Geografie 1961, 66, 326-344
Geomorphology and River Terraces on the Course of the Labe in Central Bohemia
Besides geomorphological mapping the work treats of relations and age of structural, erosional and accumulation phenomena occuring along the course of the river Labe (Elbe) on the Bohemian territory. It comprises studies of an area of about 420 km2, extending in Central Bohemia from the mouth of the Cidlina river up the stream of the Labe as far as Přelouč west of Pardubice. In this work the author starts from the investigations pursued by K. Žebera, from Quarternary investigations of the north-western part of the Železné hory (Iron Ore Mountains) by J. Sekyra, and from the studies of the environs of Kolín by L. Urbánek. When distinguishing individual terrace levels, older studies already expressed the hypothesis of subsidences of the Labe Plateau near Sadská that might have taken place in the course of the Pleistocene accumulation. In the area under investigation, the following main phases in the post-cretaceous development of surface phenomena preserved in different extent have been distinguished: 1. Modellation of surface phenomena during the regression of the Cretaceous Sea; 2. Tertiary peneplanation by Saxonian tectonic movements, the contact line between Železné hory and the Cretaceous; 3. Quarternary fluvial erosion and accumulation; 4. Fading tectonic movements in Middle Pleistocene; 5. Periglacial influences of the youngest glacials; Eolian phase in the Earliest Pleistocene and Postglacial. In the time of the Saxonian movements, phenomena left over after the regression of the Cretaceous Sea at the edge of the cretaceous plateau were quickly removed by erosion. The highest levels of the Crystallinicum plateaus of Kutná Hora near Křečhoř present more or less preserved parts of the Central Bohemian Oligocene Peneplain, lowered by denudation. Meanwhile, on the higher peneplain the monadnock range Oklika (307 m) is the only one cropping out with its lydites over the surface, under this level monadnocks occur quite abundantly. Predominantly they are exhumed cretaceous rocks (Bedřichov 279 m, near Spytovice 230 m, between Týnec n. L. and Krakovany etc.) or stripes of more resistant rocks exposed in Middle Pleistocene by a fast denudation. The majority of monadnocks - affected by periglacial weathering - were lowered during the Quarternary and levelled up with their own deluvial material, in the area of Železné pohoří also with eolian sediments (J. Sekyra 1955). The zone of young radial faults of Sudetian direction formed a narrow cretaceous graben on the southwestern side of the faulted mass of Železné pohoří. In Pliocene and Pleistocene, the relief went on being consistantly rejuvenated through movements of blocks along this line causing on the one hand the uplift of the range of Železné pohoří itself, and the subsidence of the foothill area on the other. In inaglacial phases cretaceous sediments were affected by intense mechanical periglacial weathering - espiecially solifluction - which gave the weathering soft marls the shape of round hills divided from one another with flat depressions and wide valleys. The intensity of the denudation activity in the cretaceous plateau of the Chlumec peneplain may be judged from the height of level of the Labe Pleistocene terraces occuring in shallow valleys and saving with their gravel mantle the marls substratum from destruction, especially in the Periglacial milieu (inversion of the relief through terraces). Consequently, the 100 m thick layer of cretaceous sediments may be considered the vertical extent of denudation in the Quarternary. Even if this amplitude got modified to a certain extent by tectonic movements, taking place in the Quarternary, the main importance may be ascribed to the denudation agents of predominantly periglacial character which asserted themselves in the Pleistocene (especially solifluction), and to the fluvio-erosional activity of the river Labe itself (K. Žebera 1956). Consequently, it may be assumed that cretaceous sediments covering in the past the present exposed slopes and plateaus of the northern part of the Železné pohoří, used to be of a considerable extent. In the Quarternary period of maximum erosion of the Bohemian Mass - between the Mindel and Riss glacials and in the earlier Pleistocene - the river Labe did not shorten its course as most of the rivers did but lengthened it in the southern direction. Through the subsidence of the cretaceous block between Kolín (and Velím) and Velký Osek (and Poděbrady) a local erosion base had been established. As a consequence a fresh meander was formed in the river course, which enabled the upstream erosion to come into effect and affect the older, tectonically predisposed gorge near Týnec n. L. The course of the base of the lowest gravel-sand terrace shows that the erosion step retreated as far as the contact line with less resistant cretaceous rocks east of Týnec n. L., and that the river ceased cutting down in its upstream portion. It reduced its gradient through meandering, keeping it however sufficient to enable erosion activity in the valley near Týnec n. L. Before entering it, the river deposited low extensive gravel-sand accumulations. The middle (Mindel) and lower (Riss) terraces in the subsiding Nymburk block in the Labe Basin weren destructed by the transportation activity of the river. Vast quantities of gravels had been most probably deposited in the confluence area of the Labe and the Vltava and were the main cause of the reduction of the river gradient and a consequent clogging of the river channel. The reduced erosion base of the Labe in the Nymburk Basin caused the brooks-flowing down from the Crystallinicum of Kutná Hora - to start an intense downward erosion. In this way, a rich relief was achieved through a finger-shaped drainage-system of deeply-cut valleys, at the present cut down in places through Pre-Quarternary into the Pre-Cretaceous substratum. The brooks (Pekelský potok, Výrovka etc.) in their lower reaches make an abrupt bend for the North-Eeast, vertical to the direction of the tectonic of the Železné pohoří along which also the southern cretaceous block of the Nymburk Basin subsided. On the floor of the Labe Valley between Veletov-Tři Dvory-Kolín-Veltrusy and Vrbová Lhota (also in the vicinity of Sadská as mentioned by R. Sokol) the ascertained graben is mostly filled with coarse-grained sand-gravels. The upstream aggradation of gravel-sands ironed out the erosion step in the Týnec Valley. After the deepening of the channel of the Labe by erosion had been completed, a continuous accumulation and resedimentation took place, lasting through the whole last glacial, the postglacial and - with small interruptions - up to the present day. After a not very distinct erosion the youngest gravel-filled channel towards the North to the mouth of the Cidlina and accumulate to form in the eastern and southern part of the Nymburk Plateau. The deposits extend north-east of the gravel-filled channel towards the North to the mouth of he Cidlina and accumulate to form shallow deposits of sands - only exceptionally containing gravels - on the lowered western brim of the Žehuň Plateau. In the last stadial and postglacial the river ramified into several branches. Inundations left behind shallow depressions which have become filled with coarse-grained drifted sands. In the main Würm period of the Eolian phase (Hochglacial), eolian sediments were deposited in zones. The filling of ground ice wedges and other cryopedological forms is the best evidence of the fact that drifted sands existed already in the time of the extinction of ice forms. In the postglacial finer drifted sands as well as sands from valley and low terraces were resedimented in the deserted branches and on the lowest erosion step. Loess sediments covered eastern and south-eastern slopes of the ranges and older erosion and tectonic edges, which they rounded and flattened, or levelled the irregularities in the relief. Considerable gradient (in old meanders as much as 5%%) and the covering of the drifted sands by the Holocene flood-loams in some sections of the valley of the Labe east and north-east of Kolín as well as the sinking of the lower contact-line of the mollisol, points to a recent subsidence of the cretaceous block in the valley of the Doubrava and its continuation to the Nymburk Basin. Other proofs of tectonic dynamics are the historical earthquakes that took place in the environs of Týnec n. L. and to the North of Kutná Hora (Michal 1940), as well as the horizons of mineral water on the Poděbrady spring line and its south-eastern continuation.
