Loch Skeen, north-east of Moffat in the Southern Uplands, is an excellent place to appreciate a range of glacial landforms and the spectacular Grey Mare’s Tail waterfall
Moffatdale is a fine example of a deep glacial trough carved out along a fault running NE-SW by successive ice sheets during the Quaternary Ice Age (the last 2.6 million years). Glacial erosion exploited the weaker rocks along the fault, cutting down into the floor of the pre-existing valley and forming a series of truncated spurs along its flanks. Below Bell Craig on the southern flank of the trough, a prominent rock slope failure can be seen where the glacier has steepened the slope enough to destabilise it, leading to collapse of the valley side after the ice retreated.
On the north side of Moffatdale the tributary valley of the Tail Burn, which drains Loch Skeen, forms a hanging valley some 250m above Moffatdale. Hanging valleys occur where a main valley has been deepened by glacial erosion more than its tributary valleys. When the main glaciers melt the tributary valleys are left perched above the trunk valley and are often the locations of waterfalls. The Gray Mare’s Tail, with a main fall of around 60m, is one of the highest waterfalls in Scotland. It plunges into Moffatdale over a series of cascades and plunge pools, each controlled by variations in the resistance to erosion of the Silurian greywacke sandstone and mudstone bedrock. The Grey Mare’s Tail was an early tourist attraction, visited by Sir Walter Scott and celebrated in his poem, Marmion, as the “roaring linn”.
Corrie-like features (natural amphitheatre-like basins) can be seen at the head of Loch Skeen and north of White Coomb. During a brief and intense cold episode (the Loch Lomond Stadial – 12,900-11,700 years ago) at the end of the last glaciation, glaciers and ice caps built up once more in the Southern Uplands. A plateau icefield extended over the Tweedsmuir Hills north of Moffatdale, with tributary glaciers extending down into the main valleys. At the front and along the margins of Loch Skeen, above the waterfall, a superb suite of moraines defines the changing positions of the ice margin of one of these glaciers, each one a heap of gravel, sand and silt. They can be seen as parallel, heathery ridges and hummocks. The landforms include terminal, lateral and hummocky moraines and kettleholes (depressions left by a large block of melting ice), which reflect the advance and active recession of the glacier. A particularly prominent lateral/end moraine, known locally as ‘The Causey’, runs along the east side of Loch Skeen for approximately one kilometre.
The bedrock over which the Tail Burn falls is clearly layered, with ribs of hard “greywacke” sandstone separated by crevices where softer mudstone has been worn away. They belong to the Queensbury Formation of early Silurian age. Each greywacke-mudstone pair is formed from a large submarine avalanche of gravel, sand and mud that originated in shallow shelf seas and flowed rapidly down into a deep ocean trench, where the cloud of sediment settled on the ocean floor. The strata are now steeply inclined to the north, having been tilted up as they were scraped off and piled up against the trench wall by the subducting Iapetus ocean. A similar environment exists today at the Peru-Chile trench off the west coast of South America, where the Pacific tectonic plate is descending under the Andes. Slivers of black, fossil-rich mudstone – the Moffat Shale – cross the Tail Burn; each forms the base to a package of greywackes bounded by thrust faults above and below. Nearby to the east at Dob’s Linn, the Moffat Shale is rich in graptolite fossils, making it an internationally important site defining the boundary between the Ordovician and Silurian systems of rock strata.
Text contributed by John Gordon and Simon Cuthbert
Find out more
The site is owned by the National Trust for Scotland, and there is a well-made but steep path giving access to Loch Skeen from the NTS car park in Moffatdale. https://www.nts.org.uk/Visit/Grey-Mares-Tail/
Gordon, J.E. 1993. Loch Skeen. In: Gordon, J.E. & Sutherland, D.G. (Eds), Quaternary of Scotland. Geological Conservation Review, Series No. 6. Chapman & Hall, London, pp. 578-581.
Pearce, D., Rea, B.R., Bradwell, T. & McDougall, D. (2014) Glacial geomorphology of the Tweedsmuir Hills, Central Southern Uplands, Scotland. Journal of Maps, 10, 457-465.
McEwen, L.J. & Werritty, A. (1997). The Grey Mare’s Tail, Borders. In: Gregory, K.J. (Ed.), Fluvial Geomorphology of Great Britain. Geological Conservation Review, Series No. 6. Chapman & Hall, London, pp. 39-40.
Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. (2012). British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.
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