Plateau Glaciers and their significance

Plateaus exist in many mountainous parts of the world. Although not as spectacular as high, prominent peaks, they do often have (or have had in the past) glaciers associated with them. In this vignette I describe some work done on plateaus and their glaciers from northern Norway and show how they can be useful in telling us about the current behavior of 'outlet' glaciers, and where there might have been glaciers before deglacaition. They also can tell us about pre-Quaternary landscapes in parts of the world that once had extensive glaciers. This vignette shows two examples of present-day plateaus and small glaciers from northern Norway. It also shows how inferences can be made from these landforms and suggests that there may be areas in North America that might have had plateau glaciers at one time.

Plateau Glaciers in North Norway

Figure 1 shows the north-facing glaciers from Øksfjordjøkelen, north Norway. [Author's note: Øksfjordjøkelen = Øksfjord glacier. In Norwegian, Bre, Isbre, Blauisen and Jøkul are all terms for glacier; the suffix 'en' denotes the definite article. In Icelandic, the word jøkull is used.] Lateral and terminal moraines are found in the valley and the glaciers that formed them have been retreating during the last hundred years. The most recent maximum extent of the glaciers was about 1800-1850 CE (Common Era or AD), sometimes called 'The Little Ice Age' (Grove, 2004). Although it is not easy to see, the ice comes from an area of about 40km² with the highest point of the glacier on the plateau at about 1200m above sea level (asl), a km away from the plateau edge. Valley glacier systems below the plateau descend to near sea-level.

Now compare the size and altitude of the icefield in Fig. 1 with those seen in Fig. 2, about 120km to the southwest in the Lyngen Peninsula. Here, in the background, the highest rock plateau is at about 1800m asl and supports an ice mass some 100m thick and about 2km². As in Fig 1, outlet glaciers flow to the valley floors 800–1000m below. The plateau in the foreground is about 1300m asl but supports only a very thin (10–20m) glacier that is < 0.5km². Comparisons with photographs taken by climbers in 1898 show that it was once more extensive, but still flat with virtually no outlet glaciers flowing from it off the plateau and towards the valley.

Despite the glacier recession in the area, the remnant plateau glacier in Fig. 2 is at about 1400m asl yet the plateau supporting the glacier seen in Fig 1 is only about 900m asl with the maximum height of the ice summit at about 1200m. So, why does the much lower plateau support a much larger glacier? We found the answer in a paper by the British meteorologist Gordon Manley (Manley, 1955) who suggested that altitude was not everything, despite it tending to be colder the higher up you go (i.e., the lapse rate; 5.5°/km altitude). What also has to be considered is the area on which a summit can accumulate ice to form a glacier. As long as ice can be preserved, year-on-year, to form a glacier then you can produce a large glacier at a relatively low altitude (Gellatlyet al. , 1986). This is what appears to be the case in Fig. 1; the plateau area is large despite its relatively low altitude and thus it can support a glacier over nearly all the area. Both areas are in coastal locations so a precipitation gradient is unlikely. Maritime-continental gradients are important in some Scandinavian glaciers (Whalley, 2004), but the larger glaciers are near the sea.

Now consider a plateau glacier sitting on top of the rock plateau. If it just sits on the plateau it will not produce any substantial moraines as these are normally formed from weathered rock debris that falls on the glacier's surface. Even if ice tongues descend over the plateau they still might not actually produce any moraines below. Figure 3 shows more plateau ice (in fact on the same plateau as for Fig. 1 but off the view). Here the edge of the glacier has retreated away from the edge by some 200m. If you look carefully at the edge of the plateau you can see a trace of a ridge. This is a small frontal moraine produced from the glacier that has pushed up some weathered blockfield subglacial debris. If the glacier melted completely away then, unless you knew it had been there, it might be difficult to infer the glacier's past presence if you were just looking for moraines. Even if some ice did spill over the plateau edge, a small amount of ice might be left in a very small cirque on the face of the plateau and could produce moraines. However, the moraines might lead you to think that it was only ice in the cirque that had produced them if the plateau ice had melted away (Gordonet al. , 1987). The schematic in Fig. 4 shows what this might be like. We conclude, that on plateaus, one has to be careful in presuming how big a former glacier mass was, or indeed if there was one there at all. Such plateau glacier and icefield traces have been found in areas in the British Isles (McDougall, 2001). Investigations are currently under-way in the Dartmoor area of southern England, well south of what is normally thought to have been the ice maximum extent. The idea follows on from above--that moraine traces in the landscape are so subtle and indistinct that they are easy to miss unless you look for them very carefully. The paper by Evans et al. (2002) brings together some of the findings about plateau glaciers in North Norway and fits this into a wider regional context. An overall view of plateau icefield landsystems is given by Rea et al. (2003).

Figure 5 shows the edge of the Beartooth Mountain plateau in Montana. There are some features (protalus ramparts) that are usually attributed to snowbanks rather than glaciers. But these mountains also have substantial high plateaus (approx 3000m asl) that could have had glaciers on their summits. However, as far as I know, nobody has been to look for them. Future research on the possibility of former plateau glaciers in the area may be warranted, perhaps starting with a Google Earth examination at 45° 16' N; 110° 02' W.

Revealed Blockfields and Pre-Glacial Landscapes

One final, rather intriguing, point about the Norwegian plateaus is the blockfield left after the ice has gone (e.g. in Figs. 2 and 3). These blockfields can be a metre or more deep over solid bedrock. The blockfields have been weathered from the country rock (a tough gabbro) that gives rise to the plateaus. It is not imported glacier till. So why is it still there? Should it not have been removed by the ice on the plateau or even by the maximum ice extent over this area (which was probably a kilometre above the present ice level)? The answer appears to be that the glacier ice was frozen at the bed (cold-based glaciers), so sliding of the ice did not remove the blockfield. This has a bearing on the long-term evolution of the landscape (Whalleyet al. , 2004). Clay minerals in the blockfield suggest chemical weathering in a much warmer climate than present now (mean annual air temperature, maat, about 1 ºC). We can conclude that the blockfield was probably produced by chemical weathering under pre-glacial conditions rather than periglacial (mechanical, frost action) rock weathering. The plateaus were part of a very old (perhaps 30Ma) 'Paelic surface' originally dissected by rivers and hillslope erosion. Glaciers only came later, so we should not really think of these landscapes as being just 'glacial.' They are much older than the Quaternary glaciations and are sometimes called 'palimpsest' landscapes; some old features have been left, some have eroded away. This has been termed 'erosion censoring' and illustrates that most landscapes are much older than appear at first sight.

• Evans, D. J. A., Rea, B. R., Hansom, J. D., & Whalley, W. B. (2002) Geomorphology and style of plateau icefield deglaciation in fjord terrains: the example of Troms-Finnmark, north Norway. Journal of Quaternary Science, 17, 221-239.
• Gellatly, A. F., Whalley, W. B., & Gordon, J. E. (1986) Topographic control over recent glacier changes in Southern Lyngen Peninsula, North Norway. Norsk Geografisk Tidsskrift, 41, 211-218.
• Gordon, J. E., Whalley, W. B., Gellatly, A. F., & Ferguson, R. I. (1987) Glaciers of the southern Lyngen Peninsula, Norway: a possible model for mountain deglaciation, in: V. Gardiner, (Ed.), International Geomorphology. (Chichester: John Wiley,), pp. 743-758.
• Grove, J.M. (2004) Little Ice Ages, Ancient and Modern, 2 Vols. (New York: Routledge).
  
• Manley, G. (1955) On the occurrence of ice-domes and permanently snow-covered summits. Journal of Glaciology, 2, 453-456.
• McDougall, D. A. (2001) The geomorphological impact of Loch Lomond (Younger Dryas) Stadial plateau icefields in the central Lake District, northwest England. Journal of Quaternary Science, 16(6), 531-543.
• Rea, B. R., & Evans, D. J. A. (2003) Plateau icefield landsystems, in: D. J. A. Evans, (Ed.), Glacial landsystems. (London: Hodder), pp. 407-431.
• Whalley, W. B. (2004). Glacier research in mainland Scandinavia. Earth paleoenvironments: records preserved in mid- and low-latitude glaciers. L. D. Cecil, J. R. Green and L. G. Thompson. Dordrecht, Kluwer, 121-143.
  
• Whalley, W. B., Gordon, J. E., Gellatly, A. F., & Hansom, J. G. (1995) Plateau and valley glaciers in north Norway: responses to climate over the last 100 years. Zeitschrift für Gletscherkunde und Glazialgeologie, 31, 115-124.
• Whalley, W. B., Rea, B. R., & Rainey, M. M. (2004) Weathering, blockfields, and fracture systems and the implications for long-term landscape formation: some evidence from Lyngen and Øksfjordjøkelen areas in North Norway. Polar Geography, 28(2), 93-119.