THE EFFECTS OF MORAINE DEGRADATION ON LICHENOMETRIC DATING OF LITTLE ICE AGE MORAINES

The topographic degradation of Little Ice Age (LIA) moraine ridges can be modeled quantitatively as a diffusion process. Such degradation is significant because over LIA time frames, it can affect the number, orientation, and distribution of boulder surfaces available for lichen colonization. As moraines degrade, boulders at the crest are gradually becoming exposed and possibly unseated, while boulders at the base are being buried. As a result, the average lichen population at both the crest and the base should be younger than the initial colonization age. Boulders initially deposited on the mid-slope should be the most stable and thus this area should harbor undisturbed lichen from initial colonization. Although many studies suggest the importance of moraine degrading processes on lichen populations, none has provided a quantitative framework for assessing their impact on lichenometric dating techniques. The suitability of a simple diffusion model to quantify these processes was evaluated in a preliminary study in which lichen size and distribution data were collected from two LIA terminal moraines. As predicted, the largest individual lichen on each moraine was located on the slope; however, the range and standard deviation of the lichen population at the crest, slope, and base of each moraine differs from expectations using a simple diffusion model. The distribution of lichen-bearing boulders observed on each moraine indicates that the model is incomplete because it does not redistribute unseated boulders. However, both field data and model results suggest that lichenometric dating techniques based on either selective sampling procedures or populations of lichens may result in dates that are only a fraction of the age of the moraine, an effect that is amplified with moraine age.