Quaternary, Palaeogene volcanic districts of Scotland -introduction
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Since the start of the Quaternary Period, about 1.8 million years ago, the Highlands and Islands of western Scotland have undergone several periods of glaciation during the Pleistocene Epoch. Initially, only the mountain tops were covered by ice, but evidence from offshore deposits indicates that at times during the last 750 000 years, the middle latitudes of Europe supported large ice sheets, which extended across much of the Scottish mainland (Boulton et al., 2002). The ice sheets grew and decayed in a rhythmic manner: cold periods (glacials) when the ice sheets grew were separated by shorter, warmer intervals (interglacials) when conditions were comparable with those of the present day. There were at least five glacial/interglacial cycles, each of about 100 000 years duration. Within the glacials, colder periods are known as stadials, and short, warmer periods, but probably not as warm as the interglacial periods, are known as interstadials. The most recent intense and widespread glaciation occurred during the Dimlington Stadial (Table 17) when much of the evidence of earlier glaciations, and pre-Quaternary weathering, was removed by erosion. The Quaternary record in the Hebrides is therefore very incomplete prior to about 26 000 years BP (before present). Many of the ages quoted for this period, and indeed up to about 60 000 years BP are in radiocarbon years. It should be noted that these may be significantly less than true ages in calendar years (e.g. Walker and Lowe in Gordon, 1997).
The effects of the Quaternary ice ages are visible throughout the Inner Hebrides, the western Scottish seaboard and on Arran. Morainic deposits are widespread (P580488). Valleys, hills and rock surfaces commonly exhibit signs of glacial erosion and moulding (P521672; P580489), and on the coastline numerous raised beaches and marine-cut benches provide evidence of changes in sea level (P914160; Plate 42). The large erosional features such as corries began to form during earlier glaciations, but the glacial deposits preserved on and around the mountain massifs of Skye, Rum, Mull and Arran belong largely to the Main Late Devensian glaciation, and were deposited during the Dimlington Stadial and in a subsequent, less extensive glaciation during the Loch Lomond Stadial (Table 17). The mountains attracted high precipitation, and under the prevailing cold conditions this led to the formation of glaciers and local ice caps. During the Main Late Devensian glaciation these glaciers and ice caps coalesced into ice domes, which interacted with ice from much larger sheets on the Scottish mainland (P914157).
By about 10 000 years BP, the climate had become more temperate and the local glaciers and ice caps had melted. This change defines the start of the Holocene Epoch, formerly known as the Postglacial Period, and probably another (as yet incomplete) interglacial. Shortly after this time, Man reached the area. Some of the earliest records of human activity in Scotland, of Mesolithic age, come from Staffin, Skye, and from Rum where the local bloodstone (Chapter 6) was fashioned into artefacts at Kinloch. Radiocarbon determinations obtained from charcoal and from hazelnut shells from this site range from 8590 ± 95 years BP to 7570 ± 50 years BP (Wickham-Jones and Woodman, 1998).
The Quaternary geology of the Inner Hebrides and Arran has been intensively studied over many years. The excellent field guides published by the Quaternary Research Association detail these researches for specific areas and contain useful discussions of Pleistocene processes and descriptions of numerous key localities (Mull:Walker et al., 1985; Skye: Ballantyne et al., 1991; south-west Highlands including Mull:Walker et al., 1992).