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98 S. Jewtuchowicz

widened and deepened the valleys. The erosional edge of the upper terrace, 107—119 m. high, dates from that time (Fig. 41). The retreat of the ice-sheet, diverted the course of the Vistule but did not alter the direction of all its tributaries, which for a time continued to flow west. With the retreat of the ice-sheet, the cessation of meltwater flow, and the change of the Vistule's course, the water supply to the pradolina was cut down. At that time the pradolina floor narrowed to its present day width, while only few rivers fed it with water. Nor was the water supply augmented during the Baltic glaciation.

Geological structure of river valleys shows that in the first part oł the Baltic (Wurm) glaciation the pradolina was eroded and then the old stream beds were filled in. During the Pomeranian phase of the Baltic glaciation, the formation of the watershed near Łęczyca blocked the westward flow of water through the pradolina. The erosional--accumulational terrace which was formed at that time coincides with the present day floor of the pradolina.

Although during the last cold period periglacial factors acted vigorously in the pradolina, they did not alter its generał morphologic pattern.

As shown by studies, the morphologic development of the pradolina and of the Kutno end-moraine ridge was strongly influenced by vertical movements of the Ouaternary bedrock. Association of morphological processes with tectonics is justified in that area by the occurrence of saliferous Zechstein deposits in the bedrock and their specific reaction to diastrophism. Geological sections and paleomorphological maps reveal a generał tendency toward upward movements which resulted in the exposure of bedrock forms despite the repeated advance of ice-sheet. These facts suggest that each time the ice-sheet stopped in that area, the ice loaded surface augmented the upward thrust of saliferous deposits.

The old movements of the bedrock are not known in detail. They can be only inferred from the destruction of deposits of certain geological periods. On the other hand, the study of the structure and the morphology of present day river valleys has madę it possible to determine the distribution and seąuence of vertical bedrock movements during the last cold period. The erosion of rivers coincides with the earlier phase during which the upward thrust of the bedrock occurred in the pradolina zonę. Accumulation in river valleys coincides with the younger phase of the bedrock uplift that occurred in neigh-bouring areas, i. e. in the Kutno end-moraine ridge and Ozorków hills. During the youngest phase, which coincides with the formation of the watershed near Łęczyca, the westward flow of waters through the