Esker distributions and tunnel valleys

Plate 2.3 shows the distribution of eskers on the Fennoscandian Shield. Although eskers occur beyond the Shield, they are infrequent. A similar situation applies on and around the Laurentide Shield in North America. It is not entirely clear whether eskers are simply not so well preserved in the fringing soft-sediment areas, whether subglacial tunnel flow eroded deep channels into the substratum rather than being contained in R-tunnels, or whether eskers are largely replaced by tunnel valleys, or whether, as suggested by Clark & Walder (1994), channelled meltwater beyond the shield is discharged via high pressure 'canals' rather than low pressure R-tunnels.

Boulton et al. (2001b) have suggested that on the Shield, large R-tunnels that ultimately give rise to eskers occur where ground-water flow alone is unable to discharge the subglacial meltwater flux. They suggest that the spacing of eskers is that required to discharge the excess meltwater flux, a suggestion consistent with the observed increase in esker frequency with radial distance away from the ice divide, which would also be the groundwater divide. In any case, if eskers do represent the former locations of low pressure R-tunnels, they would inevitably act as sinks for groundwa-ter, which would predominantly flow towards them. This pattern of flow is simulated in Fig. 2.14. This simulation demonstrates that the effect of tunnel drawdown would be to ensure that the

Figure 2.14 Modelled palaeohydraulic patterns based on the distribution of eskers (N-S lines) and bedrock conductivities in the area north of the Salpausselka moraines in Finland. Scale in kilometres. (a) Simulated water pressures in bedrock as a proportion of ice pressure. The pressure difference across a till lying above bedrock (see Fig 2.4) would produce larger water pressures at the ice-bed interface. (b) Groundwater flow vectors in bedrock. They are strikingly similar to the patterns that would be expected in unglaciated temperature regions.

Figure 2.14 Modelled palaeohydraulic patterns based on the distribution of eskers (N-S lines) and bedrock conductivities in the area north of the Salpausselka moraines in Finland. Scale in kilometres. (a) Simulated water pressures in bedrock as a proportion of ice pressure. The pressure difference across a till lying above bedrock (see Fig 2.4) would produce larger water pressures at the ice-bed interface. (b) Groundwater flow vectors in bedrock. They are strikingly similar to the patterns that would be expected in unglaciated temperature regions.

dominant groundwater flow vector would be transverse to ice flow and not parallel to it as suggested by Boulton et al. (1993) and Piotrowski (1997b). Although I agree with Piotrowski and Piotrowski & Marczinek (this volume, Chapters 9 & 10) that in areas such as North Germany the transmissivity of the subsurface would have been inadequate to discharge even the basal meltwa-ter flux alone to the margin via longitudinal flow, all meltwater can be discharged by groundwater provided the flow is transverse and towards esker/tunnel valley channels. The inferred hydraulic pattern would also play a major role in influencing ice-sheet dynamics through its influence on the effective pressure at the ice-bed interface.

So-called tunnel valleys that have long been regarded as products of subglacial fluvial erosion (Madsen, 1921) are common in the zone of sedimentary rocks that fringe the Shield area in Europe. Some prefer the term 'tunnel channel', that embraces both small channels and larger valleys, but I shall retain the term tunnel valley, in the recognition that they are large features that demand a special explanation. In Europe, features with this appellation are broad (0.2-5km), deep (50-400m), steep sided (up to 40° marginal slope) channels that can be up to 100 km in length. Unlike normal valleys, they rarely have till at their base, but tend to have sand and gravel fluvial sediments near their base, which are overlain by glaciolacustrine and marine sediments. Although many have been occupied during several glacial cycles, some were entirely eroded during the Weichselian. The volume of excavation that they represent would require very high rates of erosion. The larger ones in North Germany, if continuously eroded during the period of Late Weichselian glacier occupancy, would require a continuous sediment discharge rate of about 0.1 m3s-1, much larger than could be achieved if the water flux was derived from basal melting alone. It would imply either that they were eroded by short-period catastrophic floods (Wingfield, 1990) or that large quantities of surface meltwater found their way to the bed of the ice sheet and were channelled along the valleys. In the former case they may have formed by bankful discharges; in the latter a relatively small tunnel would have existed along the valley axis. Brennand et al. (this volume, Chapter 6) suggest that tunnel valleys in central Ontario are 'consistent with' an origin in which they, together with the regional drumlin fields, were eroded by a subglacial megaflood.

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