4.2 The Devensian Lateglacial period in Argyll and Bute

The last British–Irish Ice Sheet covered all of Argyll and Bute (Bradwell et al. 2008a; Hubbard et al. 2009). Current climatic models provided by high–resolution analyses of annual layers of ice in Greenland's ice–sheet (Lowe et al. 2008; Rasmussen et al. 2008) describe initial warming of the North Atlantic region at c. 12700 cal BC. Much deglaciation occurred before this in a cold but dry climate. Chironomid assemblages suggest that mean July temperatures were lower than c. 7.5°C before the Windermere Interstadial, before c. 12350 cal BC and reached c. 12°C after this (Turney, Harkness and Lowe 2007): coleopteran data (Coope et al. 1998) suggest temperatures of c. 18°C in Windermere Interstadial summers.

With warming, soils began to support vegetation and to store organic matter. Biological productivity increased in lake basins. Devensian Lateglacial pollen records in Argyll and Bute are depicted in Figure 17. They uniformly record the colonisation of, first, grasses, sedges and herbs of bare– and disturbed–ground character. A closed vegetation cover may have formed but comparative aridity maintained populations of plants we think of as coastal like sea plantain. Heathland developed rich in crowberry, a heather adapted to dry conditions. Juniper and birch migrated but a closed woodland cover probably did not form.

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Figure 17: Map of Devensian Lateglacial and earliest Holocene pollen records © Richard Tipping: Barachander (Tipping 1989b); Drimnagall (Rymer 1977); Ford (Tipping 1989b); Inverliever (Tipping 1989b); Loch a' Bhogaidh (Edwards and Berridge 1994); Loch an' t–Suidhe (Lowe and Walker 1986); Loch Barnluasgan (Tipping 1989a); Loch Cill an Aonghais (Peglar in Birks 1980); Lon Glas (Tipping 1989b); Mishnish (Lowe and Walker 1986); Na Lona Min (Tipping 1986); Pulpit Hill (Tipping 1991).

The earliest shorelines to be formed during deglaciation are found at altitudes of around 40m OD (Ordnance Datum) (Sutherland 1997) because the weight of the last British–Irish Ice Sheet forced the British Isles below contemporary global sea level, which itself reached 120m below present. With deglaciation, the land rose faster than the sea itself rose, leaving elevated shorelines. A calibrated 14C assay on the abandonment by the sea of one coastal basin in Knapdale at 31.3m OD of 15860–14820 cal BC is, if correct, the earliest for deglaciation in the region (Shennan et al. 2006). At other localities in western Scotland, however, shorelines this high and higher are younger, c. 13000 cal BC (Shennan et al. 2006).

Temperatures declined throughout the Windermere Interstadial to around 11°C. Evidence from chironomids suggests three falls in mean July temperature of between 0.5 and 3 °C. Around 11950 cal BC mean July temperatures plummeted to below c. 7.5°C. Plant communities were destroyed. Periglacial conditions promoted heavy soil erosion and on steep slopes, talus or scree accumulated (Ballantyne and Kirkbride 1987; Dawson, Lowe and Walker 1987).

Sea level fell within the Windermere Interstadial. Some areas show the sequential fall over time, as on Oronsay (Jardine 1977), in Kilmartin (Gray and Sutherland 1977) and in the Mhoine Mhor (Peacock et al. 1977). Shorelines eroded by the sea become more coherent and continuous. Contemporary shorelines are seen to fall in altitude from north east to south west (see Figure 2.1: Sutherland 1997) because the land around Rannoch Moor rose faster than land on Kintyre. A prominent shoreline, the Main Rock Platform, was mapped by Gray (1974, 1975, 1978) and Dawson (1984) falling from around 12m OD in inner Loch Etive to 0m OD around Campbeltown. This shoreline is thought to have formed in the periglacial climate of the Loch Lomond Stadial (Gray 1978).

Marine sediments relating to the higher sea–levels of the Windermere Interstadial have been recorded in a number of places, including the central valley of Bute, beneath Campbeltown and from the head of the tidal Loch Gilp at Lochgilphead to the entrance to the lower Add valley and under the Crinan Canal, and at South Shian (Peacock 1983, 1989; Peacock et al. 1977). These are the Clyde Beds. They contain marine faunas that can define sea surface and deep water marine temperatures in the Devensian Lateglacial. Peacock (1983) estimated from modern analogues that in the early part of the Windermere Interstadial summer sea surface temperatures (SST) were close to present but some 3° C at 25m depth: winter SST was near freezing. Later, perhaps around 10800 cal BC, summer SST may have exceeded 13° C.

An ice sheet centred on Rannoch Moor re–formed towards the end of the Windermere Interstadial (Golledge 2006, 2010) or if like ice sheets further north, began to readvance (Bradwell et al. 2008b). Valley glaciers advanced along Loch Etive to the confluence with Loch Linnhe (Gray 1995), down Loch Awe as far as Ford (Gray and Sutherland 1977; Tipping 1989b) and to the head of Loch Fyne (Sutherland 1984). A separate ice sheet formed on Mull (Ballantyne 2002) and a rock glacier emerged on Jura (Dawson 1977). Loch Lomond Stadial ice rucked up marine shells with an age around 9700 cal BC at the west end of Loch Creran (Peacock et al. 1989) and incorporated organic mud just east of Loch Lomond at c. 10500 cal BC (Rose, Lowe and Switsur 1988). The climatic change to the Holocene epoch at around 9750 cal BC occurred in a matter of decades or a few years (Taylor et al. 1993; Alley 2000).