Soft‐bedded ice sheet in hummocky terrain of north‐central Poland: Origin of rim ridges and subglacial processes

ABSTRACT Hummocky terrains are characterised by various geomorphological features that typically record processes associated with the downwasting of ice sheets. Common landforms in these areas include ramparts, linear ridges, kettle holes and ice‐walled lake plains. However, the origins of some of these features remain controversial, as they can variously result from sediment release from stagnant, debris‐rich ice or from ice‐marginal subglacial processes. This study investigates the internal composition and structures of rim ridges in hummocky terrain in north‐central Poland to uncover their formation mechanisms and infer the sequence of glacial processes. Sedimentological analyses were conducted on 12 rim ridges distributed across an extensive hummocky terrain. Each ridge consists predominantly of subglacial traction till interbedded with centimetres‐thick fine sand laminae or lenses, indicating episodic basal decoupling and recoupling. Clast fabric measurements reveal that, across the study area, they exhibit a broadly uniform orientation that is often unrelated to the alignment of the ridges themselves. The internal structures consistently show sediments arching into distinct anticlinal forms that mimic the morphology of the rim ridges. Ample evidence suggests that the investigated forms formed in a dead ice environment. Specifically, water‐saturated soft sediments were squeezed into narrow zones between blocks of dead ice due to localised pressure variations. Overall, this study provides insight into the processes in operation during the ice sheet advance and retreat up to its disintegration stage; in the latter, intriguing rim ridges were formed in response to dynamic interactions between ice, water, and sediment.