with shales, show a higher endurance. Their number includes, in the first place, the Letná formation forming rock outcrops in the Letná slope, the Hradčany ridge, the magnificent Vyšehrad promontory as well as the well-known outcrop of geological strata soaring above the highway near Závist. Volcanic rocks have only very small representation in the Prague Ordovician in the vicinity of Řeporyje (diabases). In the stripe proceeding from Dvorce via Bohdalec to Šterboholy the Ordovician formation is pierced with relatively frequent veins of minette.

The Silurian sedimentation links up closely with the Ordovician sedimentation. In its lower part the formation of shales prevails showing frequent imprints of the sea fauna - - the graptolites. Upwards the quota of calcium carbonate increases, as a result of which the Motol formation is characterized by the prevalence of calcareous shales grading into tuffaceous limestones in the environs of volcanic centres. These limestones prevail in two of the upper formations - the Kopaniny and the Požár (Přídol) formations comprizing rich sea fauna - and are exposed in the upper part of the Dalej Valley, near Chuchle, in the procession of the Barrandov rocks and in the Radotín Valley. The lower, mostly shaly Silurian, corresponds to the Liteň group, the upper Silurian with the prevalence of limestones to the Kopaniny and the Požár formations.
The characteristic feature of the Silurian is the basic submarine volcanism producing autometamorphic diabases and their volcanoclastics originating particularly in the interval between both principal periods. At the western city boundary there was a volcanic centre near Nová Ves, where its products have formed the conspicuous Hemrovy Skály (Hemr Rocks); the diabase veins appear conspicuously in the steep slopes of the Vltava valley near Chuchle and Barrandov.
Limestone sedimentation continued without interruption also during the Devonian where it fully prevailed in the Lower Devonian stages of Lochkov, Prague and Zlíchov, called after their sites in the Prague region, which form the principal limestone mass in the NE projection of the Bohemian Karst. It was only in the highest stratum of the Lower Devonian, the Dalej-Třebotov formation, that the limestone sedimentation was replaced with calcareous greenish and redish clayey shales with frequent fossils of tentaculites of the Dalej stage. The upper stratum consists of Třebotov limestones, appearing in large quarries in Hlubočepy where their erect banks form the compact faces of the Vysoká and above the Jezírko.
Limestone sedimentation terminates with Choteč limestones which, however, form part of the Eifel stage of the Middle Devonian. These are capped by the powerful Srbsko formation which, however, is characterized by sandy-clayey flysh development and locally contains frequent remains of primitive flora. It also belongs to the Givet stage of the Middle Devonian and represents the final stage of marine sedimentation in the Prague basin. The Silurian and Devonian formations show a widely diverse development and abound in fossils. They appear in large-scale natural and quarry exposures in the SW part of Prague, in the first place in the Prokop and the Radotín Valleys. They project to the right-hand Vltava bank only in a narrow strip in the Podolí and Braník area.

Upper Devonian and Lower Carboniferous are characterized by the Varisan orogenic phase in the course of which not only the Ordovician to Devonian formations, but also the underlying Proterozoic strata were folded and refolded. No sediments have been preserved from the younger Paleozoic (Carboniferous and Permian) or from the major part of the Mesozoic (Triasic, Jurassic and Lower Cretaceous). The Varisan mountain chain fell victim to degradation forces and the subsequent transgression of the Upper Cretaceous sea covered the flat ground.
The marine transgression in the Cretaceous was preceded by the formation of lakes and bogs producing clays, locally with ample remnants of flora and thin coal seams. It was followed by sandstone sedimentation, initially in fresh water, subsequently in the sea, with the formation of greenish glauconitic Koryčany sandstones. The described formation corresponds to the Cenomanian, the higher formations of sandy marlites to the lower part of the Turonian. Sandy limestones and conglomerates with a quantity of marine fauna sedimented around lydite knobs forming initially rocky islands in the Cretaceous sea. Pieces of hard lydites were moulded by the surf into coarse boulders which can be observed in the Ládví quarries.
Cretaceous rocks represent the last marine sediments in the Prague area. No folding took place after they had deposited as a result of which they are still placed almost horizontally with a slight fall towards the North to East. They form the highest plateau and represent only the remnants of the initially continuous cover. Here and there erosion moulded them into genuine table mountains, such as the Vidoule or the Bílá Hora (White Mountain) and the Petřín.
We have no proved traces of the Paleogene. Obviously it was a period of weathering, intensive surface lowering and levelling, as a result of which the whole Central Bohemia formed a peneplain at the end of the Paleogene. The Neogene gave rise to the sands, gravels and clays deposited by the rivers or settled in minor lakes which have been preserved to the present day. The deposits are mostly situated at high elevations, e.g. on the Bílá Hora (White Mountain), the Sulava or below the Ládví, and belong to an entirely different river network than the present one.
In the youngest epoch of geological history, the Quaternary, the Bohemian Massif was lifted gradually and subjected to erosive effect of rivers which increased in intensity particularly in the past million of years. The cutting of the water courses into the ground was not continuous and regular. It took place in phases conditioned by the climate, charaterized by temperature changes fluctuating between warm (interglacial periods, corresponding approximately with the present conditions) and cold (glacial periods), when the ice cap covered the whole Northern Europe and the climate in Bohemia was very harsh, comparable with that of the northern parts of Siberia.
In warm periods soils originated under forest cover; otherwise relative sedimentation peace prevailed. In the cold period the braided rivers eroded intensively the ground while depositing powerful gravelsand fields in other parts of their course. In dry phases wind brough fine dust which gave rise to loess, the well-known brick-maker's yellow clay or flying sand in the proximity of rivers.
In the course of the last million of years the Vltava with its tributaries cut more than 100 m into the ground, and the individual phases of this process are shown by steps of various elevations on the slopes of their valleys covered with gravelsand deposits - terraces. In Prague they form a number of stepped plateaux from Bohnice over Pankrác, Letná, Karlovo náměstí (Charles Square) to the lowest steps, such as Maniny. Loesses form vast covers on top of the plateaux and drifts in the valleys. Here and there they form whole series, where the individual loess covers from cold periods are separated by fossil soils originated in warm periods. Loesses are most widely spread on the left-hand bank of the river Vltava and in the North of the city. On the right-hand river bank heaps of flying sand can be found here and there. The most recent period, corresponding to the last ten thousands of years, is characterized by deposits in the floodplains of the rivers which continue to form even at present, and the calcareous tufa near some karst springs e.g. in the Čertova strouha (Devil's Ditch) in Malá Chuchle or below Zadní Kopanina.

Stratigraphic table of the Ordovician (according to J. Klomínský et al. 1995).

Stratigraphic table of the Silurian (according to J. Klomínský et al. 1994).

Stratigraphic table of the Devonian (according to J. Klomínský et al., 1995).

Stratigraphic diagram of deposits of the Bohemian Cretaceous basin in Prague environs (adapted from S. Čech et al., 1980). 1 sandy marlites, 2 calcareous siltstones and claystones, 3 saltwater sandstones, 4 freshwater to brackish sandstones, 5 claystones, 6 basic conglomerates.

CONTENTS