ice sheets and landscape evolution

ice sheets and landscape evolution The surface of the Earth has been profoundly modified by the action of ice sheets. On several occasions during the Quaternary period of the past 2 million years, ice sheets and glaciers covered approximately one-third of the Earth's surface, and parts of every continent bear the imprint of glaciation. Ice sheets modify the landscape in two principal ways. First, they act as powerful agents of erosion, scouring out and removing large volumes of rock and sediment; and second, this eroded material is laid down by glacial deposition, either beneath the ice sheet, along the ice edge, or beyond the ice margins. Ice sheets therefore erode new landscapes, or bury old land surfaces beneath new glacial landforms. In some circumstances, ice sheets can also protect the underlying land surface from erosion or deposition, preserving ancient landscapes beneath their frozen covering.

The most important control on erosion and deposition beneath an ice sheet is the temperature at the base, in particular whether the ice is at or below the local melting point (see glaciers and glaciology). If the ice is below the melting point (which may be below 0 °C if the ice is under high pressure), it will be frozen to the underlying land surface and the base of the ice sheet will be immobile. (The high adhesive strength of cold ice can easily be appreciated while clearing a car windscreen on a frosty morning.) In contrast, if the basal ice is at the melting point the ice sheet can slide over a thin film of meltwater at the bed, and significant erosion and deposition can occur as rock fragments are dragged along beneath the ice. Warm-based conditions tend to occur below thick, rapidly moving ice, or where the surface climate is relatively mild, whereas cold-based conditions are encouraged beneath thin, sluggish ice in extremely cold environments. Warm- and cold-based conditions can occur simultaneously below different parts of the same ice sheet, depending on variations in ice thickness, flow rates, and climate. Glaciated landscapes can thus provide important insights into patterns of freezing and melting conditions at the base of former ice sheets.

Landscapes of glacial erosion

Five basic landscapes of glacial erosion were identified by Professor David Sugden of the University of Edinburgh. Models of erosion by ice sheets are illustrated in figure 1. First, landscapes of areal scouring are extensive tracts of subglacially eroded bedrock, consisting of hundreds of overdeepened rock basins and intervening rock knobs. In Scotland, such terrain is known as knock and lochain topography, from the Gaelic words cnoc, meaning ‘knoll’ and lochain, meaning ‘small lake’. The widespread evidence for abrasion and plucking in such landscapes indicates that they develop below extensive areas of sliding, warm-based ice. Spectacular examples of areally scoured landscapes occur in north-west Scotland, especially in Sutherland and the Isle of Lewis.

Second, in landscapes of selective linear erosion, glacial erosion is confined to deep, linear troughs incised into unmodified plateau surfaces. Such landscapes develop where warm-based ice streams occur between areas of slowly moving or cold-based ice. This situation is common in parts of Greenland and Antarctica today, where ice streams drain the greater part of the ice that accumulates on the great inland snowfields (Fig 2). Third, landscapes of little or no glacial erosion consist of essentially unmodified preglacial landscapes that have survived one or more periods of glacial occupancy. Sometimes the only indication that such landscapes have been glaciated at all is the presence of rare erratic boulders, or evidence for glaciation in surrounding regions. Landscapes of little or no glacial erosion persist beneath cold-based ice where sliding is non-existent or insignificant. Examples include Buchan in north-east Scotland and parts of the Canadian Arctic islands, where Tertiary landforms and sediments have been preserved despite repeated occupancy by glacier ice during the Quaternary.

The fourth and fifth landscape types are alpine landscapes, consisting of branching networks of troughs separated by ridges; and cirque landscapes, in which separate cirques (or corries) are set in an upland massif. These landscapes develop in mountainous terrain, where ice cover is insufficient to bury summits and ridges. Mature alpine landscapes exhibit many of the ‘classic’ features of glaciation, including troughs, hanging valleys, truncated spurs, and narrow arêtes rising to narrow rock peaks. These are most spectacularly developed in high mountain environments, such as the Himalayas, Rockies, Andes, and the Alps of New Zealand and Europe (Fig. 3). Cirque landscapes reflect more restricted glacial conditions than alpine landscapes.

Landscapes of glacial deposition

Landscapes of glacial deposition tend to develop in the outer parts of ice sheets, where melting ice releases its sediment load or deforming sediment accumulates at the base of the ice. Depositional landscapes also predominate in areas underlain by weak sedimentary rocks, which are easily reworked by glacial action. Examples include the spectacular drumlin fields of lowland Britain, and the ice-pushed hills of northern Europe and the northern Prairies of North America.

Continent-wide patterns of glacial landscapes reflect long-term influences of pre-existing topography and geology, glacier and ice-sheet morphology, basal temperatures, and ice-flow rates, and systematic variations in these factors can explain many major landscape patterns, particularly in the mid-latitudes. A glance at the map of Canada, for example, reveals a broad arc of large lakes, from the Great Bear Lake to Lake Ontario, which is thought to reflect the distribution of former warm-based zones below the Laurentide Ice Sheet. In the west, the fiordlands and alpine terrain of British Columbia attest to vigorous glaciation of high-relief mountains in a snowy, maritime climate. In the north-east, the plateaux and troughs of Labrador and Baffin Island record the selective action of ice streams in a colder, more arid environment. In contrast, depositional landscapes predominate in southern Alberta and Saskatchewan, where soft sedimentary rocks crop out near the southern limit of the ice sheet.

Glacial landscapes develop cumulatively over multiple glacial cycles, and are therefore more likely to relate to ‘average’ glacial coverage during the Quaternary than to ‘instantaneous’ ice configurations at glacier maxima. This average ice cover, which lies about halfway between present-day and ice-age maximum conditions, represents the principal control on much Quaternary landscape evolution, not only beneath the ice sheets themselves, but also the development of erosional shore platforms, tropical atolls (because of the correlation between ice cover and global sea level), and long-term sediment yields to the ocean basins.

Douglas I. Benn

Bibliography

Benn, D. I. and and Evans, D. J. A. (1998) Glaciers and glaciation, Chapter 9. Arnold, London.
Bennett, M. R. and and Glasser, N. F. (1966) Glacial geology: ice sheets and landforms. John Wiley and Sons, Chichester.