TUNNEL VALLEYS OF OLD GLACIAL LANDSYSTEMS IN POLAND

Abstract

Tunnel valleys are among the largest erosional structures in the Pleistocene cover of the Polish Lowland and other areas around the Baltic Sea. They have been fairly well recognised within the extent of the Weichselian (Vistulian) Glaciation. Research conducted in Northern Poland show that these forms may have originated as a result of either glacifluvial erosion, glacial erosion or both processes (glacial-glacifluvial troughs). In old glacial landscape areas, shaped during the Sanian (Elsterian) and Saalian Glaciations (Odra and Warta Glaciation), the outlines of tunnel valleys are now highly obliterated in the relief. No trough lakes have been preserved in these areas – such characteristic features in the extent of many tunnel valleys within the area of the LGM limit. So far, deep erosional incisions in sediments of the Neogene, the Cretaceous and even the Jurassic, have been regarded as river palaeovalleys – particularly in the transitional area between the Polish Lowland and the uplands of Southern Poland. Even during field charting, related to the preparation of the Detailed Geological Map of Poland, these forms were frequently passed over in the legend. Only several investigators mentioned the possibility of the existence of old troughs, mentioning isolated cases thereof. Analyses of drilling profiles, various archival geological and geophysical documentation and contemporary outcrops, allow for distinguishing over two hundred large structures of this type in the old glacial landscape belt – mostly buried. They were essential for subglacial drainage in the marginal part of the ice-sheet, by for­­­ming a several dozen kilometre wide parallel pattern. Some meltwater which eroded the substrate might have used old valley depressions, which were formed before the Pleistocene or in Eopleistocene. Petrographic composition of the gravelly debris was considerably modified under the influence of the local bedrock material. Some tunnel valleys were filled still during the deglaciation, some were transformed by fluvial processes between the Eemian Interglacial and the Holocene, others are inversely indicated in the landscape in the form of eskers. The identified forms here are on average smaller than the ones documented e.g. within the extent of the Weichselian Glaciation in Germany and in the North Sea. They reach from several to several dozen kilometres in length, from several to several dozen metres in depth and a considerable width – up to several kilometres.

Keywords

• tunnel valley
• old glacial landscape
• glacifluvial sediments
• polish lowland
• warta glaciation