Morfogeneza Jeziora Lednickiego i jego obramowania (doniesienia wstępne)

The Lednica Lake basin was formed as a result of subglacial erosion about 18.000 yr BP. The ice cover decayed over that period due to oscillations. One of them was the so-called Dzwonowsko-Lednogórska oscillation whose marginal forms surround Lednickie Lake basin to the south. The later oscillation, the so-called Gniezno oscillation, affects the region under discussion to the north. During the recession of the continental glacier the basin concavity was filled with blocks of dead ice. As late as about 12.000 yr BP the Lednica Lake formed as a consequence permafrost degradation and dead ice melting. It is presumed that a shallow water basin which was more extensive than the modern one appeared as permafrost became ultimately degraded. The formation of the lake shelves occurring about 3 and 4 m above water level can be linked to the high water stage. At the turn of the Late Vistulian the water table level in the basin was markedly lowered and the lake extent resembled the present one. In the last 9.000 years the Lednica Lake has changed periodically its water level as well as its extent and shoreline. This resulted in sets of basic lake shelves (upper shelves 65 - 105 cm, lower shelves 20 - 40 cm and submerged shelves 30 - 50 (80) cm below the present level) and in sets of tiny ledges of the so-called second generation, indicative short though significant changes in the water level. Abrasion shelves were mostly produced on the eastern lake shores. They remain indistinct on the sloping western shores. On the other hand, local bays with deeper portions at the bottom keep a record of a variety of intense biogenic accumulation processes. The present paper is preliminary in its nature and calls for further geomorphological studies, especially palaeoecological research which will use radiometric dating.