Aerial view of the árka Brook canyon.
Ground Surface
After the retreat of the Cretaceous sea Prague basin consisted in a monotonous plain covered with almost horizontal sediments. At that time the Cretaceous strata formed a continuous cover of the older, folded bedrock consisting of Paleozoic and Proterozoic rocks. For the long period of the youngest Cretaceous and the older Tertiary this levelled ground was exposed to intensive degradation proceeding in tropical and subtropical climate which accelerated the origin of deep weathering. The younger phase of Older Tertiary (Oligocene) gave rise to a flat ground on top of which we find sand and gravel accummulations with clayey strata which, according to preserved flora remains, belong to the Neogene. The topmost gravels, preserved e.g. on the Sulava near Černoice or on the White Mountain, belong to Miocene, the lower accumulations uncovered e.g. in sand pits near Kobylisy to the youngest Tertiary - the Pliocene.
Into these high strata the valleys of the Vltava and its tributaries have cut deeply. In the course of the youngest Tertiary and particularly the Quaternary their erosive activities moulded a richly articulated ground relief lending Prague its picturesque natural frame. In the inner Prague the old Tertiary plain was removed almost completely - with the exception of the Petřín hill the top of which is still covered with Cretaceous sandstone and sandy marlite. The forces of erosion denuded the Cretaceous sub-base consisting of folded rocks of diverse endurance and moulded highly articulated ground in which basins alternate with flat steps and plateaux at various elevations above the valley floor, as well as steep slopes and ridges soaring conspicuously above their surroundings. In the hard Proterozoic rocks north and south of the city the Vltava made a narrow, rocky canyon. Also the valley of the Radotín and Dalej Brooks, cutting deeply into the limestone of Silurian and Devonian formations, acquire canyon character locally with remarkable rock formations due to different characteristics of the limestones forming them.
The picture of inner Prague is enlivened markedly by the strips of hard Ordovician quartzites projecting like sharp ridges in the broad Motol valley, forming the Vítkov hill, the Bílá Skála (White Rock) in Libeň, conspicuous ridges on the Lower Rokytka and a number of other elevations. Even more registant are the Proterozoic lydites in which the árka Brook has produced the wild Dbán gorge and the rock formations of the Dívčí Skok (Girl's Jump) and the abák, ranking among the tourist attractions of the Prague nearest surroundings, similarly as the wild ridge of Kozí Hřbety and the Holý Vrch in the Tiché Údolí (Quiet Valley) below Únětice. Rock formations characterize also the Silurian diabases, forming the rock face below the church in Chuchle or the Calvary in Motol.
This diverse ground relief originated mostly in the course of the youngest geological epoch, the Quaternary, when the rise of the Bohemian Massif accelerated which resulted in more intensive deep erosion. The witnesses of these processes are the terraces of the Vltava, i.e. the remainders of gravelsand aluvia which had formed the valley floor in the past, but were cut through by subsequent erosion and were preserved locally in the form of steps of various elevations on the sides of the valley, particularly in deeply cut meanders.
Prague is characterized by the large meander of the Vltava flowing around the Letná, the plateau of which is covered with the gravels of the Letná terrace of the Middle Pleistocene and the lowest step of which is represented by the large area of the Maniny terrace in Holeovice. It is possible to say that most stepped plateaux in Prague are the Vltava terraces of various elevations. This is proved by the fluvial gravelsand covers on the flattened top of the Braník Rock, on the Vyehrad, below the Charles Square, as well as in the 90 - 100 m heights above the canyon of the Vltava in Bohnice or Lysolaje.
This step, developed in considerable thickness particularly in the vicinity of Suchdol and Lysolaje, is situated above the deep incision of the Vltava canyon and represents a significant divide. It is the first terrace of Quaternary age which separates the Vltava canyon, created by intensive down cutting in the course of the Quaternary, from the shallow, broad valley which had existed here before the creation of this canyon and the edges of which were situated frequently at a considerable distance from the present-day valley. An example is the lydite ridge of the Kozí Hřbety and the Holý Vrch, forming the cut bank of this terraced step, below which also the old river arms filled with ample fossil fauna (recalling the present fauna of the pools in the Labe lowlands) have been preserved.
The western environs of Prague are characterized by horizontal plateaux consisting in the remainders of the sub-horizontally placed Cretaceous sediments. From the initially continuous tableau erosion has moulded table mountains here and there on the sides of which particularly Cenomanian sandstones crop out, covered by a plate of marlstones. Typical examples of such relics are particularly the Vidoule and the Petřín hills together with the White Mountian. Here and there the Cenomanian sandstones form a sharp edge in which minor rock groups have been preserved as a start of future rock cities. An example of this development are the protected Petřínské Skalky as well as the minor canyon in the Vinoř valley. To the south the Cretaceous cover disintegrates into minor relics, often formed by Cenomanian sandstones, such as in the environs of Slivenec, above Chuchle and in the Klánovice wood.
On top of the Devonian limestones of the Bohemian Karst minor karst forms have originated in the Prokop and Radotín Valleys. The biggest of them was the Prokop Cave, some 100 m long and relatively spacious, a pilgrimage place in the past which, however, fell victim to the expansion of the local limestone quarry. The Corral Caves in the rock amphitheatre above the former swimming pool near Klukovice are the reminder of a bigger cave system.
The number of karst phenomena includes also resurgences, in the first place the karst spring in Zadní Kopanina, one of the biggest in the Bohemian Karst area, which created thick strata of calcareous tufa in the Kopanina gorge, at present cut through by the youngest erosion. This erosion still takes place in the steep lateral gorges, such as the Čertova Strouha (Devil's Ditch) near Malá Chuchle or the Bendovka in Podhoří. In major valleys, however, the water courses deposit flood loams and the fill at the valley floor attains a thickness of as many as 10 m. This applies also to the Vltava floodplain in which loose meanders, at present straightened by canalization, used to exist in the past. The Malá Říčka in the Stromovka park or the old Berounka - Krňák below the Zbraslav castle are their remainders.
From the number of minor formations mention should be made of the ravines linked with easily eroded rocks and soils, particularly old weathered waste. In the natural reserve of Prokop Valley the Bílá Rokle (White Ravine) near the Hlubočepy cemetary is protected at present. Northwest of Prague their number includes the Housle ravine in the walls of which pleistocene loesses, Cretaceous sandy marlites and sandstones (from top to bottom) can be found with the tropical weathered detritus of the Proterozoic sub-base at the bottom. Ravines have been eroded also in ancient westhered wastes on the left hand side of the Tiché Údolí (Quiet Valley), at present concealed with woods.
The relief of Prague bears also numerous traces of human activities. The selective extraction of limestones of various types in the Hlubočepy area has created remarkable landscape scenery exquisitely disclosing the variegated structure of the Silurian and Devonian formations. By way of example let us mention the smooth faces of the Vysoká, consisting of erect strata of Třebotov limestones, the artificial canyon of the Jezírka originated by the extraction of a layer of the same limestone, as well as the so-called cliffs at the boundary of Hlubočepy below the cemetery. Also the Branická Skála, the parade of the Barrandov Rocks as well as the famous Prokop Rock below the former church, exposing the sharp boundary between the Prague and the Zlíchov Lower Devonian formations, belong to this group.
In conclusion a few words should be said also about the Quaternary accumulations softening the sharp erosion edges moulded in the bedrock. On the right hand river bank they include primarily the windblown loess loams and loesses the powerful drifts of which frequently cover the cut banks preserved above the terraced steps (the Juliska in Podbaba), on the left hand river bank flying sands. Where the brooks have been left to spontaneous development in major valleys, the accumulation activities connected with the origin of meanders have been renewed (Tiché Údolí, árka Valley below the Vizerka).
Aerial view of the árka Brook canyon.
Morphological representation of inner Prague showing the remainder of the Cretaceous table, Vltava terraces and loess drifts (terrace and loess occurrence according to Q. Záruba, 1948, J. Králík, 1983 and I. Chlupáč, 1988).
k - tabular Cretaceous sediments (Cenomanian - Turonian).
n - remainders of Neogene gravels.
Block diagram of Prague and nearest environs (according to the data of the Czech Geological Institute).