Spaced Out ::

Hillier, J., Smith, M.J., Clark, C.D., Stokes, C.R. and Spagnolo, M. (2013)
Geomorphology

Subglacial bedforms preserved in deglaciated landscapes record character-istics of past ice-sediment flow regimes, providing insight into subglacial processes and ice sheet dynamics. Individual forms vary considerably, but they can often be grouped into coherent fields, typically called flow-sets, that reflect discrete episodes of ice flow. Within these, bedform size-frequency distributions (predominantly height, width and length) are currently described by several statistics (e.g., mean, median, standard deviation) that, arguably, do not best capture the defining characteristics of these populations. This pa-per seeks to create a better description based upon semi-log plots, which reveal that the frequency distributions of bedform dimensions (drumlin, MSGL, ribbed moraine) plot as straight lines above the mode (f). This indicates, by definition, an exponential dis-tribution, for which a simple and easily calculated, yet statistically rigorous, description is designed. Three descriptive parameters are proposed: gradient (the exponent, characterising beforms likely least affected by non-glacial factors), area-normalised y-intercept (quantifying spatial density), and the mode (f). Below f, small features are less preva-lent due to i) measurement: data, sampling, mapping fidelity ii) possibly post-glacial degra-dation or iii) genesis: being created, or not, sub-glacially. This new description has the beneift of being insentitive to the impact of potentially unmapped or degraded smaller features and better captures properties relating to ice flow. Importantly, using lambda, flow sets can now be more usefully compared with each other across the all deglaciated re-gions and with the output of numerical ice sheet models. Identifying the charactersitic exponential and that it is typical of ‘emergent’ subglacial bedforms is a new and poten-tially powerful constraint on their genesis, perhaps indicating that ice-sediment interac-tion is fundamentally stochastic in nature.