Palaeoglaciological Reconstructions

As an example of how these principles can be used to make a simple palaeoglacio-logical reconstruction we start with a map of the distribution of landforms on a hypothetical ice-sheet bed (Figure 12.9A) and outline how these landforms can be used to infer the basic shape and dimensions of a former ice sheet (Figure 12.9B).

1. Terminal moraines. Large terminal moraines mark the outer limit of the ice sheet. In some places the terminal moraine is a single ridge; in other places it may be a series of subparallel ridges. There may be large sandar (outwash plains) leading away from the terminal moraines wherever subglacial or supraglacial streams emerged from the ice margin. The terminal moraines can be used to reconstruct the size of the ice sheet at its maximum extent.

2. Unmodified preglacial landscape. In the core of the former ice sheet is a landscape dominated by the preservation of largely unmodified preglacial landforms on uplands. These landforms were created by subaerial weathering and erosion processes, and include well-developed tors and associated saprolites, boulder fields and boulder depressions. The preservation of these landforms is used to infer that this is the location of the main (first order) ice divide. Here former icesheet velocities were low and the ice sheet was cold-based, allowing preservation of components of the preglacial landscapes.

3. Streamlined subglacial landforms. Streamlined subglacial landforms are common on the former ice sheet bed. In the east, there are four separate systems of streamlined subglacial landforms, each representing a different flow set. Since these landforms are created parallel to former ice flow and because each set has a different orientation, we can infer that they indicate four distinct flow sets, each separated by subsidiary ice divides. In contrast, the streamlined subglacial land-forms in the north and south have sharply delineated lateral margins. The maximum extent of the southern set is marked by large lobate moraine systems that project beyond the general extent of the ice-sheet terminal moraines. These south-flowing outlet glaciers would therefore be interpreted as the sites of palaeo-ice streams. There is no terminal moraine associated with the northward-flowing set

Figure 12.9 Example of a palaeoglaciological reconstruction. (A) Overview of mapped landform evidence. (B) Reconstruction of the former ice sheet based on the landform record. Note that the reconstruction assumes that the mapped glacial landforms formed more-or-less synchronously and near the time of the ice-sheet maximum, that is, there is little information about patterns of ice recession.

Figure 12.9 Example of a palaeoglaciological reconstruction. (A) Overview of mapped landform evidence. (B) Reconstruction of the former ice sheet based on the landform record. Note that the reconstruction assumes that the mapped glacial landforms formed more-or-less synchronously and near the time of the ice-sheet maximum, that is, there is little information about patterns of ice recession.

of streamlined subglacial landforms, but ice-rafted debris on the ocean floor indicates that there was a calving glacier or ice shelf here. These landforms could therefore be interpreted as the sites of palaeo-ice streams feeding into an ice shelf. On the western side of the ice sheet there is an area of streamlined subglacial landforms, this time without sharp lateral boundaries, no terminal moraine and with ice-rafted debris on the ocean floor. This again indicates a marine setting, but we cannot reconstruct a palaeo-ice stream in this location because the landforms lack the diagnostic abrupt lateral boundaries. Instead, the inference is that a fast-flowing tidewater glacier drained into a calving bay here.

This example is highly simplified and for scale reasons it omits many of the landforms listed in Section 12.3, which would also provide useful information about the former ice sheet. The 'real world' is likely to be much more complex for the following reasons.

1. We have illustrated an ice-sheet bed with a near-complete landform record but it is much more likely that the landform record will be fragmentary in nature. There will be gaps in the record where landforms are missing, as well as areas where landforms have been modified, overprinted or erased by changes in ice-flow direction or thermal regime.

2. We have assumed that the mapped glacial landforms formed more-or-less synchronously, at or around the ice-sheet maximum. This is not the case in the real world because landform development beneath ice sheets is time-transgressive; that is landforms are created, re-orientated, overprinted or erased as the ice sheet changes its configuration and thermal regime over time (e.g., during ice-sheet growth and during deglaciation). Changes in ice-flow direction would arise from migration of the ice divides (e.g., during ice-margin retreat), or as ice streams switch on and off in response to changing subglacial conditions and changes in ice-sheet basal thermal regime. Landforms relating to ice-sheet inception and icesheet growth would probably be destroyed during ice-recession so it is difficult to extract information about these time periods directly from the landform record.

3. Our simple model does not account for wholesale changes in the basal thermal regime of the ice sheet through time; that is, the effects of changes in the location of zones of basal melting and freezing as the ice sheet expands and recedes across the landscape. These changes have the potential to create, re-orientate, overprint or erase existing landforms.

4. Deglaciation is the last in a series of events during the lifetime of an ice sheet and it is likely that most of the landforms would probably date from deglaciation, not from the ice-sheet maximum as we have illustrated. Where the landform record contains features such as recessional moraines, eskers and ice-marginal melt-water channels, these could also be used to reconstruct patterns of ice-recession during deglaciation.

Notwithstanding these limitations, this simple model illustrates some of the land-forms we might expect to encounter on a former ice-sheet bed, their spatial distribution and their relationship to one another.

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