One of the long-standing mysteries in glacial geomorphology is how drumlins, elongated hills built usually of some combination of clay, silt, sand, and gravel, develop beneath glaciers. Because the size, shape, and composition of these landforms can vary dramatically, even within the same drumlin field, researchers have developed competing theories of how these streamlined hills form.
Although previous quantitative models have striven to reproduce observed drumlin patterns, they have not been based on measurements from an active drumlin field. Here Iverson et al. present a new mathematical model of drumlin formation that is based on thousands of measurements from a drumlin field in front of Múlajökull, a wet-based outlet glacier of the Hofsjökull Ice Cap that has surged eight times in the last 94 years.
The modeling results indicate that the Múlajökull drumlins grow during episodes of erosion that occur during prolonged quiet periods of normal flow between surges. During these quiet intervals, the slip of the ice across topographic undulations and the flow of water along the glacier’s base toward channels between drumlins create a stress field that causes the drumlins’ heads and flanks to preferentially erode. Together, argue the authors, these processes enable gentle topographic undulations to grow at an increasing rate and gradually migrate downstream over the course of multiple surges.
These findings are consistent with field measurements that show that subglacial stresses are higher on sediments in and near water channels than within the drumlins themselves, as well as independent evidence that erosion occurs preferentially in such areas. The model also produces drumlin heights and stratigraphy like those observed at Múlajökull.
This model represents an important step forward in understanding how the Múlajökull drumlins evolved. Just how applicable it will be to other drumlin fields, however, is an outstanding question that the authors argue will be difficult to resolve until scientists have collected comparable data sets from other drumlin fields. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1002/2017JF004354, 2017)
—Terri Cook, Freelance Writer