Moraine formation

Ridges, mounds and hummocks formed at the margin of glaciers are generally termed moraines. The study of moraines is particularly useful as it can shed light on the physical processes occurring at both active and former ice margins1,2 and because moraines are markers of former glacier extent, so can be used to track glacier change (e.g. size) over time3.

Moraine ridge forming at the terminus of Easton Glacier, Washington, USA. Photo: W. Siegmund

How do moraines form?

Moraines form through several main processes, which may vary from glacier to glacier, on a temporal (e.g. seasonal basis), and with changes in climate. The key moraine-forming processes are shown in the diagram below and explained through this page.

Summary of the three main moraine-forming processes. Push moraines (top) form during periods of ice-front stillstand or advance that bulldoze proglacial sediments. Dump moraines (middle) consist of rock and sediment that fall, flow and slump from the ice margin by gravity. Ablation moraines (bottom) form due to the varying rates of ice melt across the snout. Where debris cover is sparse (i.e. where the ice is ‘clean’) melting is relatively rapid and the glacier surface lowers quickly. Where surface debris cover is thick, the ice is insulated from melting and ice-cored moraines can exist. Created by J. Bendle.

Push moraines

Push moraines form at the snout of active glaciers. Rock and sediment debris at the ice margin is moulded into ridges by the bulldozing of material (ice pushing) by an advancing glacier4,5.

Due to the nature of their formation, push moraines tend to take on the shape of the ice margin during the time at which they formed4,5 (see image below). They are often found at the margin of active temperate glaciers (such as those found in southern Norway and Iceland) that experience brief periods ice-front stability or advance despite a general pattern of recession4,5. In some cases, a series of annual push moraines may form, where low-relief ridges are formed during winter advances of the glacier snout, leaving behind a detailed record of glacier extent over time6-10.

Push moraine ridges formed at the retreating terminus of Skálafellsjökull, Iceland. The moraines display a ‘sawtooth’ planform that closely mimics the ice margin geometry. These push moraines have been shown to form annually, driven by local climate conditions (Chandler et al., 2016; ref. 10). Image from GoogleEarth.

Debris squeezing

As well as the bulldozing of debris, sediment may also be squeezed out from beneath the glacier margin, either as a glacier advances in winter, or in the ablation season when till becomes water-soaked and easily displaced by the weight of overlying ice4,11. This process also contributes to the formation and growth of push moraines.

Dump moraines

Dump moraines form where debris flows or falls from a glacier surface due to gravity and accumulates at the ice front or side as a ridge. They form where the ice front is stationary and there is a regular supply of debris to the snout, normally due to the melt-out of rock debris stored in the ice4.

Dump moraine size is related to the amount of debris accumulating at the snout and the length of time the glacier margin is stationary. The volume of debris on the glacier surface is high where (i) the debris content within the ice is high; and (ii) where ice velocity is high, as faster flowing ice can transfer more debris to the margin11.

Loch Lomond Stadial moraines

Debris dumped from the ice front may be bulldozed into push moraines by advance(s) of the glacier margin2,12. Moraines formed by a combination of both the dumping and pushing of debris include those constructed by certain Scottish cirque and valley glaciers during the Loch Lomond Stadial2,13 (see image below). These moraines are similar in their genesis and morphology to those created by Icelandic glaciers today, which suggests that Loch Lomond Stadial glaciers in Britain were likely temperate and active during deglaciation2,13.

Loch Lomond Stadial moraine ridges formed by a combination of the dumping and bulldozing of rock and sediment debris, Coire Ardair. Photo: B. Davies.

Ablation moraines

Ablation moraines form where rock and sediment debris accumulate on the glacier surface near the margin and subsequently undergo melt-out4,11. The accumulation of dark-coloured material on the glacier surface lowers the ice albedo (i.e. its reflectiveness) and increases the amount of solar radiation absorbed at the glacier surface, which causes ice melt to speed up. However, where the debris layer is more than a few centimetres thick it insulates the ice surface from heating, slowing the rate of ice melt. Where the debris cover is extensive across a large part of the snout, the ice margin may detach completely from the main body of the glacier and become stagnant (see image below).

The debris-covered and stagnant ice margin of Exploradores Glacier in central Patagonia, Chile. Photo: J. Bendle.

When a debris-covered snout melts over time material is gradually let down from the ice surface to produce an area of ‘hummocky moraine’. This melt-out process can produce a variety of moraine types, from a chaotic assortment of sediment mounds and hollows (see image below)1 to more regular transverse ridges (often termed controlled moraines) that reflect the former pattern of debris in a parent glacier14.

Example of chaotic mounds and hollows in southern Patagonia, South America, which are interpreted to have formed by ice stagnation (see Darvill et al., 2017; ref. 15).

References

[1] Kjær, K.H. and Krüger, J., 2001. The final phase of dead‐ice moraine development: processes and sediment architecture, Kötlujökull, Iceland. Sedimentology48, 935-952.

[2] Lukas, S., 2005. A test of the englacial thrusting hypothesis of ‘hummocky’ moraine formation: case studies from the northwest Highlands, Scotland. Boreas34, 287-307.

[3] Schomacker, A. 2011. Moraine (Eds.) Singh, V.P., Singh, P. and Haritashya, U.K. Encyclopedia of Snow, Ice and Glaciers. Springer.

[4] Benn, D.I. and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder Education. 

[5] Boulton, G.S., 1986. Push‐moraines and glacier‐contact fans in marine and terrestrial environments. Sedimentology33, 677-698.

[6] Sharp, M., 1984. Annual moraine ridges at Skálafellsjökull, south-east Iceland. Journal of Glaciology30, 82-93.

[7] Bradwell, T., 2004. Annual moraines and summer temperatures at Lambatungnajökull, Iceland. Arctic, Antarctic, and Alpine Research36, 502-508.

[8] Beedle, M.J., Menounos, B., Luckman, B.H. and Wheate, R., 2009. Annual push moraines as climate proxy. Geophysical Research Letters36.

[9] Lukas, S., 2012. Processes of annual moraine formation at a temperate alpine valley glacier: insights into glacier dynamics and climatic controls. Boreas, 41, 463-480.

[10] Chandler, B.M., Evans, D.J. and Roberts, D.H., 2016. Characteristics of recessional moraines at a temperate glacier in SE Iceland: Insights into patterns, rates and drivers of glacier retreat. Quaternary Science Reviews135, 171-205.

[11] Bennett, M.M. and Glasser, N.F. 2009. Glacial Geology: Ice Sheets and Landforms. John Wiley & Sons.

[12] Boulton, G.S. and Eyles, N., 1979. Sedimentation by valley glaciers: a model and genetic classification. Moraines and varves33, pp. 11-23.

[13] Jones, R.S., Lowe, J.J., Palmer, A.P., Eaves, S.R. and Golledge, N.R., 2017. Dynamics and palaeoclimatic significance of a Loch Lomond Stadial glacier: Coire Ardair, Creag Meagaidh, Western Highlands, Scotland. Proceedings of the Geologists’ Association128, 54-66.

[14] Evans, D.J.A., 2009. Controlled moraines: origins, characteristics and palaeoglaciological implications. Quaternary Science Reviews28, 183-208.

[15] Darvill, C.M., Stokes, C.R., Bentley, M.J., Evans, D.J. and Lovell, H., 2017. Dynamics of former ice lobes of the southernmost Patagonian Ice Sheet based on a glacial landsystems approach. Journal of Quaternary Science32, 857-876

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