Plateau icefields: Geomorphology and dynamics

Many mountain regions contain high plateaux which may act as accumulation areas for icefields. These plateaux often have outlet glaciers which spill from higher level accumulation areas down into adjacent valleys. The mass balance sensitivity of these outlet glaciers is controlled to a large extent by the size of the feeder plateau/plateaux and fluctuations of the ELA (equilibrium line altitude) around its edge. Plateau relief is characterized by gently rolling topography with slope angles steepening only towards tots (which may be nunataks during ice coverage) and valley-heads. The low slope angles combined with thin ice cover result in small driving stresses. In some cases, recession of plateau ice has revealed areas that have experienced little or no subglacial erosion. Some of these areas may have blockfields containing a relict (with respect to present climate) weathering mineral assemblage. These 'protected' areas correlate well with the low ice surface angles, low ice creep rates and low basal shear stresses. They may also reflect the presence of cold-based and thus non-erosive ice. in the 'draw down' regions where ice flows off a plateau the outlet glacier beds and ice surface slope angles increase, resulting in an increase in the basal shear stress and thus higher strain rates. The resultant increase in strain heating may be sufficient to raise the basal ice temperature above the pressure melting point (PMP) and thus initiate sliding. Erosion of any soft sediment and subsequently bedrock will occur at this sliding interface. Former plateau icefields and their outlets can be reconstructed even though the plateau geomorphology is often very subtle. For example, outlet glaciers extending to tower altitudes may produce prominent moraine systems due to their greater erosional capacity and supraglacial debris supply from the sides of outlet valleys. From observations of modern plateau glacier environments in Arctic Canada and north Norway combined with observations from the deglaciated Lake District of England a diagnostic assemblage of landforms is derived. From this work we suggest that some moraine systems formerly attributed to corrie glaciers are better explained as outlets from plateau icefields thereby implying a considerably greater extent of ice and and modified ELAs (predominantly higher).