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Since the advent of satellite imagery, geomorphologists have used remotely sensed images to gain a better understanding of large-scale landform assemblages. The main benefits of using satellite imagery are clear: the large field of view and the range of display scales, both allowing a higher speed of coverage. Many features undetectable on aerial photographs at large scales become readily apparent on LANDSAT images when viewed at a scale of 1 : 100,000 or more. Unfortunately, few studies use both types of imagery in tandem. This study shows the advantages of combining modern satellite images with aerial photographs and traditional field survey techniques to address problems across a wide range of scales.

The geomorphology of Lower Annandale, between Lockerbie and Moffat, is very intriguing. When viewed from the ground or low-flying aeroplane the ground looks unremarkable—rolling farmland punctuated by occasional streams, typical of much of

Figure 7.1 Subset of a LANDSAT 5 image of Southern Scotland (bands 4, 5 and 7). Lockerbie is just off the southern edge of the image and Moffat is just to the north. The region is underlain by Silurian greywacke sandstones that strike NE-SW. The extent of the Permian sandstone in the south is also shown. Strong large-scale linear features cross-cut the geological 'grain'. The best developed, highly elongated, ridges and grooves are found in the central portion of the image, in the Annan Valley. The arrow shows the direction of palaeo-ice flow. B, Beattock; J, Johnstonebridge.

Figure 7.1 Subset of a LANDSAT 5 image of Southern Scotland (bands 4, 5 and 7). Lockerbie is just off the southern edge of the image and Moffat is just to the north. The region is underlain by Silurian greywacke sandstones that strike NE-SW. The extent of the Permian sandstone in the south is also shown. Strong large-scale linear features cross-cut the geological 'grain'. The best developed, highly elongated, ridges and grooves are found in the central portion of the image, in the Annan Valley. The arrow shows the direction of palaeo-ice flow. B, Beattock; J, Johnstonebridge.

lowland Scotland. When viewed by satellite, however, 700 km above the earth, subtle patterns emerge. The LANDSAT 5 image in Fig. 7.1 is ca. 20 km across, has a pixel resolution of 30 m, and clearly shows the strong north-south lineations in the wide valley of the River Annan. The positive elongate features are in the same family of subglacial bedforms as drumlins: namely streamlined, elliptical, mounds of sediment or bedrock associated with icesheet flow. However, the large spindle-shaped landforms in Lower Annandale, typically 1-3km long with length:width ratios up to 10:1, are probably best referred to as megadrumlins and megaflutes (sensu Rose, 1987). These features probably formed beneath the last ice sheet to cover the area, during the Late Deven-sian stage.

Using 1:24,000-scale aerial photographs, a more detailed inspection of the landform assemblage is possible. Further examination reveals a complex ridge-and-groove topography in Lower Annandale, with megaflutes and drumlins representing 'islands' defined by a large-scale network of interconnected grooves. Once recognized, these large grooves, often very shallow in form and gradient, can be mapped across the whole valley in detail. They occur on a range of scales (Figs 7.2 & 7.3), with spacings of tens to hundreds of metres, and cut through both bedrock and superficial deposits. The overall trend of the linear grooves in Lower Annandale is due north-south, whereas the strike of the steeply dipping greywacke sandstone is orientated NE-SW. A primary bedrock control on groove formation therefore can be ruled out.

Detailed walkover survey of the ground between Beattock and Johnstonebridge (Fig. 7.1) confirmed the existence of shallow, linear, depressions surrounding low, elongate, ridges. On the valley side, above ca. 200 m OD, topographic high points tend to be composed of bedrock (Fig. 7.2). On the valley floor, between the Rivers Annan and Kinnel Water (Fig. 7.3), hills are more commonly composed of glaciogenic sediments, typically lodgement till or coarse gravelly diamictons. The extensive intervening depressions are often gravel filled. These linear depressions, or grooves, are interpreted as glacial meltwater channels. The channels undulate in long profile, implying a subglacial origin. There is no possibility that these wide, largely dry, channels could have been cut during the post-glacial period. Furthermore, when mapped from aerial photographs, the channels represent an interconnected, anastamosing, network (Fig. 7.3). Given that the grooves cut into bedrock southwest of Beattock are subglacial meltwater channels, and that these same channels also cut through glacial sediments to define large drumlins and megaflutes (Fig. 7.2), it is argued that many of the streamlined landforms are erosional remnants.

It is the author's view that large volumes of debris-laden subglacial meltwater, acting under high pressure at the base of a mobile ice sheet, were responsible for carving the network of channels seen in Lower Annandale. The residual landforms are drumlins and megaflutes in bedrock and glacial sediments. The size and elongation of these streamlined forms increase gradually down-valley from Beattock to Johnstonebridge, possibly indicating increasing basal-meltwater flow linearity at higher velocities.

This case study echoes the conclusions made by Sharpe & Shaw (1989), working in the Canadian Shield, namely that the role of subglacial meltwater may be more significant than previously thought in shaping the glacier bed. It should be stated, however, that these findings are in sharp contrast to the more accepted theories of drumlin formation (cf. Boulton, 1987; Rose, 1987; Menzies et al., 1997). Presently, there is no evidence to suggest whether the channel networks identified in this case study are the

Figure 7.2 The geomorphology of the area immediately southwest of Beattock, Annandale. In the north of the area closely spaced subparallel channels define the outline of remnant, drumlinoid, bedrock hills. Individual channels can be traced over distances of 2-3 km. Further south these channels dissect glacial deposits and bedrock alike, resulting in elongate megadrumlinoid forms. Contours at 10 m vertical intervals; drumlinoid forms shown in dark grey; bedrock at surface shown as stipple; modern alluvium shown in light grey. See Fig. 7.1 for location. An aerial photograph is included at www.blackwellpublishing.com/knight

Figure 7.2 The geomorphology of the area immediately southwest of Beattock, Annandale. In the north of the area closely spaced subparallel channels define the outline of remnant, drumlinoid, bedrock hills. Individual channels can be traced over distances of 2-3 km. Further south these channels dissect glacial deposits and bedrock alike, resulting in elongate megadrumlinoid forms. Contours at 10 m vertical intervals; drumlinoid forms shown in dark grey; bedrock at surface shown as stipple; modern alluvium shown in light grey. See Fig. 7.1 for location. An aerial photograph is included at www.blackwellpublishing.com/knight

Figure 7.3 The geomorphology of the area north of Johnstonebridge, Lower Annandale. Subtle interconnected channels, cut into superficial deposits and bedrock, delimit the extent of streamlined drumlinoid ridges. These N-S glacial lineations are found widely across Lower Annandale. See Fig. 7.1 for location.

Figure 7.3 The geomorphology of the area north of Johnstonebridge, Lower Annandale. Subtle interconnected channels, cut into superficial deposits and bedrock, delimit the extent of streamlined drumlinoid ridges. These N-S glacial lineations are found widely across Lower Annandale. See Fig. 7.1 for location.

result of catastrophic subglacial flooding, as favoured by Shaw et al. (1989), or high volumes of basal meltwater associated with a fast-flowing ice stream, as found elsewhere by 0 Cofaigh et al. (2002a) and preferred by this author.

Finally, this case study highlights an example of equifinality in glacial geomorphology. Although landforms such as megadrum-lins may appear similar, particularly when viewed on satellite imagery, they may be the result of very different geological processes.

Conclusions

1 A complex network of large-scale subparallel channels and streamlined ridges occur in Lower Annandale, southern Scotland.

2 The bedrock-cut channels are subglacial meltwater features aligned parallel to former ice-flow. They are not structurally controlled.

3 The existing landscape in Lower Annandale has been greatly influenced by subglacial meltwater erosion—many of the drumlinoid ridges and megaflutes being erosional remnants of bedrock and glacial deposits.

4 The nature and distribution of these landforms indicate that ice-sheet flow in this part of southern Scotland was convergent and faster flowing than in neighbouring areas, probably aided by high volumes of subglacial meltwater.

Acknowledgement

T. Bradwell publishes with permission of the Executive Director, British Geological Survey (NERC).

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