Dr. Stephan Harrison, University of Exeter: ‘The Glaciation of Dartmoor’
The 2012 Dartmoor Society Research Lecture was held at the Dolphin Inn in Bovey Tracey on 9 November 2012. It was presented by Dr Stephan Harrison, University of Exeter (Cornwall campus), one of four co-authors of a paper entitled ‘The Glaciation of Dartmoor: The southernmost independent Pleistocene ice cap in the British Isles’ published earlier this year.1 74 people, Society members and non-members, were in attendance.
Dr Harrison began by summarising the subjects he intended to address: the limits of glacial ice in the UK and Ireland; past views on the glaciation of Dartmoor; new insights, including an introduction to plateau ice fields; evidence for glaciation on Dartmoor; and finally the results of independent numerical modelling.
The consensus in the professional community over at least the last 70 years has been that the highlands in southwest England were not glaciated during the Pleistocene, and that the southern edge of the British continental icecap was in the vicinity of Bristol. The Irish ice cap did extend down as far as the northern part of the Scilly Isles, however. Papers published as recently as 2011 and 2012 indicate that Southwest England was not glaciated.
Because the area was not covered by the main ice cap, however, does not mean that it didn’t have any ice cover. Dr Harrison noted a number of papers going back to 1865 that suggested something more than periglacial activity had occurred on Dartmoor. The most recent and most detailed of these papers, by R. Pickard, was published in 1943 and was based on his presidential address to the Devonshire Association.2 Pickard’s evidence included worn boulders, the terrain (e.g. roche moutonnée), gravels, the presence of moraines, grooved rocks and slabs. Pickard’s work has never been refuted, but this is most likely because the paper probably wasn’t peer reviewed, not because other researchers accepted his findings.
Dr Harrison’s interest in the glaciation of Dartmoor was piqued as a result of a holiday on Exmoor in the late 1990s during which he identified numerous glacial features in the landscape. He reckoned that if Exmoor, which is lower in elevation than Dartmoor, had glacial features, Dartmoor and Bodmin Moor certainly should have as well. He published his first paper on the glaciation of Dartmoor in 2001.3 He did say, however, that although he has spent much time looking for glacial features on Bodmin Moor, he has yet to find any. The recognition of plateau ice fields as a new form of glaciation and numerical models done by colleagues suggested that northern Dartmoor probably had been glaciated at least during the last glacial maximum.
Until recently, only two types of glaciation were generally known – the huge, thick continental ice sheets such as those covering most of Britain and Ireland, and the smaller valley glaciers that form on steep slopes. Plateau ice fields are cold based and thin, relatively speaking, and move very slowly over terrain with low slope angles, and, as a result, have little erosive energy. Because they are rather gentle features, there is often very little evidence that they existed – most commonly evidence is comprised of glacial features associated with the little fingers of ice that moved off the edge of the plateau, e.g., various types of moraines, outwash features. Modern analogues of plateau ice fields can be found in Norway and Greenland.
Dr Harrison cited evidence for plateau ice field glaciation in several areas on northern Dartmoor, including the areas around Sittaford Tor, Tavy Hole, Great Varracombe, and in the West Okement valley (near Slipper Stones).
Glacial features were mapped using aerial photography and then checked on the ground. The kinds of features cited as evidence include melt water channels, terminal and recessional moraines, the presence of glacial erratics and roche moutonnée, scoured bedrock, hummocky terrain in valley bottoms and other constructional landforms including kames and kame terraces, and highly compacted sediment profiles (diamicts) with oriented fabrics.
The distribution of tors can also be used as an indicator of the presence and extent of the ice fields. Three types of tors were identified: 1) Larger, castellated tors (e.g. Vixen Tor) located on narrow crests thought to be too fragile for ice cover to survive; 2) Smaller tors positioned on subsidiary summits around the edges of the north Dartmoor plateau; and 3) summits where the tors either never existed or were removed by glaciation located on the highest of the Dartmoor plateaus (e.g. Hangingstone Hill/Cut Hill). The assumption is that the ice fields would be located where there were no tors.
The final pieces of evidence described for the glaciation of Dartmoor result from independent numerical modelling done using standard glacial models. This work, done by Alan Hubbard and published in 2009, indicated that ice fields were far more extensive in the British Isles than previously thought, and that ice should have been present on northern Dartmoor, Bodmin Moor and Exmoor.4 Furthermore, the proposed distribution of ice fields on Dartmoor coincides with the distribution of tors as described above. In addition to models generated to show possible positions of the ice fields, others were generated for ice thickness and speed of movement. These indicated thin ice and very slow movement, both of which are characteristics of plateau ice fields.
Finally, Dr Harrison noted that, although they have no direct evidence of the age of the Dartmoor glaciation, it probably occurred in the late glacial period, about 25,000–26,000 years ago. The kinds of feature he has identified on Dartmoor are characteristic of that period, the Younger Dryas. Determining age will be the subject of future research
A number of questions were asked after the presentation, and Dr Greeves concluded the formal part of the evening by thanking Dr Harrison for this fascinating lecture. The evening ended with a light buffet.
- Evans, D.J.A., Harrison, S., Vieli, .A, and Anderson, E., 2012, The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap in the British Isles: Quaternary Science Reviews, vol 45, pp 31–53.
- Pickard, R. 1943, Glaciation on Dartmoor: Transactions of the Devonshire Association, vol 75, pp 25–52.
- Harrison, S., 2001, Speculations on the glaciation of Dartmoor: Quaternary Newsletter, vol 93, pp 15–26.
- Hubbard, A., Bradwell, T., Golledge, N., Hall, A., Patton, H., Sugden, D., Cooper R., and Stoker, M., 2009, Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British-Irish ice sheet: Quaternary Science Reviews, vol 28, pp 759–777.