The aim of this study was to delimit lacustrine deposits underlaying present peatlands. On this basis, the location of water bodies in late Pleistocene and early Holocene was recognized. The lakes’ occurrence was presented on the background of geomorphological conditions. Lacustrine deposits occur mainly in depressions of the northern part of the Knyszyńska Forest. They are placed in upper parts of the Czapielówka River, Jałówka River, middle Sokołda River and upper Kumiałka River catchments. The thickness of gyttja varies between 0.4 and 2.5 m. These are detrital, calcareous and clay-calcareous gyttjas. Lacustrine sediments fill the bottoms of various meltout depressions. The origin of these depressions, as well as the whole glacial relief of the terrain, is often linked to deglaciation of the Warta ice sheet. However, kame deposits in the Janów village are younger than Warta glaciation. Moreover, the catchment relief of the upper Kumiałka River is similar to the relief which originates from Vistulian glaciation. Besides, there are boulder deposits directly under the lacustrine deposits. These three facts indicate a younger age of the melt-out depressions in the upper Kumiałka River catchment.
The Vistulian decline was a period of rapid environmental events. The authors correlated ages of the last Scandinavian Ice Sheet limits in northern Poland with ages of prominent events adapting the conditions of periglacial environment of Central Poland in response to the Late Vistulian climate warming. Ages from previous thematic geological and palaeogeographical studies were collected. The approach used indicates that despite methodological uncertainties and sometimes inconsistency of ages, it is especially helpful in timing of first warming signals (ca. 19–18 cal ka BP) and establishing of environmentally bipartite 3 millennia of the Oldest Dryas in the extraglacial zone. Abrupt warming at the onset of the Bølling-Allerød is well registered in biotic and abiotic archives available from Central Poland and remains in agreement with the large recession of the southern ice sheet margin.