Drumlinization and drumlin-forming instabilities: viscous till mechanisms

Glacially induced flow naturally tends to thin and extended till cover thro ugh shock formation, even in the absence of longitudinal gradients in the a pplied stress. Thicker till cover has an increased effective pressure at it s surface and base, a lower sliding velocity or deformation rate and above a critical thickness, a decrease in wave velocity with thickness, leading t o reverse-facing shocks moving downstream. For sliding and for some rheolog ies of internal deformation, a decrease in sediment flux with thickness occ urs, implying backward-moving kinematic waves and reverse-facing, reverse-m oving shocks. Downstream-facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally. Eventually, sh ocks coalesce, leaving an upstream-facing shock for sliding and a downstrea m-facing shock for internal deformation. It is observed that some drumlins have downstream blunt ends only. Fairly realistic three-dimensional drumlin shapes can be produced from symm etric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice-sheet flow. The fact the viscous theories produce drumlinoid forms suggests that on thi s scale till behaves viscously and the lower length scale for drumlins repr esents the plastic/viscous transition scale.