Landscape evolution degrades the geologic signature of past glaciations

Abstract

Glaciers on land leave behind characteristic landforms and bedrock topography. The spatial extent of this signature and dating of the erosional and depositional features has led to a good understanding of temporal and spatial shifts of past glaciers and climate. At best the landscape contains considerable information on past glacial processes in the form of moraines and other depositional landforms. However, evidence suggests that all unconsolidated landforms degrade, manifested in the general degradation and smoothing of the topography over time. To determine the relative amount and pattern of degradation on glacial moraines we use a well-documented moraine degradation model that uses both local slope and a proportionality constant to determine the diamicton transport rate. This formulation results in diffusion type topographic smearing and smoothing of the initially angular topographic features. More specifically, diamicton is eroded at the crest of a moraine, transported downhill, and deposited on the gentler lower flank. Collectively this produces moraine cross-profiles that closely resemble observations in the field. By assuming an initial constant boulder frequency per unit volume of the diamicton, the model tracks the surface lowering and the related exhumation of boulders to the moraine surface. This produces a tightly constrained set of testable predictions: (1) modelled moraine cross-profile; (2) modelled spatial pattern of surface boulders (no boulders on the lower flank); and (3) relative frequency of surface boulders (smallest frequency of surface boulders at the inflection point, highest frequency at the crest). These characteristics were measured in the field of alpine, tropical, and Antarctic moraines. The correspondence between model predictions and field data is remarkably good, and strongly supports our understanding of the time dependent evolution of glacial landscapes. These results highlight the evolving nature of glacial landforms and indicate the complete removal of original moraine surfaces and postglacial emergence of spatial pattern in boulder frequency.