The characteristics of ribbed moraine and assessment of theories of their genesis
Ribbed (Rogen) moraines are large subglacially formed transverse ridges that cover extensive areas of the beds of the former Laurentide, Fennoscandian and Irish ice sheets. Since the flow speeds and stability of ice sheets are known to be sensitive to conditions operating at the bed, a full understanding of the processes of ribbed moraine genesis are critical if we are to appreciate their role in ice sheet dynamics. Several theories of ribbed moraine genesis have been published, however, these could not be tested due to the paucity of data on ribbed moraines. This thesis addresses this deficit by producing the first representative data set on ribbed moraine characteristics. Various remote sensing and GIS techniques were used to record the morphological, morphometric and spatial characteristics of ribbed moraines in Ireland, Canada and Sweden, over a combined area of 81,000 km2 • This established that some published accounts were inaccurate or untrue, and that ribbed moraine morphology is more complex than was hitherto reported. This thesis demonstrates that ribbed moraines form independent of topographic influences, are not always curved down-ice, do not have accordant summits, can have both steep proximal and distal sides, have undulating crests and resemble waves, are not always anastomosing and do not necessarily fit neatly together like a jigsaw. This thesis also provides the first quantitative database of ribbed moraine ridge length, width, height and wavelength, and demonstrates they exist over a larger scale range than was previously thought. The above data were used to test the various ribbed moraine theories. This led to the rejection of the topographic model of shear and stack and undermined the credibility of all other shear and stack hypotheses, the two-step hypothesis, the megaflood hypothesis and the thennal fracturing model of fonnation. Ribbed moraine wavelength data were used to test the only numerical computer model of ribbed moraine formation, which argues that they are the product of instability in a deforming subglacial till. Extensive tests failed to falsify the model and it is concluded that it remains the prime candidate of explanation. However, if future tests or observations do falsify this model, we argue that because ribbed moraines share many common properties of other natural instabilities, it will be another instability mechanism that will emerge as being successful is explaining their genesis.