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.

This paper presents results of investigation on peat and lacustrine sediments from the Kładkowe Bagno peat-bog located in the Puszcza Knyszyńska Forest. Using analysis of plant remains from sediment samples, vegetative and generative finds were identified which allowed describing peat units. Basing on these results, reconstruction of subfossil vegetation and palaeoenvironmental changes in the mire was made. Altogether 4 subassociations of Sphagnetum magellanici were described, which delivered information about humidity of the mire surface during peat forming processes. Stages of deposit development were dated by radiocarbon method. Accumulation of the oldest sediments in the southern basin took place in the Late Glacial. Peat of the northern basin started to accumulate in the Atlantic period. The both parts of the mire aggregated probably 400 years ago.