| Summary: | DEVOLOPMENT OF THE PLEISTOCENE VALLEY OF THE MIDDLE VISTULA RIVER, CONSIDERING ESPECIALLY ITS GAP IN THE SOUTH POLISH UPLANDS Summary In this paper, referring to the publications of Pożaryski [18, 19] published forty years ago, we take into account results of new chronostratigraphic studies. Basic materials for this study are: a) unpublished data of Pożaryski's field investigations and examination of several hundred borings made in 1953 along cross lines ofthe Vistula valley (Fig. 1); b) partially published [20] results of TL datings of the basic Pleistocene profiles. It has been found that in the axial part of the Vistula river gap in the South Polish Uplands, between Zawichost and Puławy (Fig. 1), four main deposit series of the river flowing from south occur. They are separated by erosion surfaces and denoted by letter indices F, M, LE and V (Figs 4,5). The age of these series has been defined in relation to the tills occurring in the valley sides. On the basis of the TL datings the tills have been correlated with the glaciations: Odranian (Saalian I), Sanian 2 (Elsterian II) and Sanian 1 (Elsterian I). Under the last of the tills mentioned we can find residua of still older glacial deposits, probably of the Nidanian (pre-Elsterian) glaciation. On the basis of paleogeomorphologic criteria we can divide the oldest fluvial deposits (F) into two subseries which, however, are not differentiated in regard to lithology (so they are not separated in Fig. 5). The first subseries was formed after retreat of the Nidanian ice sheet, and it was connected with the valley 2 - 3 times narrower and 25 – 35 m deeper than present one. The second subseries is younger than Sanian 1 tills and was accumulated in the valley of a width similar to the present one; its bottom was about 20 m below the present one. Three following series (M, LE, V) occur at smaller depths, accumulated during successive glacial-interglacial cycles in the valley of a width similar to the present one (Figs 4, 5). This valley was especially intensely filled above the level of the present bottom during the advances of Sanian 2 and Odranian ice sheets, which reached the Sandomierz Basin south of the gap. During two subsequent glaciations (Wartanian and Vistulian) the ice sheet did not reach the Vistula river gap in the South Polish Uplands; therefore, the filling levels were lower. In the Sandomierz Basin within the valley of the present Vistula river, up-stream from the gap, we can find only two younger series of fluvial accumulation, i.e. LE and V (compare Figs 1, 6, 7). This shows that between the San and Wisłoka river mouths, along the north margin of the Sandomierz Basin, the Vistula valley was eroded after the retreat of the Odranian ice sheet. However, counterparts of the older series of fluvial deposits (M, F) distinguished in the gap occur in the valley of the lower San river, i.e. the last Carpathian tributary of the present Vistula river (Figs 1, 6, 8). This indicates that during the earlier erosion-accumulation cycles Carpathian waters ran towards the gap along the south margin of the Sandomierz Basin and the valley of the lower San river (vide Fig. IB). Our basic data only partially deal with the development of the valleys in the discussed area during the preglacial time, i.e. in the lower Pleistocene and eo-Pleistocene. These data show that the eo-Pleistocene valley breaking through South Polish Uplands was about 10 - 15 m shallower than the present one. However, at that time Carpathian river waters already ran to the gap and not to the valley of the Dniestr river (Black Sea basin) as it is suggested by other authors [7, 25]
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