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Origin o the Warsaw-Berlin Pradolina 97

the development of smali valleys in the pradolina is to study the history of the pradolina itself.

The formation of the pradolina bed and of other valleys shows traces of two processes: erosion and accumulation (Figs. 30, 31). Erosion phase points to the upward movement of the pradolina zonę, while accumulation is indicative of upwards movements in the bedrock of neighbouring areas. These two phases coincide with two terraces of rivers and of the pradolina.

During accumulation phase the streams became shallow and valleys were filled in. This phase also coincides with the formation of dunes which stretch along stream valleys in the pradolina. Some dune fields lie in dead valleys abandoned by rivers. From this it may be concluded that after the accumulation phase the pradolina rivers were diverted from their course. They abandoned their beds and flowed in new ones. Ali this was caused by the formation of the watershed in the pradolina near Łęczyca, which was produced by the uplift movement of salt structures in the bedrock of pradolina zonę.

The detection of links between the dunes and the old stream valleys allows to define the period when the rivers became shallow and their course changed. In the investigated area the dunes of Witów are the best known. Pollen analysis and radiocarbon data have shown that the dunes began to form in the Older Dryas period. Thus the accumulatión of sand in the valleys, the change of river beds, and the formation of the watershed in the pradolina must have preceded the dune formation. Changes in the pradolina resulting from salt structure movement started already during the Pomeranian phase of the Baltic glaciation.

Traces of old valleys allow to reconstruct the original river pattern in the pradolina (Fig. 40). At that time the rivers flowed to the west.

As indicated by the analysis of geological structure, the pradolina followed a erosion depression which strongly controlled the direction of drainage in this area.

It can be concluded from morphological studies that during degla-ciation, the ice-sheet margin did not stop in the investigated area and thus neither end-moraine nor outwash plain was formed. During the deglaciation the river flow across the Łęczyca environment and drained meltwaters.

Perhaps J. Siemiradzki [61] and S. Lencewicz [42] were right in their observation that when the northward course of the Vistule was blocked by ice, it flowed westward through the Warsaw--Berlin pradolina. The tributaries of the Vistule ran in the some direction. River- and meltwater flowing thraugh the tectonic depression

7 — Geneza pradoliny

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