Glaciers that carry little to no rock or sediment debris at their surface are known as ‘clean’ or ‘uncovered’ glaciers1.
Truly ‘clean’ valley glaciers represent an ideal end member in the range of valley glacier types, which differ as a result of local basin topography, the amount of surface debris they carry, mass balance, and the amount of meltwater they produce (visit this page for more detail).
Many glaciers may only be partially uncovered, or fall somewhere between being truly clean and completely debris-covered. Therefore, the landsystem of any individual valley glacier may contain only some of the characteristic features of clean glaciers, as well as other landforms not always seen at uncovered glaciers.
Dynamics of clean glaciers
With limited debris at the surface, clean valley glaciers tend to respond quickly to shifts in climate, where changes in glacier volume (due to mass loss or gain) cause oscillations (advance and retreat) of the terminus1,2.
The active nature of clean valley glaciers can be explained by the role of debris cover on energy exchange and melting at the glacier surface3. Thick debris cover forms a barrier between the glacier and the atmosphere that insulates the ice surface from melting. Where the debris layer is absent or sparse, on the other hand, a glacier and the atmosphere are able to interact freely, and there is more energy available for melting at the ice surface.
For this reason, clean glaciers react rapidly to atmospheric conditions (e.g. cooling or warming) by gaining or losing mass, and then advancing or retreating1,2. As we will see later, this behaviour has a large impact on their interaction with the landscape.
Where are clean valley glaciers found?
Clean valley glaciers are found in most glaciated mountain ranges, although they are more common in low- to moderate relief mountains, and in areas of hard (to erode) bedrock, where the debris supply from valley side mass movements is minimal1.
Excellent examples of clean valley glaciers can be found in the Scandinavian mountains of western Norway and Sweden, in the coastal mountains of British Columbia and the Canadian Rockies, the Andes of Patagonia, and the European Alps.
Landforms of clean valley glaciers
Together with the large-scale features of glaciated valleys2 – which may include U-shaped valley profiles, arêtes, hanging valleys, and ribbon lakes – clean valley glaciers are known to produce a distinctive suite of landforms and sediments1. The most typical are outlined below.
Latero-frontal moraines are formed at the outer limit of clean valley glaciers1. Largely, they are the result of ice pushing and the squeezing of waterlogged sediments from beneath the ice margin, with few dumps of material from the ice surface4-6. Much of the material that makes up the moraines formed by clean valley glaciers, therefore, derives from subglacial erosion (evidenced by the presence of faceted, striated, and sub-angular rocks in moraine deposits), or is picked up from the foreland during glacier advance4-6.
The active nature of most clean valley glaciers – meaning that they oscillate readily in response to changes in climate – often leads to the formation a large number of moraine ridges on the valley floor, with each ridge representing a period of glacier stability1,2. The size and spacing of these recessional moraine ridges give some indication of the duration and frequency of glacier stillstands during retreat7.
Typically, the latero-frontal moraines formed by clean valley glaciers are relatively small; often, they are less than 10 metres high1,2. This is result of low supraglacial debris supply that limits the amount of material for moraine formation4-6, oscillating snouts that spread debris across large areas of the valley floor2, and the role of meltwater, which may flush out large volumes of debris from beneath valley glaciers and transport it away from the ice margin in proglacial streams.
While generally consistent in size, lateral moraines – those formed at the glacier sides – are sometimes larger than those deposited in the valley centre because of the increased supply of debris from valley walls, in the form of rockfalls, landslides, and slumps1,2,4,5,6. Similarly, variations in catchment geology (e.g. areas of weak or strong bedrock) or glacier dynamics may lead to variations in debris supply and, therefore, moraine volume1,2.
The former thickness of valley glaciers can often be identified using trimlines on the valley side2,8,9. These trimlines, which mark the upper limit of recent glacial erosion on the valley wall, can be identified by the contrast in vegetation cover on either side of the limit, with bare rock or pioneer vegetation found below the trimline where glacial erosion has occurred, and well-vegetated or forested slopes above it.
The upper extent of valley glaciers may also be inferred from the boundary between frost weathered debris (e.g. talus) above and the ice-moulded bedrock below. This is known as a periglacial trimline8,9.
Subglacial erosional processes are active at most clean valley glaciers. Zones of ice-moulded bedrock, roches moutonnées, whalebacks, and striations, for example, are often seen emerging from beneath retreating glacier snouts, providing evidence for abrasion and quarrying of the bed by warm-based and sliding ice1,2.
In addition to erosional landforms, the beds of clean valley glaciers may contain flutings (flutes)10, usually on the valley floor between recessional moraines, as well as other streamlined deposits that fill in the hollows behind lumps and bumps in the bedrock and trail away in a downglacier direction. These latter features are known as lee-side cavity fills2.
Glaciofluvial landforms and sediments are common in the proglacial zone of clean valley glaciers, particularly those occupying maritime mountain ranges where the amount of glacial meltwater produced each year is high1,2,11. At valley glaciers in dry, arid climates, on the other hand, glaciofluvial features are less abundant.
In valley glacier systems, the movement of meltwater streams is restricted by the valley walls. Therefore, it is common for outwash (sandur) deposits (also known as valley trains) to build up in the valley bottom, or in the low points between recessional moraines1,2,11.
Where glaciofluvial processes are especially active, meltwater streams may completely rework moraines and other glacial deposits, so that only minor traces of former terminus positions remain9. Over time, river terraces may form in glaciated valleys as a result of fluvial incision into valley fill deposits1,2,11.
The landsystem of clean valley glaciers
The unique features of clean valley glaciers – i.e. that they carry limited surface debris, that most debris they transport comes from erosion of the bed, and that their snouts fluctuate in response mass loss or gain – leads to the formation of a distinct set of landforms1,2.
In summary, this includes: numerous low-relief moraine ridges crossing the valley floor (occasionally with larger lateral moraines where rock debris falls from the valley walls); areas of ice-moulded bedrock with roches moutonnées, whalebacks, and striations; and the outwash deposits (e.g. valley trains) of proglacial streams.
Spatial pattern of landforms
At some (but not all) clean valley glaciers, the landforms formed by clean valley glaciers are organised into several zones2. This includes an inner erosional zone in the upper valley, where ice-moulded bedrock is extensive; an intermediate zone characterised by both bed erosion and deposition; and an outer depositional zone in the lower reaches of a glaciated valley, where moraines and outwash deposits are most extensive.
This arrangement of landforms, which is not restricted to valley glaciers, reflects the downglacier transport of debris from areas of net erosion higher up in the valley towards the ice margin.
 Benn, D.I., Kirkbride, M.P., Owen, L.A. and Brazier, V., 2003. Glaciated valley landsystems. In Evans, D.J.A. (ed.) Glacial Landsystems, pp. 372-406.
 Benn, D.I., and Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder-Arnold, London.
 Nakawo, M. and Young, G.J. 1981. Field experiments to determine the effect of a debris layer on ablation of glacier ice. Annals of Glaciology 2, 85–91.
 Matthews, J.A. and Petch, J.R., 1982. Within‐valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway. Boreas, 11, 225-247.
 Shakesby, R.A., 1989. Variability in Neoglacial moraine morphology and composition, Storbreen, Jotunheimen, Norway: within-moraine patterns and their implications. Geografiska Annaler: Series A, Physical Geography, 71, 17-29.
 Benn, D.I. and Ballantyne, C.K., 1994. Reconstructing the transport history of glacigenic sediments: a new approach based on the co-variance of clast form indices. Sedimentary Geology, 91, 215-227.
 Eyles, N. 1983. The glaciated valley landsystem. In Eyles, N. (ed.) Glacial Geology. Pergamon, Oxford, 91–110.
 Ballantyne, C.K., 1997. Periglacial trimlines in the Scottish Highlands. Quaternary International, 38, 119-136.
 Ballantyne, C.K. 2007. Trimlines and palaeonunataks. In Elias, S.A. (ed.), Encyclopedia of Quaternary Science. Elsevier, Oxford, 892–903.
 Evans, D.J., Ewertowski, M. and Orton, C., 2017. The glaciated valley landsystem of Morsárjökull, southeast Iceland. Journal of Maps, 13, 909-920.
 Maizels, J.K., 1995. Sediments and landforms of modern proglacial terrestrial environments. In Menzies, J. (ed.), Modern Glacial Environments. Butterworth-Heinemann, Oxford, 365–416.