The leading hypothesis for the formation of drumlins--elongate hills that form beneath sediment-floored glaciers--is that they are nucleated where low pore-water pressure in the sediment bed inhibits its shear. These parts of the bed may resist erosion and prompt local deposition, causing hills to form. However, a drumlin field recently exposed by the retreat of the surge-type Icelandic glacier, Múlajökull, provides a compelling alternative hypothesis: that drumlins form in zones of low effective stress (total ice pressure minus pore-water pressure) associated with swarms of longitudinal crevasses in the glacier margin. These two hypotheses lead to predictions of effective-stress and bed-deformation patterns that are conspicuously different. During two field seasons, these patterns will be studied at Múlajökull by trenching drumlins and their intervening swales and then collecting intact till specimens for two kinds of analyses. Some samples will be compacted in consolidation tests, a well-established technique for inferring the past maximum effective stress on former glacier beds that has not yet been applied systematically to drumlins. Other samples will have their anisotropy of magnetic susceptibility (AMS) measured, and preferred orientations of susceptibility will be used to determine patterns of deformation in the drumlins and intervening areas. If past effective stresses were anomalously low in drumlins, the leading hypothesis for their formation would be contradicted, and the crevasse hypothesis would be supported. The same would be true if AMS patterns were either not related to drumlin morphology or indicated convergent shear of bed sediments toward the up-glacier ends of drumlins.
The origin of drumlins, which cover vast areas of Canada, Scandinavia, Britain, Ireland, and the northern U.S., is one of the longest standing problems in Earth science. Although drumlins have been the subject of over 1300 scientific contributions since their study began ~150 years ago, the origin of no drumlin field is known definitively. By focusing on a drumlin field that is "active" in the sense that it has been shaped by the current glacial regime and studying drumlin sediments in new ways, this research in central Iceland will help illuminate the processes that sculpt one of the most emblematic and mysterious of glacial landforms. Results will also provide guidance for predicting the distribution of drag at the beds of ice sheets, a fundamental factor in estimating their speeds. This project will support the education of two graduate students and will provide international independent study projects for six undergraduate students from underrepresented groups. A request will be made for a high school teacher, supported by the NSF-supported PolarTREC program, to participate in the field work. This teacher would stream his or her experiences daily back to multiple classrooms with the goals of exposing K-12 students to glacial geologic research and interactively answering their questions.
This project is co-supported by the Geomorphology and Land Use Dynamics program and the Office of International Science and Engineering.
