Deposition

Glacial erosion and glacial deposition are often treated separately, but Kleman et al. (2008) provide a neat example of how these two subjects can be combined in a palaeoglaciological reconstruction. These authors examined spatial differences in both glacial erosion, using the patterns of glacial scouring, fjords, lake basins and deep linear erosion, and glacial deposition, using the thicknesses of Quaternary deposits (termed drift: mainly tills) across Fennoscandia. Their ultimate aim was to investigate the relative roles of mountain ice sheets (MIS) and the full-sized Fennoscandian Ice Sheet (FIS) in shaping the landscape. Fjords and deep non-tectonic lakes were used to delineate zones of deep glacial erosion, and relict landscapes were used to mark areas characterised by former frozen-bed conditions. The amount of exposed bedrock in the landscape and drift thickness was used to create a three-fold landscape classification comprised of: (i) thick drift; (ii) intermediate drift thickness; and (iii) absence of drift/ice-scoured zones. The figure below illustrates this. Part A is a map of drift cover thickness and distribution, different types and intensity of glacial erosion, and occurrences of relict landscapes in Fennoscandia. Relict landscapes are defined as consisting of pre-Late Weichselian landforms and deposits (such as boulder fields). Part B shows the location of thick drift cover zones and scouring zones along a NW-SE transect across the area covered by the former Fennoscandian Ice Sheet. Kleman et al. (2008) then tried to relate the distribution of these different landscapes to former glaciological regimes. They inferred that the western fjord zone of deep glacial erosion formed beneath both MIS- and FIS-style ice sheets during the entire Quaternary, whereas the eastern (lake) zone of deep glacial erosion was exclusively related to MIS style ice sheets, and formed largely during the early and middle Quaternary. The scouring zones formed under conditions of rapid ice flow towards iceberg-calving margins of FIS-style ice sheets. A centrally placed zone of thick drift could not be explained by deposition under FIS-style ice sheets, so was interpreted to be the combined result of marginal deposition of fluctuating

MIS-style ice sheets, and the inefficiency of later FIS-style ice sheets to evacuate this drift because their core areas were characterised by low ice velocity and frozen-bed conditions.

Source: Kleman, J.K., Stroeven, A.P. and Lundqvist, J. (2008) Patterns of Quaternary ice sheet erosion and deposition in Fennoscandia and a theoretical framework for explanation. Geomorphology, 97, 73-90. [Modified from: Kleman et al. (2008) Geomorphology, 97, figure 7, p. 80].

Techniques are also being developed that allow us to date palaeoglaciological events more accurately (Box 12.8), which will in turn allow us to improve and refine icesheet reconstructions.

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