Middle and Late Weichselian stratigraphy and palaeoenvironments in central Poland

The moraine plateau near BetchatÓw (central Poland) was incised at the beginning of the Middle Weichselian. Valleys 20–35 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 ‐ 40,000–30,000 BP ‐ lacustrine sediments with an obvious colian 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. 30,000–22,000 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 (22,000–14,350 BP), characterized by the most severe climatic conditions with strong periglacial activity. Finally ‐ (14,350‐12,700 BP)‐ eolian sediments were deposited, reflecting milder climatic conditions at the time of the abrupt ice sheet decay.