The Loch Lomond Readvance on north Arran, Scotland: glacier reconstruction and palaeoclimatic implications COLIN K. BALLANTYNE * School of Geography and Geosciences, University of St Andrews, Fife, Scotland, UK Ballantyne, C. K. 2006. The Loch Lomond Readvance on north Arran, Scotland: glacier reconstruction and palaeoclimatic implications. J. Quaternary Sci., Vol. 22 pp. 343–359. ISSN 0267–8179. Received 24 February 2006; Revised 1 June 2006; Accepted 19 June 2006 ABSTRACT: Geomorphological mapping of northern Arran provides evidence for two advances of locally nourished glaciers, the younger being attributable to the Loch Lomond Stade (LLS) of ca. 12.9– 11.5 k yr BP, primarily through the mutually exclusive relationship between glacial limits and Lateglacial periglacial features. The age of the earlier advance is unknown. Inferred LLS glacier cover comprised two small icefields and eight small corrie or valley glaciers and totalled 11.1 km 2 . ELAs reconstructed using area–altitude balance ratio methods range from 268 m to 631 m for individual glaciers, with an area-weighted mean ELA of 371 m. ELAs of individual glaciers are strongly related to snow-contributing areas. The area-weighted mean ELA is consistent with a north– south decline in LLS ELAs along the west coast of Great Britain. This decline has an average latitudinal gradient of 70 m 100 km À1 , equivalent to a mean southwards ablation-season temperature increase of ca. 0.428C 100 km À1 . Mean June–August temperatures at the regional climatic ELA, estimated from chironomid assemblages in SE Scotland, lay between 5.7 Æ 0.18C and 4.1 Æ 0.28C. Empirical relationships between temperature and precipitation at modern glacier ELAs indicate equivalent mean annual precipitation at the ELA lay between 2002 Æ 490 mm and 2615 Æ 449 mm. These figures suggest that stadial precipitation on Arran fell within a range between þ8% and À33% of present mean annual precipitation. Copyright # 2006 John Wiley & Sons, Ltd. KEYWORDS: Loch Lomond Stade; equilibrium line altitudes; Scotland; palaeoclimate. Introduction The Isle of Arran (55825 0 –55843 0 N; 05804 0 –05825 0 W) is located in the Firth of Clyde in west-central Scotland (Fig. 1). The northern half of the island is underlain by a nearly circular granite outcrop, which occupies an area of ca. 145 km 2 and is flanked by Dalradian schists and Devonian sandstones (Tyrrell, 1928; MacDonald and Herriot, 1983). Much of the western and central part of the granite outcrop comprises fine-grained granite that underlies rounded hills, 350–711 m high, separated by broad cols and steep-sided glacial troughs. Surrounding the fine-grained granite core is a broad zone of more massive coarse-grained granite. In the west this underlies much of the Beinn Bharrain massif (721 m), a steep-sided ridge with deep corries on its western flank. The eastern zone of coarse-grained granite is much more rugged and alpine in character, with narrow are ˆtes, deeply indented by corries, joining seven summits over 700 m and culminating in Goatfell (874 m; Fig. 1). Striae around the coasts of Arran indicate that at the last (Late Devensian) glacial maximum (LGM), glacier ice moving southwards from the Scottish Highlands was diverted both westwards and eastwards around the island. Allochthonous erratics occur only at a few coastal locations, suggesting that at the LGM the mountains of north Arran nourished an independent ice dome from which ice flowed radially towards the coasts and fed ice southwards into the encircling Firth of Clyde ice stream (Tyrrell, 1928; Gemmell, 1971). Glacially- transported ‘perched’ granite boulders occur on several of the highest summits, implying total ice cover at the LGM, despite the presence of tors along some summit ridges. Moraines in the valleys and corries of northern Arran are mentioned in several early accounts (Ramsay, 1841; Bryce, 1859; Gunn, 1903; Scott, 1918) and have been interpreted as representing readvance of glacier ice subsequent to the LGM (J. Geikie, 1894; A. Geikie, 1901; Gregory, 1926; Tyrrell, 1928). Charlesworth (1955) depicted an extensive Lateglacial local glaciation centred on the granite hills of northern Arran, but his account appears to be entirely speculative and is largely unsupported by field evidence. A more thorough survey of local glaciation was carried out by Gemmell (1971, 1973), who attempted to relate post-LGM retreat stages and readvances to the distribution and altitude of JOURNAL OF QUATERNARY SCIENCE (2007) 22(4) 343–359 Copyright ß 2006 John Wiley & Sons, Ltd. Published online 21 November 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jqs.1059 * Correspondence to: C. K. Ballantyne, School of Geography and Geosciences, University of St Andrews, Fife KY16 9AL, Scotland. E-mail: ckb@st-and.ac.uk Contract/grant sponsor: The Carnegie Trust for the Universities of Scotland.