FINE TUNING MORPHOLOGIC DATING OF HILLSLOPES

Morphologic dating matches an observed hillslope profile with one generated by an appropriately calibrated model. The model most commonly used for dating assumes the downslope debris flux is a function (any function) only of gradient and is independent of scale. If this assumption is correct, morphologically derived dates made along a terrace scarp or scarps presumed to be the same age should be roughly the same. Not only is this not the case for profiles of terrace scarps measured near Jenny Lake in Teton National Park, Wyoming, but the derived ages are a linear function of scale (i.e., scarp profile height) with a very high coefficient of correlation. Despite the problematic and unanticipated influence of scale on morphologic age, the morphologies predicted by a variety of models fit the observed pattern of hillslope degradation quite closely. Models based on a debris flux being linearly proportional or a power function of gradient produced similar results: close fits to observed morphology but a strong influence by scarp height.

Normalizing for scarp height permits an easily applied means for accounting for the effect of scale. Morphologic dating of two populations of profiles taken from scarps of known age (Glacial Lakes Nipissing and Algonquin wave-cut bluffs along the Lake Michigan shoreline) produced a relatively accurate age relationship between the populations when compared with the morphologic ages made without compensating for scale. Although normalizing for scale is not possible for scarps in which there is not a fairly broad range of scarp heights, for appropriate scarps it results in a substantial improvement in the accuracy of morphologic dating. This revision in methodology should be considered a “short-term fix” and not a substitute for a robust model for hillslope degradation able to account for the influence of scale while still producing close matches to observed hillslope morphology.