THE EVOLUTION OF TILL FABRICS AND THEIR IMPLICATIONS FOR SUBGLACIAL DEFORMATION AND LANDFORM GENESIS

Till fabrics are a fundamental tool in glacial geology and have a long history of use and application. We present three styles of fabrics frequently encountered in the literature: macrofabrics, microfabrics, and magnetic fabrics. Macrofabrics are produced from the orientations of pebble-sized elongate clasts within the till. These are possibly the most published and oldest till fabrics, but suffer from a small sample size (usually 50-100 pebbles) and possible user bias. Microfabrics measure the orientation of sand grains using oriented thin-sections. They suffer from the same sample size and operator bias shortcomings. The most recent style of till fabric is determined through the measurement of the anisotropy of magnetic susceptibility (AMS) of oriented till samples. The resulting magnetic or AMS fabrics provide a robust, volume-averaged and objective measure of the orientation of thousands of microscopic particles (usually maghemite) within the till. We observe that, in drumlins and flutes, all three fabric types produce very similar and consistent results – particles are aligned parallel to regional ice flow and plunge up glacier. Our research reveals that the entirety of subglacial till – from boulders to microscopic particles – undergoes pervasive shearing that faithfully orients all particle sizes parallel to ice flow. The objective of our most recent research is to assess whether AMS microfabrics record local variations in ice flow possibly caused by divergent, parallel and convergent flow around obstacles (drumlins and flutes), and to apply these results to inform hypotheses of subglacial bedform genesis. To that end, 16 backhoe pits were excavated in one drumlin in the Weedsport Drumlin Field of North-Central New. Twenty-five oriented till samples were collected from each pit for AMS microfabric analysis. The resulting fabrics are strong, plunge upglacier, are typically oblate and parallel to within 30 degrees of the regional ice flow direction (165°), as determined by drumlin orientation.