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Origin ot the Warsaw-Berlin Pradolina 95

Three boulder-clay horizons distinguished in the Pleistocene represent three ground moraines. The oldest moraine coincides with the Cracow (Mindel) glaciation, while the other two coincide with the Middle-Polish (Riss) glaciation (Fig. 12—17).

Geological sections show big erosion gaps. In several places the youngest ground moraine of the investigated area is underlain by jurassic formations.

The division of the Pleistocene is also illustrated by the map of boulder-clays (Fig. 18). Their distribution shows that 4.2°/o of the inves-tigated area contains no boulder-clay at all, 44% contains only the Warthe stage boulder-clay, 31.6% showed of Drenthe stage boulder--clay and 20.2% revealed three horizons of boulder-clay from the Mindel and Riss periods.

These figures indicate that the accumulation of each boulder-clay was followed by intensive denudation. Eefore the advance of the Riss ice-sheet, 79.8% of the boulder-clay deposited during the preceding glaciation was destroyed. The period of the heaviest destruction coincides with the Great Interglacial (Mindel-Riss).

The attempt to reconstruct changes in the Pleistocene relief was based on the height of the bottom horizon of boulder-clays which represent particular glaciations. Paleomorphological maps, illustrating this problem (Figs. 19, 20) show that after each glaciation the relief showed tendency to recur to the pre-Pleistocene surface.

The influence of the bedrock on the development of the present day surface was studied. The adjustment of the relief to the morphology of the bedrock which was repeated after each glaciation (cf. paleomorphological maps), madę us search for similar phenomena in present day morphology. The comparison between the present day landscape and the pre-Pleistocene relief points clearly out that the development of the bedrock controlled recent morphology. This dependance was deter-mined by the occurrence of the salt structures in the bedrock of the investigated area. The upward movement of the salt structures resul-ted in the uplift of the Pleistocene cover and conseąuently favoured the action of destructive processes. As a result, the bedrock was outcropped and the morphology of the new surface was controlled by sub-Quaternary relief. The tendency to expose the bedrock relief is observable even to day.

According to K. K e i 1 h a c k [30] the formation of the Warsaw-Berlin pradolir.a coincides with the position of the ice-sheet margin along the end-moraines of the Leszno (Brandenburg) stage. In the investigated area, a zonę of hills called the Kutno end-moraine ridge lies between the end-moraine of the Leszno stage and the pradolina. As stated above,