As early as 1981, Denton and Hughes (1981) recognized the role ice streams likely played in the dynamics of former ice sheets and presented a hypothetical map of potential ice-stream locations (Fig. 9.2). These were predicted on the basis of obvious topographic troughs and on ice-sheet geometry. The map is highly speculative, as at the time there was little or no evidence for ice-stream existence, but it made the point that ice streams probably existed and significantly affected the topography, flow geometry and behaviour of the ice sheets. Since this early work, there has been a drive to find more substantial evidence for their existence and significant progress has been made. A wide range of reconstructions that argue for the existence of specific ice streams have been hypothesized, at various locations within the Cordilleran, Laurentide,
British-Irish, Icelandic, Scandinavian, and Barents ice sheets. These have been reviewed by Stokes and Clark (2001), who comment that: 'unfortunately, hypothesised locations have tended to outweigh meaningful evidence because our understanding of ice stream geomorphology is limited' (Stokes and Clark, 2001, p. 1455). Evidence used to hypothesize specific ice streams ranges widely and includes: topographic troughs, intense glacial scour and streamlining of bedrock, specific erratic dispersal 'plumes', drumlin patterns indicative of fast flow or characteristic ice-stream flow patterns, mega-scale glacial lineations indicative of fast flow, sedimentary and tectonic evidence of pervasive deformation in glacial sediments, hummocky topography indicative of wastage following ice stream stagnation, large till deltas or trough mouth fans indicating spatially focused sediment delivery, and so on. Over fifty specific palaeo-ice streams have been hypothesized some of which have a strong basis in terms of evidence, but most of which are highly speculative.
To enable us to reliably identify a palaeo-ice stream there is a clear need for the various types of geomorphological and geological evidence to be synthesized into an idealized template of what they might leave behind. Rather than building such a landsystem model based on palaeo evidence, we firstly outline some of the problems in identifying palaeo-ice streams and turn to the characteristics of contemporary ice streams as a guide.
9.4 PROBLEMS IN IDENTIFYING PALAEO-ICE STREAMS
Unfortunately, the term 'ice stream' is often used very loosely to describe a range of fast ice flow phenomena. What are the differences, for example, between an ice stream, a surge and a surging ice stream? Does a transient ice stream qualify as a surge? This has led to many ambiguities in the literature. Ice streams have been used to account for terrestrial lobes protruding from the ice sheet margin (e.g. Patterson, 1997a), and to describe zones of fast flow within a lobe protruding from the margin (e.g. Boyce and Eyles, 1991). The term 'ice stream' has also been interchanged with the term 'tidewater glacier' (e.g. Merritt et al., 1995), and it has been used to describe short-lived surge behaviour in a marine environment (e.g. Kaufman et al., 1993), and surging lobes on land (Eyles et al., 1994).
It is recommended that the definition of an ice stream given by Paterson (1994) should be adhered to. That is: 'a region in a grounded ice sheet in which the ice flows much faster than in regions on either side'. This carries no implication as to whether the flow is transitory or continuous, or to the mechanism that promotes fast flow. The term 'surge' is usually taken to describe flow acceleration of a temporary (and non-steady state) nature, which in an ice sheet context is potentially confusing. It is probably best to restrict use of'surging' to just glaciers, whereby a large part ofthe glacier experiences cyclical flow acceleration, and use the term 'ice stream' for zones of fast flow within an ice sheet. These ice streams may be in continuous operation or transitory. An 'ice-sheet surge' would thus be reserved for a dramatic flow acceleration and possible margin advance ofthe whole ice mass, and which happens at regular cycles.
Little theoretical work has been carried out to predict the landforms and landform assemblages that we should expect an ice stream to produce. In essence, we have no clear criteria on which to base our assumptions. Mathews (1991) emphasized the need for diagnostic criteria when attempting to find palaeo-ice streams. There are two main problems:
1. a wide variety of evidence has been used to infer former ice streams (see above and Stokes and Clark, 2001), and
2. such evidence has rarely been scrutinized in detail, despite its often subjective nature.
Because ice streams are distinct features within an ice sheet, it seems logical to suggest that they will, in general, leave behind a distinct imprint of their activity, a 'geomorphological signature' or footprint. However, it seems that in some cases the identification of a distinct flow pattern (i.e. of drumlins) is regarded as ample justification for postulating the location of an ice stream. We would argue that the real challenge lies in demonstrating why the distinct flow pattern was produced by an ice stream.
All contemporary ice streams are marine-based, leaving us with the problem that we do not actually know what a terrestrial-terminating ice stream looks like. This poses a number of important questions when trying to reconstruct their configuration, not least of which is how does the ice stream evacuate ice so rapidly? Because contemporary ice streams feed ice shelves or terminate in open water, the removal of ice at the terminus is rapid and this maintains a high velocity. They can maintain a high velocity without advancing. In contrast, a terrestrial ice stream has a much less effective method for removing ice and presumably, therefore, has to advance producing a large splayed ice lobe at its terminus. This explains why many former ice streams have been postulated for the southern margin of the Laurentide Ice Sheet, the lobate configuration of which is thought to represent the distal portion of a number of palaeo-ice streams. However, is the fast flow restricted to a narrow zone along the central axis of the lobe, or does the divergence of flow result in a more uniform but slower regime of flow velocities? We presume it to be the latter case, with a lobe of slower moving ice (i.e. below typical ice stream velocities) protruding beyond the overall ice margin.
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