Clast ploughing, lodgement and glacier sliding over a soft glacier bed

Rarely‐preserved features indicative of clast lodgement are exceptionally well preserved near Peoria, Illinois, on the contact surface between Illinoian till and underlying glacifluvial sand due to synsedimentary cementation of the substrate contact. Features preserved on the cemented contact surface record a history of particle transport, lodgement by ploughing into a deformable substrate, and subsequent overriding by abrasive debris‐rich ice. Linear grooves and frontal sediment prows suggest that clasts embedded in the glacier sole ploughed through the soft, deformable bed. Increasing form resistance by the enlargement of sediment prows that developed on the lee side of clasts and deeper penetration of the clasts into the substrate eventually exceeded the force exerted on the clasts by ice flow, and the clasts lodged. Subsequently, clasts were abraded on their up‐ice flanks and plucked on their down‐ice flanks, resulting in stoss‐lee morphology. These features offer direct information on the nature of the interface between aglacier and a soft, deformable substrate such as characterized large areas of former ice sheets.