Ribbed moraine formation

Ribbed (Rogen) moraines are conspicuous landforms found in interior parts o f formerly glaciated areas. Two major theories for ribbed moraine formation have been suggested in recent years: (i) the shear and stack theory, which explains ribbed moraine formation by shearing and stacking of till slabs o r englacially entrained material during compressive flow, followed by basal melt-out of transverse moraine ridges, and (ii) the fracturing theory, acc ording to which ribbed moraines form by fracturing of frozen pre-existing t ill sheets, at the transition from cold- to warm-based conditions under deg laciating ice sheets. In this paper, we present new data on the distributio n of ribbed moraines and their close association with areas of frozen-bed c onditions under ice sheets. In addition, we show examples of ribbed moraine ridges that fit together like a jig-saw puzzle. These observations indicat e that fracturing and extension of a pre-existing till sheet may be a predo minant process in ribbed moraine formation. In summary, we conclude that al l described characteristics of ribbed moraines are compatible with the frac turing theory, while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detai led morphology of ribbed moraines. One implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen-bed to thawed-bed cond itions under former ice sheets. (C) 1998 Elsevier Science Ltd. All rights r eserved.