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Letter to the Editors

GLACIAL MELTWATER CHANNELS IN THE NORTHERN ISLES OF : COMMENT

SIRS Flinn's painstaking work on the history of glaciation in Shetland has led him to the conclusion that, though unexpected, there was an ice front close offshore from Yell and across or just offshore from the northern tip of Unst at the Weichselian glacial maximum (Flinn 1983). This conclusion can be supported by evidence from borehole and sea bed samples collected by BGS in their 1:250,000 Regional mapping programme of the area. Inshore, to the west and east of Shetland, till deposits are present at the sea bed and can be mapped for some 75 km offshore to the east (Fig. 1) forming an incomplete cover on rockhead. These tills are frequently grey to the west of Shetland and greyish to reddish-brown to the east. It is presumed that these tills represent the occurrence of the last glacial advance, the Weichselian. In the north there is a different sediment pattern with patches of sand, till and soft clay distributed in a manner which agrees well with the pattern to be expected in areas where glacial meltwater channels, as suggested by Flinn, debouched at an ice front. The thickness of Quaternary deposits is variable to the north of Shetland and bedrock is also present within one metre of the sea bed at some sites. Samples of a compact stony grey till are restricted to a narrow area northwards from Bluemull Sound (Fig. 1). Within (Fig. 1) and offshore from it, only sands with some gravel have been recovered. However seismic evidence shows that, in a restricted area, up to 30 m of Quaternary material is present within a hollow in the rockhead and comprises firm sandy clay with scattered pebbles of presumed glacial age with a thin cover of soft, post glacial clays (Chesher, 1984). Outside these two areas, only sands and gravels occur, except in an area about 25 km north-east of Unst where soft pink clays have been sampled. Between the two till areas noted offshore to the north of Mainland Shetland between Uyea and Point of (Fig. 1), outcrops of rock, sand and boulders suggest the possibility of the presence of an additional area of meltwater channels. Flinn's map of glacial striae gives some indication of a divergence of the ice sheet along the northern coast of Mainland, with striae orientated due northwards at the Point of Fethaland and north-westwards at Uyea. The offshore till distributions confirm the presence of a Shetland ice cap of probable Late Weichselian age and suggest that it had limited extent to the north. It would appear that the ice occurred as discrete lobes to the north of Shetland. The previous expectation that the ice front lay farther to the north of Shetland than suggested here (Boulton et al. 1977) would not be consistent with the results

Scott. J. Geol. 21, (2), 222-224, 1985 Downloaded from http://sjg.lyellcollection.org/ by guest on September 25, 2021

LETTERS TO THE EDITORS 223

FIG. 1. Distribution of offshore till material as deduced from BGS surveys. Sea bed sample and shallow core positions and geophysics traverse lines are not shown. Numbered locations: 1. Uyea, 2. Point of Fethaland, 3. , 4. Yell Sound, 5. Bluemull Sound.

indicated by the BGS sampling. Flinn's observations also agree with BGS results obtained from other areas of the northern North Sea, where the present 1:250,000 mapping programme has identified localised, late Weichselian till or till-like deposits. These results suggest that, at least at the late Weichselian maximum, the extent of glacier and ice cap areas was confined to Shetland and and eastwards from the Norwegian Channel. The abundance of periglacial features on Ronas Hill (Mykura 1976; Ball and Goodier 1974) may also be explained by the closeness of this eminence to the Downloaded from http://sjg.lyellcollection.org/ by guest on September 25, 2021

224 LETTERS TO THE EDITORS postulated ice front. A thin ice cover over the islands would not have submerged the hill, leaving it exposed as a nunatak, causing the formation of felsenmeer, although this may be more recent in age than the late Weichselian maximum (Mykura, 1976). Patterned ground has also been described from areas of Fetlar and Unst (Ball and Goodier, 1974) at lower altitudes than on Ronas Hill, but no record can be found relating to such periglacial features on Saxa Vord or Hermaness Hill, on the north coast of Unst. These areas would also have been ice-free at the time of the late Weichselian maximum. This letter is published with permission of the Director, British Geological Survey.

REFERENCES

BALL, D. F. and GOODIER, R. 1974. Ronas Hill, Shetland: A preliminary account of its ground pattern features resulting from the action of frost and wind. In Goodier, R. (ed) The Natural Environment of Shetland. Proceedings of the Nature Conservancy Symposium, 29-30 January 1974, 89-106. The Nature Conservancy Council, Edinburgh. BOULTON, G. S., JONES, A. S., CLAYTON, K. M. and KENNING, M. J. 1977. A British ice-sheet model and patterns of glacial erosion and deposition in Britain. In Shotton, F. W. (ed), British Quaternary Studies, recent advances, 231-46, Oxford University Press. CHESHER, J. A. 1984. Shetland 60°N 02°W, Sea Bed Sediments and Quaternary Geology, 1:250,000 map series, British Geological Survey. FLINN, D. 1983. Glacial meltwater channels in the northern isles of Shetland. Scot. J. Geol. 19, 311-20. MYKURA, W. 1976. British Regional Geology: Orkney and Shetland, HMSO.

British Geological Survey D. LONG Marine Geology Research Programme A. C. SKINNER Murchison House West Mains Road Edinburgh EH9 3LA

MS. accepted for publication 10th October 1984