The Limits of Glaciation: Was Dartmoor’s Landscape Formed by Ice?

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Distribution of tors on Dartmoor gives clues to the existence of an ice cap: Image by Nigel Cox

Have our previous estimates of glacier limits been wrong?

Generations of British geography students have drawn a line on the map between the rivers Severn and Thames to represent the southernmost limit of the last glacial ice sheet.

Now, a new study has produced evidence of glaciation further south, on the bleak Devon moorland of Dartmoor.

Dartmoor: Beyond the Ice?

In his book, The Hidden Landscape, geologist Richard Fortey distinguishes between the granite masses of northern Britain which “have an almost terrifying monumentality in the face of assault by ice and weather” and those of the south-west, which “lay beyond the margin of the massive ice advance … have had time to respond to the elements at comparative leisure.”

This is the conventional wisdom – that Dartmoor was shaped not by ice but by periglacial forces operating close to an ice mass. It is to these conditions, allied to earlier chemical weathering, that we attribute the formation of Dartmoor’s famous granite tors, the exposed rock molded into fantastic shapes.

Evidence for Glaciation of Dartmoor

The new research looks at several different lines of evidence for glaciation, not least the distribution of the tors themselves. These tend to occur on the edges of the moor: their absence at its center suggests that a small ice cap covered the higher central areas, either destroying existing tors or preventing their formation.

Using aerial photographs, the study looks in detail at landforms on the moor which, being poorly-developed and blurred by later erosion, haven’t previously been attributed to glacial action. Though both erosional and depositional features have been identified, it is the glacial moraines which, according to lead author David Evans, provide the most important evidence.

According to Professor Evans, the report’s findings are significant, demonstrating “that glacier ice existed in Britain much further south than previously thought and that this was on a landscape that was previously regarded as never touched by ice and therefore an iconic ancient weathered landscape.”

Redrawing the Map of the Ice Age

So, how have geomorphologists been mistaken for so long? Looking at the landscape, it makes perfect sense that a high, bleak plateau area just to the south of a major ice sheet should develop its own ice cap. According to Professor Evans, the failure to read the rocks is entirely understandable.

Could Dartmoor once have been covered by an ice cap? Image by Michael Haferkamp

Previous researchers had indeed missed the glacial features,” he told Decoded Science. “This is as much to do with having pre-conceived notions about the evolution of the Dartmoor landscape than anything else. So, very much a case of not seeing it because you don’t believe it, but also a case of no glacial specialists spending any time there because they thought there was nothing to interest them.”

Following on from this research, then, is it possible that other ice caps also formed beyond the main body of ice? It certainly seems possible. With other research showing signs of ice action on Exmoor, Professor Evans notes that “we fully expect the other upland areas to be potentially glaciated too.” So it seems that we’re all going to have to learn to redraw that map…

Sources

Dartmoor National Park Authority. The Formation of Tors on the Dartmoor Granite – an outline. Accessed June 14, 2012.

Evans, D.J.A., et al. The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap in the British Isles. (2012). Quaternary Science Reviews.

Fortey, R. The Hidden Landscape. (1993). Published by Pimlico.

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