The moraine plateau near Beichatow (central Poland) was incised at the beginning of the Middle Weichselian. Valleys 2&35 m deep were formed and then infilled with silts, sands, gravels, organic sediments and slope diamictons of the Piaski Formation, which is characterized by highly varying sedimentary environments. Initially -40,00&30,000 BPlacustrine sediments with an obvious eolian supply (re-worked loess) and locally fluvial suites were deposited in depressions. Slope sediments were simultaneously deposited at the valley margins. The Middle Weichselian was relatively mild and dry, but the milder interstadial periods have not been precisely established. Semi-anastomosing, stable-channel, highly aggrading and ephemeral streams of a semi-arid climate (subpolar desert) formed c. 3 0 , m 22,ooO BP. Continuous permafrost is implied from cyclic sedimentary processes reflecting summer melting and from periglacial structures. This was succeeded by a highly aggrading, high-energy and ephemeral braided fluvial system in an arid climate (polar desert), reflecting a complete lack of vegetation and increased eolian activity. These braided rivers formed during a period which may be directly correlated with the time of the maximum extent of the last ice sheet (22,00&14,350 BP), characterized by the most severe climatic conditions with strong periglacial activity. Finally -(14,35&12,700 BP) -eolian sediments were deposited, reflecting milder climatic conditions at the time of the abrupt ice sheet decay.
The Pliocene-Pleistocene boundary, defined using pollen analysis, has been recorded in sections at the Betchat6w outcrop, Kleszczow Graben, central Poland. The boundary is located at the top of the green clay subunit of the Tertiary Clayey-sandy unit. A pollen flora of Reuverian C type, dominated by pine, 10-20% of 'Reuverian elements' and scarce thermophilous Tertiary trees, was found directly below this subunit. Cold stage floras of Praetiglian type were, in turn, recorded in the tekilisko Formation, lying directly on the green clays. The Pliocene-Pleistocene transition at Belchatow demonstrates the characteristic features of this boundary in western Europe, that is, the disappearance of Tertiary thermophilous tree pollen taxa, followed by a cooling to open-forest or forest-steppe conditions. Both pollen analysis and geological data suggest continuous sedimentation at least from the Upper Pliocene to the PraeTiglian, with no change of sediment sources and no marked hiatuses. The fluvial deposits of the Praetiglian Cekilisko Formation, although Pleistocene in age, mark a final period in the Tertiary evolution of the Kleszczow Graben. Later, there is a hiatus to the deposition of the first glacigenic sediments in the region possibly of Elsterian age. jourml of Quaternary Science
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