PATTERNS OF GLACIER SLIDING ON DRUMLIN SURFACES, MULAJOKULL, ICELAND

Many subglacial landforms, including drumlins, form at the sole of glaciers far from eyes of geologists. As a result, drumlin formation is poorly known although there has been decades of research. These glacial landforms are of particular interest because they provide insight into how glaciers deform subglacial sediment and whether drumlins form zones of concentrated basal shear stress that resist glacier flow.

At Mulajökull, a surging outlet glacier of the Hofsjökull ice cap located in central Iceland, drumlins consist of many till layers thought be some to have each been deposited during separate surge events. A review of historical aerial photos reveals that drumlin location was strongly correlated former to swarms of longitudinal crevasses, indicating a reduced normal stress on bed till that may shielded it from erosion. In addition to ongoing studies of the till’s magnetic fabric, examination of striations on bullet boulders lodged at drumlin surfaces should indicate final flow patterns during the last surge.

Using a compass and a GPS receiver, striations and orientations of boulders were measured on 45 bullet boulders at three different drumlins. Results indicate that the majority of the boulders and striations follow the form of the drumlin, with divergence around the head and convergence near the downstream end. The data indicate that during the late stages of surging, sliding was a prominent mechanism of ice motion, in addition to a poorly known component of subglacial till deformation.

Sliding patterns at the bed surface may be useful in a proposed numerical model of drumlin formation as a tuning parameter for three-dimensional ice flow past drumlins.