Cave under Willow – a cave on the slope of Kępa Forteczna, on the border of the Grudziądz district of Małe Tarpno and the village of Nowa Wieś. It is located within the same sandstone that forms the Maple Cave. It is located in the mesoregion of the Grudziądzka Valley, which is part of the macroregion of the Lower Vistula Valley. The cave was formed in Pleistocene conglomerates and sandstones of various grains as a result of water erosion, weathering and gravitational sliding of rock packs.
