Drumlins

Drumlins around Lago Viedma

Although the Patagonian Icefields aren’t generally associated with drumlins (Glasser et al., 2008), there are some around Lago Viedma in the South Patagonian Icefield. They have been described in detail (Ponce et al., 2013) but they show up beautifully in the Landsat map below. The mapping below is by me (Bethan Davies) and Glasser and Jansson (2008). Drumlins are probably rare in Patagonia as the temperate ice masses release large amounts of meltwater, which may destroy any bedforms.

Drumlins around Lago Viedma, South Patagonian Icefield. The background image is Landsat 7 ETM+ from 2001. Panels C and D are to the same scale.

Drumlins around the world

Drumlins have been observed at the beds of former palaeo ice-sheets across the world. They are found across the Pennines of Britain, in Anglesey, and in the Lake District (Livingstone et al., 2008; Hughes et al., 2010, 2014). In the photo below there is a drumlin on Anglesey. The farmer has helpfully put a stone wall along the long-axis of the drumlin.

Drumlin in Anglesey. Photograph: Bethan Davies

The most ubiquitous subglacial landforms

Drumlins are therefore one of the most ubiquitous landforms formed underneath ice sheets (Clark et al., 2009).  They are typically oval-shaped hills, with a long-axis parallel to ice flow. The up-ice (stoss) face is typically steeper than the down-ice (lee) face (Stokes et al., 2011). They are typically between 250 – 1000 m long, 120-300 m wide, and 1.7 to 4.1 times as long as they are wide (Clark et al., 2009). They also generally occur in clusters, or swarms, as can be seen in the images from around Lago Viedma above.

Drumlins. Top: cross-profile, Second: view from above. Third: a swarm of drumlins. Adapted from work on Wikimedia Commons.

Drumlins in palaeo-ice sheet reconstruction

Glacial geologists frequently use these swarms of drumlins in palaeo-ice sheet reconstruction, because they can be directly related to the direction of former ice flow. They can therefore be used to reconstruct the dynamic behaviour of former ice sheets (Livingstone et al., 2010; Livingstone et al., 2012).  Their length may be related to ice velocities, with a tendency to become more elongated under fast ice-flow conditions. At their longest, they grade into Mega-Scale Glacial Lineations, typically found under former ice-streams.

The mystery of how drumlins are made

Although drumlins have a typical morphology, they can be made up of lots of different kinds of internal sediments, ranging from mainly bedrock, to mainly till, to mainly sorted sediments (Stokes et al., 2011). A diagnostic process for their formation is therefore challenging to deduce.

Cross-section through a drumlin on Anglesey. This one is mostly made of glacial till. Photo: Bethan Davies

One theory growing in popularity is the ‘instability theory’, which states that small perturbations in the bed of the ice sheet grow under a positive-feedback mechanism into the large landforms we see on land (e.g., in Britain and in Patagonia) today (Stokes et al., 2013). Such instabilities tend to grow exponentially, with a rate dependent on wavelength.  Essentially, the process amplifies the relief at the ice-bed interface and results in the formation of bedforms like drumlins in recognisable patterns.

Further reading

References

Clark, C.D., Hughes, A.L.C., Greenwood, S.L., Spagnolo, M., Ng, F.S.L., 2009. Size and shape characteristics of drumlins, derived from a large sample, and associated scaling laws. Quaternary Science Reviews 28, 677-692.

Glasser, N., Jansson, K., 2008. The Glacial Map of southern South America. Journal of Maps 4, 175-196.

Glasser, N.F., Jansson, K.N., Harrison, S., Kleman, J., 2008. The glacial geomorphology and Pleistocene history of South America between 38°S and 56°S. Quaternary Science Reviews 27, 365-390.

Hughes, A.L.C., Clark, C.D., Jordan, C.J., 2010. Subglacial bedforms of the last British Ice Sheet. Journal of Maps 6, 543-563.

Hughes, A.L.C., Clark, C.D., Jordan, C.J., 2014. Flow-pattern evolution of the last British Ice Sheet. Quaternary Science Reviews 89, 148-168.

Livingstone, S.J., Evans, D.J.A., Ó Cofaigh, C., Davies, B.J., Merritt, J.W., Huddart, D., Mitchell, W.A., Roberts, D.H., Yorke, L., 2012. Glaciodynamics of the central sector of the last British–Irish Ice Sheet in Northern England. Earth-Science Reviews 111, 25-55.

Livingstone, S.J., Ó Cofaigh, C., Evans, D.J.A., 2008. Glacial geomorphology of the central sector of the last British-Irish Ice Sheet. Journal of Maps 2008, 358-377.

Livingstone, S.J., Ó Cofaigh, C., Evans, D.J.A., 2010. A major ice drainage outlet of the last British-Irish Ice Sheet: the Tyne Gap, northern England. Journal of Quaternary Science 25, 354-370.

Ponce, J.F., Rabassa, J., Serrat, D., Martínez, O.A., 2013. El campo de drumlins, flutes y megaflutes de lago Viedma, Pleistoceno Tardío, provincia de Santa Cruz. Revista de la Asociación Geológica Argentina 70, 115-127.

Stokes, C.R., Fowler, A.C., Clark, C.D., Hindmarsh, R.C.A., Spagnolo, M., 2013. The instability theory of drumlin formation and its explanation of their varied composition and internal structure. Quaternary Science Reviews 62, 77-96.

Stokes, C.R., Spagnolo, M., Clark, C.D., 2011. The composition and internal structure of drumlins: Complexity, commonality, and implications for a unifying theory of their formation. Earth-Science Reviews 107, 398-422.

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