Tunnel Valleys of the Tuchola Forest and Kashubian Lake District

This chapter presents one of the most characteristic landforms of the glacial landscape of northern Poland—tunnel valleysTunnel valley. Three tunnel valleys located in the forefield of the maximum limit of the Scandinavian Ice SheetIce sheet during the Pomeranian Phase of the last glaciation (the outwash plain of the Tuchola Forest), and three tunnel valleysTunnel valley located behind this limit (the Kashubian Lake District) are analysed. All the landforms were created by subglacialSubglacial erosional processes, mainly associated with the channelised flow of meltwatersMeltwater flowing under hydrostatic pressure. One tunnel valleyTunnel valley (the Wiecka tunnel channel) was formed in a single catastrophic outburst event involving bank-full subglacialSubglacial discharge; another is a composite tunnel valleyTunnel valley formed due to the coexistence of glacial and meltwater erosion over at least three last glaciations (the Żarnowiecka tunnel valleyTunnel valley), and the remaining ones are complex systems of tunnel valleysTunnel valley formed due to steady-state meltwater erosion of the ice-sheet bed. All tunnel valleysTunnel valley were infilled with dead-ice blocks immediately after their formation, which protected reaches located within outwash plains against backfilling with sandur deposits. Outwash flow also significantly modified their morphology; hence, such landforms located within a sandur are called “palimpsest” tunnel valleysTunnel valley. Melting of buried dead-ice blocks in warm periods of the Late Glacial and at the beginning of the Holocene revealed the tunnel valleysTunnel valley within the glacial landscape, as well as the formation of lakes, which in most cases were incorporated into the existing fluvialFluvial systems. In this way, a period of intensive fluvial and lacustrine transformation of tunnel valleysTunnel valley began.