Associations of flutings, drumlins, hummocks and transverse ridges

Abstract

Associations of flutings, drumlins, hummocks and transverse ridges were created in the inner marginal zone of the ice sheet, where, nearer the ice margin, the ice was already thinner and the former higher flow velocity had slowed down due to the decreased volume of transported ice. However, as the ice flow mechanics tend to favour certain higher flow velocities, which can, to some extent, even be self-supporting, the speed of the ice did not slow down homogenously throughout the ice mass. Certain parts of the ice continued to move at the higher speed but an increasing number of units appeared, where the velocity had dropped down to a considerably lower level. In the resulting flow pattern, with fast and slow units of ice flowing side by side, a drag effect contributed to a spiral secondary flow between the flow units of differential velocity.

Both depositional and erosional processes were involved and, depending on the balance between them, the resulting bed configuration could be formed by either one or by a combined effect of both of these processes. However, faster flow regime more often favoured erosion and in areas of lower regime deposition more often prevailed. Consequently troughs were formed in the areas of higher velocity, while ridges and other positive landforms were created in the areas of lower velocity. The spiral flow frequently transferred drift from the trough area to the area of lower flow regime, and therefore the flute-ridges and drumlins were often formed depositionally by the combined effect of the slow parallel flow and the spiral flow component. Rogen ridges indicate an undulating flow. This could also exist simultaneously with the other kinds of flow and many landforms owe their characteristics to their combined effect.