5.3.2.1 The environmental context

The last British-Irish Ice Sheet covered all of Argyll and Bute (Bradwell et al. 2008a; Hubbard et al. 2009), with the possible exception of the Rinns of Islay (Dawson and Dawson 2000) (Figure 34). 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, 2014) describe initial warming of the North Atlantic region at c. 14,650 BP. Much de-glaciation 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 Late Glacial Interstadial (GI-1), before c. 14,300 and reached c. 12ºC after this (Turney, Harkness and Lowe 1997); coleopteran data (Coope et al. 1998) suggest temperatures of up to 18°C in GI-1 summers.

Figure 34: Ice extent and sea level at the Last Glacial Maximum (GS-3) and the Loch Lomond Stadial (GS-1) in NW Europe. Limits of ice sheets and Lateglacial sea levels re-drawn from Ballantyne (2004, his Figure 2.1), Bjartmann Bjerck (2008, his Figure 3.1) © copyright

With warming, soils began to support vegetation and to store organic matter. Biological productivity increased in lake basins. Late glacial pollen records in Argyll and Bute 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.

The earliest shorelines to be formed during de-glaciation are found at altitudes of around 40m above mean sea level (Sutherland 1997) because the weight of the last British-Irish Ice Sheet forced the British Isles below contemporary global sea level, which itself reached up to 120m below present. With de-glaciation, 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 above msl of 17,810-16,770 BP is, if correct, the earliest for de-glaciation in the region (Shennan et al. 2006). At other localities in western Scotland, however, shorelines this high and higher are younger, c. 15,000 BP (Shennan et al. 2006).

Temperatures declined during the latter stages of the Late Glacial Interstadial (GI-1a-c/Allerød) to around 11°C. Evidence from chironomids suggests three falls in mean July temperature of between 0.5 and 3°C. Around 13,900 BP 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 Late Glacial 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 northeast to southwest (see Figure 2.1: Sutherland 1997) because the land around Rannoch Moor rose faster than land on Kintyre. Gray (1974, 1975, 1978) and Dawson (1984) mapped a prominent shoreline, referred to as the Main Rock Platform, falling from around 12m above Mean Sea Level (MSL) in inner Loch Etive to 0m above MSL around Campbeltown. This shoreline is thought to have formed in the periglacial climate of the GS-1 cold stage (equivalent to the European Younger Dryas), referred to in Britain as the Loch Lomond Stadial (Gray 1978).

Marine sediments relating to the higher sea-levels of the Late Glacial 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 of the Lateglacial. Peacock (1983) estimated from modern analogues that in the early part of GI-1 summer sea surface temperatures (SST) were close to present but some 3°C at 25m depth: winter SST was near freezing. Later, perhaps around 12,750 BP, summer SST may have exceeded 13°C. An ice sheet centred on Rannoch Moor re-formed towards the end of GI-1 (Golledge, 2006, 2010) or if like ice sheets further north, began to re-advance (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 1989 b) 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). Younger Dryas (GS-1) ice incorporating organic mud formed to the east of Loch Lomond at c. 12,450 BP (Rose, Lowe and Switsur 1988).

The climatic change that denotes the start of the Holocene at around 11,700 BP occurred in a matter of decades or even a few years (Taylor et al. 1993; Alley 2000; Mayewski et al. 2004; Tipping et al. 2013). Abrupt climatic fluctuations then continued during the Early Holocene creating short ‘moments of crisis’ separated by longer periods of relative stability. Such events include short-lived deteriorations in climate such as the Pre-Boreal oscillation at c. 11,250 BP (Björck et al. 1997) and the 9.2 ka event at c. 9250 (Fleitmann et al. 2008). During the Late Glacial the sea level is modelled to have fallen to -5m below present on Skye, to around 0m on Arisaig but to have remained above present tidal limits, around 2m above present in Knapdale (Shennan et al. 2006), although empirical evidence to substantiate this model is currently lacking. On Islay, Dawson, Dawson and Edwards (1998) date the lowest early Holocene RSL at around 0m above msl to c. 11,300 BP, as Dawson et al. (2001) do on northern Coll. Dawson et al. (1998) then describe a rapid RSL rise on Islay to 3m above msl by c. 9,910 BP, slowing to c. 5m above msl by the Neolithic period. On Oronsay, Jardine (1977) thought sea level reached c. 7m above msl before this time. Sequences on the Scottish west coast currently lack the chronological precision to test the suggestion (Clarke et al. 2003) that the collapse of the Laurentide ice sheet caused a nearly instantaneous global RSL rise of around 1.4m.