Geografie 1961, 66, 305-325
Geomorphology of the Valleys of Small Tributaries to the Vltava North of Prague
In 1960-1961 we occupied ourselves with geomorphological studies of small tributaries to the Vltava. Geomorphologically, the relief of the area under investigation is formed of the precretaceous peneplain of the Tursko Plateau in the West with lydite ranges and monadnocks. The whole area used to be covered with sediments of the Upper-cretaceous Sea at the present preserved only in the form of islands. The eastern part is formed by the highest terrace plateau of the Vltava. The area is almost entirely covered with loess. Approximately in the centre of the region the Vltava deeply cut down its channel. Along its present course 11 Pleistocene terrace steps have been preserved. The whole area, under the Quarternary mantle, is predominantly composed of Algonkian rocks (slates, lydites, spilites) and chalk. The most usual type of soil is loess on which developed Central Bohemian černozem. Less often occur sandy, argillaceous loams and loams (on terraces and Algonkian slates). From a larger number of small tributaries, we studied more carefully the valley of the Kamýk Brook, the Letky Valley, The Kocanda (left bank), and the Klecany, Větrušice, Máslovice and Zlončice valleys (right bank). On longitudinal profiles of all above-mentioned valleys striking changes in gradient may be perceived, repeating themselves approximately at similar levels above the surface of the river Vltava. They are not only due to local conditions (petrological composition of the substratum) but to certain erosion periods in the development of the Vltava valley (local erosion base). At their beginning all the valleys are shallow, bowl-shaped or prolonged rills. These are the oldest parts of the valleys. Investigations carried out in the field resulted in geomorphological and pedological maps of those valleys on which we drew the present level and position of steps and the corresponding cuts. When determining the relative age of valleys and steps on their axes, we started from the present height and situation of the terraces of the Vltava, especially their bases. We succeeded in reconstructing the original gradient condition under individual steps because through the activity of a whole series of agents (backward erosion, loess accumulation, debris) their present level and distance from the Vltava differs from the original one. The valley of the Kamýk Brook we consider the oldest. It was formed as early as in Pliocene as a tributary of the Vltava which was flowing over cretaceous sediments at that time. Its mouth has been shifted in Pleistocene to south-west in a similar direction as the undercut slope of the Podmoráň meander. Its declivity course is balanced, highest steps have been preserved 70-80 m above the surface of the Vltava and correspond to the erosion preceding the deposition of the middle terraces of the Vltava - the Vinohrady (M 1) and the Letná (M 2) terraces. The lowest step - 27 m above the river surface - dates from the youngest Pleistocene (erosion period preceding the deposition of the terraces IIIc, IVa, IVb). The oldest valley - cut in terraces - is the Klecany Valley on the right bank of the Vltava. Its upper reaches are composed of Pliocene Zdiby gravels and date from the Pleistocene. The Větrušice, Máslovice, Zlončice and Letky valleys date aproximately from the sametime as the Suchdol (D 2 + 3) terrace. At levels of about 235-260 m steps have been preserved in them which date most probably from the intensive erosion period preceding the deposition of the middle terraces on the Vltava. Outstanding steps occur also at 20-30 m above the surface of the Vltava. They are considered the youngest, dating from the erosion periods of the lowest terraces of the Vltava, i. e. the Veltrusy (R 2), Maniny (W 1) and valley (W 2) terraces. The youngest valley under investigation is the Kocanda Valley. Its upper reaches were formed in the period of the Kralupy terrace (G 2+3). Quite outstanding are steps occuring at 69 m above the Vltava surface (Vinohrady terrace) and 27 m above the river dating from the time of the Veltrusy, Maniny and valley terraces. The results of the studies prove the closest relation between the development of the Vltava valley and the valleys of its short tributaries.
