I have often wondered whether ideas about the non-glaciation of Dartmoor were too simplistic. I am not experienced in glacial geomorphology but I was a physical geographer who specialised in palaeoenvironmental change. Some of the valley morphologies and the valley fills always left a question mark in my head. Surely the proximity of the Dartmoor upland to an enormous ice sheet would have meant it experienced some glaciation? One only has to look at temperature maps of the UK to see how comparable Dartmoor temperatures are to northern and upland areas of the UK to appreciate that during glacials, the altitude of Dartmoor would have imapcted on ice cover.
k. image from https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-actual-and-anomaly-mapsHowever, look at any book that considers Dartmoor’s deeper past – its Quaternary history – and you will see a virtual consensus that Dartmoor is a periglacial landscape that has never been glaciated. Ideas on this may be changing. I am going to have a go at summarising recent thoughts on this by examining a paper by:
Perglaciation
Certainly, during previous glaciations, Dartmoor must have experienced considerable periglacial activity. Periglacial environments are those exemplified by extremely cold conditions, and subject to repeated freezing and thawing. This freeze-thaw action is responsible for a particular suite of processes and landforms, and occurs in zones adjacent to ice sheets. Whilst periglacial areas are extremely cold, significantly, they are just that little bit too warm to enable sustained annual ice accumulation. They therefore do not lead to the formation of glaciers and ice sheets.
Quaternary Glaciation
The Quaternary (2.6 Ma to present) is the geological period that we are in now. It is characterised by the onset of a climate regime of cyclical glaciations, interspersed with inter-glacials. We are currently in an interglacial that is known as the Holocene. In Britain, the last glacial is given the name the Devensian (115 Ka to 11.7 Ka). Devensian glaciation saw the greater proportion of the British Isles under an ice sheet that spanned, at its greatest extent (called variously the Late Glacial Maximum (LGM) or the Dimlington Stadial, 26 to 13 Ka), an area north of a line from the Isles of Scilly, along the coast of South Wales, and traversing the Midlands to the NE and over to the Wash (Chiverrell and Thomas, 2010). Earlier glaciations are thought to have extended even further south and may have clipped the top of North Devon at what is known at the middle Pleistocene maximum. And so, given this understanding of ice sheet extent in the British Isles, combined with a lack of any definitive glacial landforms on Dartmoor, it has been widely accepted that Dartmoor was not glaciated.
An icy question mark
But this story of British glaciation is devoid of nuance. It states that everything north of the ice sheet limit was glaciated, and everything south of it was not. This would be a reasonable assumption if we leave the role of elevation out of the equation. But of course, altitude has a marked effect on temperature as we saw above. Dartmoor is an upland area. Reaching elevations of up to 621m asl, it has average summer temperatures around 3°C lower than those at sea level. So, is it really reasonable to assume that Dartmoor, only about 50 miles south of the ice sheet, was un-glaciated at any point?
Modelling Evidence
Using a simple model that assumes a constant equilibrium-line altitude (ELA) at 550 m, Evans et al (2012) show how, within a period of 1400 years, a significant ice cap is able to form over northern Dartmoor. The ELA is the line that marks the boundary between ice accumulation above, and ice loss below a line of altitude. The authors use the evidence of the most obvious palaeo-glacier on Dartmoor, at the Slipper Stones in the West Okement valley, as the basis for determining this ELA. The modelling of this glaciation is shown in the figure below. It is a rather simplistic diagram that I have drawn, based on the maps in the paper. Please reader, take a look at the actual maps in the original paper. They are so much better than my scribble.
The paper only considers north Dartmoor and so makes no claims about glaciation on the southern plateau of the moor. In assuming a constant ELA neither does the paper claim to show how glaciation may have responded to fluctuating temperatures during the last glaciation. However, what it does show is how feasible it would have been, and over what time scales would have been required, to develop ice caps and glaciers. Given the Late Glacial Maximum lasted in the order of 13 thousand years, this modelling, from what I can see, is conservative and realistic for their chosen ELA.
Modelling shows initial ice accumulation between Great Links Tor and the West Okement valley; around the highest point of the moor emanating from High Willhays and Yes Tor; and the largest extent, between Fur Tor and extending NE around Hangingstone Hill. If ice accumulation is then enabled to build for 1400 years, these three zones of ice accumulation join to form a contiguous ice sheet extending approximately from Rough Tor (S) to Rowtor (N), and from Arms Tor (W) to Middle Tor (E). The data in the paper show a maximum ice accumulation of up to 750cm of ice, which by ice cap standards is quite thin. This has the consequence that ice movement is generally slow at less than 10 m/yr with the exceptions being in some of the radiating valleys, in particular the West Okement Valley and the Taw Valley.
Morphological Evidence
Are there any glacial geomorphological features left in the landscape to support this exploratory modelling? Evans et al (2012) cite a number of features. These are all subtle because of the marginal and shorter lived conditions under which they formed and this is one of the reasons why glaciation has been discounted for Dartmoor. But of course, at the fringe of the ice sheet, understated landforms are exactly what we should expect. Among the glacial features the Dartmoor explorer may want to look out for are the many valleys ringing the north moor massif that have a weakly U-shaped form; arcuate and linear boulder ridges and hummocky valley floor drift in the drainage basins. In places there are lateral-frontal moraine deposits in the vicinity of outlet lobes and some of these show recession sequences and meltwater channels. The paper goes into much more detail on the glacial morphological forms in various locations, supported by annotated photographs, so those who are interested can enjoy using the paper to try and locate these features in the field.
Impact of glaciation on tors
And what impact did this proposed glaciation have on tors? On the high plateau, under the ice sheet, and in the zones occupied by ice for the longest period of time, these areas are devoid of tors, scoured by the ice. In peripheral locations to the ice sheet, where ice build-up was not significant enough to generate erosion, or where summits stuck up like islands between outlet ice lobes in the valley heads, the tors we see today were able to persist. These tors have evolved into castellated forms as a consequence of prolonged periglacial weathering and mass wasting. Evans et al (2012) also note a further tor type that occurs at lower altitudes on subsidiary summits and around the margins which have a less castellated form.
A new way to view Dartmoor
For me, this paper chimed with my own views and understanding of palaeoclimate and the possibility of minor Dartmoor glaciation. I look forward to seeing further research into the potential glaciation of Dartmoor by different authors in order to build a critical body of evidence one way or another. When I walk the moor and use my knowledge to interpret what I am seeing, this paper adds another layer – a layer of ice – for me to enjoy the landscape.
References
Chiverrell R. C., Thomas G. S. P. 2010. Extent and timing of the Last Glacial Maximum (LGM) in Britain and Ireland: a review. J. Quaternary Sci., Vol. 25 pp. 535–549
Evans, D.J., 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, 45, pp.31-53.
Let’s hope you get your answer. Here on Gotland there are plenty of signs that we were under the ice sheet.
Nice writing!
Hi Lucinda. Thanks so much for looking at it. I really appreciate the interest. The hinterlands around ice sheets are much more difficult to interpret in terms of landscape features. Luckily my writing has enabled me to connect with a local geologist so I am hoping to be able to go out with her when all this lockdown is over and we can go on a geomorphological hunt! Hope you are all ok and managing as best you can with our changed lives.
I am not a professionally trained geologist and so have an amateur opinion. I was looking around Sandy Hole Pass on the E Dart river. I considered that if there was a ice dam around there, it would be impossible to differentiate it from medieval mining activities with the creation of “lakes” and then downstream flooding as the dam was broken.
I consider it likely that N Dartmoor was glaciated but at least some of the evidence for it may have been eradicated by miners.
That’s an interesting perspective David. I often think about medieval mining destroying evidence of prehistoric and early historic mining, but hadn’t thought about it destroying evidence of valley bottom palaeoenvironmental evidence. It would be fascinating to see the valley bottoms before the ramping up of human impacts in the last couple of millennia.