The Mystery of Drumlins

For over a century researchers have puzzled over one of the most common landscape features in Ireland – Drumlins. Their name derives from the Irish word droimnín, meaning ‘little ridge’ and tens of thousands of these tightly packed hillocks can be found in Ireland, especially in the northern half of the country.

Their shape is one of their defining characteristics, often compared to an inverted spoon or half buried egg but their internal composition varies widely and so, therefore, do theories on their formation.  While it is clear that they are related to glacial activity, generations of both leaving certificate and undergraduate students have poured over contrasting theories, such as those of John Shaw in the 1980s who postulated that catastrophic outburst floods are the cause of their formation.

Similarly, Boulton suggested differential erosion and deposition over deformable sediments as a formation process. However, none of these theories could explain the entire range of internal sedimentary structures found, which varies from homogeneous glacial till to distinct stratifications, including internal deformation in some cases.

This was until 2010, when scientists discovered an active Drumlin field on the margins of Múlajökull, a surge type glacier in Iceland. Not only were they able to map over 50 drumlins emerging from the ice over time, but they also shed light on the processes leading to their formation. General warming has led to increased activity of Múlajökull leading to several surges; that is, ice advancing over the past two decades. As the ice retreated after these surges, drumlins were revealed along the glacier’s margins.

Aerial photography and LiDAR imaging then revealed that drumlins formed in areas of former longitudinal crevasses. Thus, it is thought that horizontal expansion and longitudinal compression leads to a preferential deposition of material in lower pressure zones, i. e. crevasses.

Furthermore, repeated glacier surges over previously formed drumlins showed a clear link between glacial advance and new till layers, creating the distinct stratification observed in many Irish Drumlins.

Continuous study at this site has and will reveal more detailed information about the processes leading to the formation of Drumlins, though a first important step has been taken in solving this mystery forever.

Authored by Andrea Waitz, PhD student, TCD Geography

Johnson,  M.D. et. al (2010) Active drumlin field revealed at the margin of Múlajökull, Iceland: A surge-type glacier. Geology. v. 38, p. 943-946

Pic: Drumlin partially eroded by the sea, Clew Bay, Ireland

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