Geografie 1961, 66, 6-22
Longitudinal Section and Notes on the Genesis of Lower-Situated and Valley Terraces on the Middle Course of the Labe
The lower part of a considerably wide valley of the middle course of the Labe (beyond its confluence with the Vltava) displays a rich variety of extensive terraces especially on the right bank of the river. A comparatively thick network of deep bores enabled the drawing of a detailed longitudinal section of terraces situated between Lysá nad Labem and Dolní Beřkovice in the vicinity of Mělník, as well as the following of the correlation of individual terrace elements in their cross-sections. The results achieved in some places differ from the general conception of accumulation of the youngest terraces, as becomes evident from the comparison with the terrace system on the Lower Vltava (Q. Záruba, 1942). Terrace I, forming the highest level in the longitudinal section has been preserved only in two places on the Labe in the section under investigation at a height of 38-20 m and 34-17 m (upper base) and corresponds to terrace IIIa (pre-Riss) and IIIb (Riss). The age of terrace I (Riss 1) is best evidenced by the presence of interglacial Mindel-Riss molusc fauna in basal deposits of this level at Čilec near Nymburk (J. Petrbok, 1934; V. Smetana, 1934). Terrace II forming the main accumulation in the valley of the Labe represents from the stratigraphic viewpoint the principal terrace level dating from Riss. The accumulation surface of the terrace lies at a hight of 24-26 m, the base being situated approximately at the level of the water surface. Besides, a lower erosion surface has been developed in the deposits of the terrace at a height of about 20 m. In places the level of the rocky base has been raised to as much as 10 m. The terrace on the Vltava which is one of the worst preserved and less thick terrace levels (Q. Záruba, 1942) is in a complete contradiction with conditions prevailing in the valley of the Labe. It most probably represents the eroded remains of the originally thicker deposits overlying the upper level of the rocky base. It is remarkable that the base of the Vltava terrace IVa (Würm 1) is situated exactly at the level of the Labe terrace II. The structure of the youngest terrace III is rather interesting. It has been on the one hand developed as a 20 m thick accumulation containing the filling of an overblowed furrow (base 10 m under the surface of the river). On the other hand, it reaches approximately half the thickness of the sediments the base of which lies at the level of the rocky base of terrace II. Besides the accumulation surface (IIIa) at about the height of 12 m, the river formed four lower levels (IIIb-IIIe), predominant by of an erosion origin, in the deposits of terrace III (including the river flood plain). The accumulation of terrace III dates from Würm 1. The cutting down of the river into the deposits and the origin of secondary erosion levels date from the period succeeding the accumulation of the terrace. The development of terrace III has not been finished yet. Another half of the sediments has remained to be cut through by the stream. On the river Vltava, the IVa and IVb levels correspond to terrace III. In the area under investigation not a single proof has been found so far to substanciate the theory that the deposits filling the furrow on the floor of the present valley (Q. Záruba, 1960) date from the Riss age. On the other hand, however, several relevant facts speak for a genetic relation of the furrow to terrace III (Würm 1). From the studies of the youngest terraces carried out on the lower course of the middle portion of the Labe, the conclusion may be drawn that the amount of genetically independent terrace accumulations existing in the valleys of Bohemian rivers will highly exceed the estimated figures.
