Slope stability and valley formation in glacial outwash deposits,North Norfolk

Dry valleys and gullies in fluvioglacial sands and gravels in North Norfolk are fossil forms. Interpretation of their origin demands detailed morphological analysis, but the inevitable ambiguities of form require additional information on the sedimentology of the deposits and associated regolith. Soil mechanical data and stability analysis predict the observed modal slope angles, assuming the water table to be at or near the surface. This suggests that valley formation involved impeded drainage, and slope processes such as shallow slab slides and solifluction over permafrost. The debris was subsequently evacuated from the valleys by meltwater. The stability analysis also explains the different modal slope angles observed at two locations. Slope angle frequency distributions are composite, however, and include data from varying locations up-valley. Up-valley variations in slope angle are seen to reflect variations in the nature of the regolith and consequent changes in soil mechanical properties, as well as changes in valley relief. The up-valley trends in the nature of the regolith are commensurate with a model of headward valley erosion, with a ‘younger’, less weathered regolith at the present valley head.