Geomorphology of the Environments of Říp and Mělník (Central Bohemia)
Authors: Balatka Břetislav , Loučková Jaroslava , Sládek Jaroslav 
Published: January 2024

The present paper treats of the results of a detailed geomorphological investigation carried out along the lower course of the Vltava, and in the wide environment of the Labe, up stream from its confluence with the Vltava between Mělník and Roudnice n. L. Characteristic features of the morphology of the area under investigation are erosion-denudation phenomena on the one hand, and accumulation phenomena on the other. Erosion-denudation phenomena are represented by structural denudation plateaus occuring especially on the right bank of the Labe, and developed at several different levels. Their approximate age may be determined by their relation to the development and the genesis of the valley of the Labe, and especially, to the terraces. The oldest structural denudation plateaus occuring at a height of 325 m date from Later Tertiary (Pliocene), others are of Quarternary age (predominantly Early Pleistocene, rarely Middle Pleistocene, only exceptionally Late Pleistocene). In the north-eastern part of the mapped area, structural and tectonic conditions of cretaceous rocks played an important part in the origin of pictoresque sandstone cities as well as in the development of deeply incised canyons, which most often, follow the course of fissures and faults in the cretaceous sediments. The place of confluence of two main Bohemian rivers - the Labe and the Vltava - in the area between Říp and Mělník was shifted several times in the Quarternary. The most effectual influence upon the intricate development of water streams in this area exercised the basalt hill Říp nowadays situated in the middle of accumulations of Eearly Pleistocene terraces. The terrace material had been deposited mostly by the Vltava after it had left in the Algonkian its close valley before Kralupy n. V. Besides its petrological composition, this terrace material differs strikingly with its coarser grain from that deposited by the Labe. The uppermost river sands and gravels at northern foot of Říp and on the Sovice hill (at a height of 278 m-130 m above the surface of the Labe) most probably date from the Latest Neogene (Pliocene). Pleistocene terraces have developed here at all levels. A detailed study enabled us to determine in this area a terrace system different from the one on the Vltava (compiled by Q. Záruba 1942). Seven large Pleistocene terrace accumulations were distinguished which is less than Q. Záruba had distinguished on the Vltava. From these terrace accumulations we have excluded levels formed by lateral river erosion. In this way 15 terrace levels could be determined on the whole. From a comparison with the longitudinal profile drawn by Q. Záruba, becomes evident that his terraces Ib and II a form a single terrace accumulation (our terrace III), just as terraces IIIa and IIIb (our terrace V) and terraces IVa and IVb (our terrace VII). The Eolian accumulation in the area under investigation was composed of loess cover and loess drifts mostly of Middle and Late Pleistocene age, and of sedimented sands which form sand dunes mostly of west-eastern direction reaching in places the height of 5-6 m. They were blown here in Würm from river deposits. Periglacial processes resulted in ice-wedges and solifluction which asserted itself most strikingly at the foot of the basalt hill Říp where it formed a continuous solifluction cover of Early and Middle Pleistocene age strewn with boulders mixed with cretaceous debris and Eolian sediments. Periglacial processes together with the Eolian activity caused the asymmetrical development of some valleys (the best example is the valley of the brook Čepel in the place where it heads from south to north). The river network in this area passed through an intricate development, especially in the place of the confluence of the Vltava and the Labe. Changes of similar kind can best be traced on enlarged surfaces of terraces. In the Earliest Pleistocene at the time of terrace I, the Vltava skirted the eastern foot of Říp and devided into two branches at the time of accumulation of terrace II. The western branch headed from Velvary northwards, the eastern branch followed the western brim of the Horní Beřkovice Plateau and the northern side of the Krabčice Plateau towards the Labe which it joined most probably east of Libkovice p. Ř. At the time of origin of terrase III, the Vltava headed from Veltrusy towards north-north-west to Roudnice n. L., and farther towards north-west to the Lower Ohře. Its second branch most probably flew along the eastern side of Říp. The river Vltava shifted its stream permanently at the time of terrace IV. This terrace follows the lower course of the Vltava and the Labe along its present valley. At the time of terrace V the river started forming a large meander north-west of Cítov. At that time also the Roudnice meanders started developing (and have kept on doing so up to the present), as indicated by B. Zahálka (1946). B. Zahálka presumes the main cause is the hill Sovice built of rocks hardened with basalt. The Cítov meander was abandoned by the river as early as in the erosion phase between terrace V and VI. Simultaneously with the origin of terrace VI the basalt Jenišovice hill was exhumed. The largest part of the Labe stream flew through the tectonic depression of Mělník, the side stream most probably through its present valley. At the time of terrace VII, the Vltava skirted on all sides the foot of the Jenišovice hill with the remains of the plateau of terrace VI. At the time following the origin of the terrace level VIIb, the Vltava was captured by the Labe in the place of their present confluence under Mělník, whereas before that the place of their confluence had been constantly moving up-stream. At the time of terrace VI both rivers most probably joined somewhere in the vicinity of Dolní Beřkovice. The most important factor for the development of relief in the area under investigation was the erosion and accumulation activity of rivers, which gave rise to terrace levels. In our opinion - based upon studies carried out in this area - during the deepening of the valley, after the accumulation of the terrace sediments, the river started cutting down into its own deposits, and forming - through lateral erosion - lower levels on the gradually narrowing valley floor. Through a further intense deepening, the river cut through its own sediments and their substratum down to the comparatively narrow floor of a more or less outstanding depression in the valley bottom, and is consequently the lowest place of the next terrace. In the period of accumulation, the river deposited first of all its sand-gravel load in this depression; upwards, valleys widened to the detriment of the deposits of higher-situated terraces and the cretaceous substratum. The upper layer of the terrace sediments was deposited as high as the level of the accumulation surface of the terrace. Processes of accumulation and lateral erosion participate in the origin of terraces. The main erosion phase of the development of the valley dates from the time succeeding the deposition of terrace IV, and from the time of the origin of Middle Pleistocene terraces, at which time valleys of main rivers and their tributaries were deepened. Valleys (mostly dry) and valley depresions, dividing plateaus of Middle and Early Pleistocene terraces in the area of the Říp plateau, originated already in Earlier Pleistocene and acquired their present form in Middle Pleistocene. In Later Pleistocene numerous short valley depressions were deepened no more. There are mostly fossil phenomena formed in periglacial climate in conection with the erosion phases of the main streams. The oldest network of valleys dating from as early as Younger Tertiary occurs on the right bank of the Labe in the Jizera Plateau and Polomené Hills. Practically no changes in the course of valleys took place in Pleistocene, they kept just on cutting down most intensively. The youngest erosion processes manifested themselves in the deepening both of valleys and valley depressions, and in the formation of deep gorges.