Scenic Saturday: a pilgrimage back to the grand granitic tors of Dartmoor

A post by Chris RowanThe high and rugged scenery of Dartmoor is as wild and untamed a landscape as you’re likely to find in the United Kingdom, and would seem to have more in common with the Scottish Highlands than the prim and proper south of England. Yet not an hour’s drive from the crowded beaches of the ‘English Riviera’*, you can find yourself in a stark, windswept landscape dominated by the tors: granite-topped hills that could easily be mistaken for ruined castles, but are instead the product of millions of years of weathering.

Hound Tor, Dartmoor. Click to enlarge. Photo: Chris Rowan, 2011.

Around 300 million years ago, the Variscan orogeny, the collision between Gondwana (Africa and South America) and Laurussia (North America, Northwest Europe and the UK) that marked the final assembly of the supercontinent Pangea, produced the wickedly folded and metamorphosed rocks that can be found on the coasts of Cornwall, Devon, and South Wales, and thickened the crust enough to cause the lower reaches to melt. This produced a large granite intrusion – a batholith – that runs down the buried spine of Devon and Cornwall all the way down to Lands End. In various places, it pokes above the surface, the largest of these outcrops being Dartmoor.

Geological Map of SW England

Geological Map of SW England. Granites are garish pink. Source: BGS (click image to go to their online map viewer).

I first visited Dartmoor when I was around 6 or 7, and I remember being fascinated by the tors, which were looming and mysterious and great for causing my mother minor heart attacks as I clambered around them in a death-defying manner. As someone who grew up on the flat coastal plains of East Anglia, this was probably one of the first times that I realised that landscapes could be interesting. Thus was born a life-long love affair with pointy places and the interesting rocks you find there. Last summer, 25 years or so later, it was great fun to revisit the tors and look at them as the geologist that they helped inspire me to eventually become. And find some rather impressively large feldspars whilst I was at it.

Larger versions of all these images can be seen by clicking on them.

Hound Tor, Dartmoor

Photo: Chris Rowan, 2011.

Hound Tor, Dartmoor

Photo: Chris Rowan, 2011.

Hound Tor, Dartmoor

Photo: Chris Rowan, 2011.

Hound Tor, Dartmoor

Photo: Chris Rowan, 2011.

Plagioclase in granite, Dartmoor

A large plagioclase phenocryst visible in the weathered and lichen encrusted granite of Dartmoor. Photo: Chris Rowan, 2011.

*Not an ironic designation, as far as I can tell. Because hey, it doesn’t rain all the time…

Categories: geomorphology, outcrops, Palaeozoic, photos, rocks & minerals
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Comments (6)

  1. Bruce Stout says:

    Nice write-up and pics. I went to Dartmoor a couple of years ago. Fantastic wind-swept place.
    I am intrigued about batholiths though and their relationship to subduction-related volcanism, or more precisely, caldera-forming volcanism like in the Altiplano, the Taupo Volcanic Zone, Toba, etc.
    Generally these places tend to be extensional regimes and thinning crust, yet you mention crustal thickening here as a source of deep crustal melt. Obviously there is more to batholiths than meets the eye (well, doh).
    Can we assume that these regions of caldera formation are also associated with nascent batholiths at the base of the crust?

    • Bruce Stout says:

      err.. let me take back that bit about thinning crust… isn’t the crust under the altiplano in the region of 70 km thick? that said isn’t there some kind of back-arc extension going on at the same time?

    • Chris Rowan says:

      Extension in the Altiplano is probably driven by gravitational collapse of the orogen – the mountain range/plateau can’t grow any higher without collapsing under its own weight, so it grows outwards.

      Granites only occur where you get substantial melting of continental crust.

  2. Michael says:

    To Chris Rowan,
    It’s very good of you to post this interesting parent article about the Dartmoor tors because it is actually very related to Anne Jefferson’s posting of her parent article about the Driftless Area. There, I introduced my paper, Iannicelli (2010) which is about the origin of the Driftless Area and in this paper, I discuss how the origin of tors is directly related to terraces of cryoplanation & cryopedimentation. The unit “summit backwall & terrace” or “tread & riser” then shrinks in time from snowmelt erosion and solifluction that eventually evolves the “riser” into a cryoplanation “tump” (see Iannicelli, 2010). The tump then eventually evolves into the unit known as the “tor” or “tors”. When this finally breaks down from weathering & erosion, then the ultimate old age landscape of a summit cryoplanation plain is formed.

    Reference:
    Iannicelli, MIchael (2010). Evolution of the Driftless Area and continguous regions of midwestern USA through Pleistocene periglacial processes. The Open Geology Journal, vol. 4, pp. 35 – 54. (Note this can be viewed, downloaded and printed for free from the website of The Open Geology Journal which is a peer-reviewed geology journal.)

    • Chris Rowan says:

      Wow – an interesting connection, thanks for bringing it to our attention! I’ve added a link to your paper for those interested.

      Although things might be complicated by some just-published research (press release, paper) that challenges the long-held view that the erosion of Dartmoor was nothing to do with glacial processes. Dartmoor was beyond the southern limit of the main British ice sheet during glacial periods, but it appears it may have had its own small ice cap.

      • Michael says:

        Hi Chris,
        I’m sure that these other landforms are not moraines & drumlins but instead are paha landforms. Iowa had the same problem at one time with what they thought was a separate “ Iowan Drift “ because they saw that many parts of the pre-Illinoian till had hills that looked like fresh drumlins & moraines, so that they called these paha. They believed they were looking at constructional landforms but in reality, they were only looking at erosional landforms. For instance, a series of transverse snow dunes can create a pattern of linear depressions & snowmelt interfluves (Iannicelli, 2000; Iannicelli, 2003; Iannicelli, 2010). This pattern does resemble a bunch of parallel moraines but in order to differentiate, we can use the wind-direction, so that the landforms lie only transversly or perpendicularly to the wind direction.
        Snow dune erosion is even evident in the Dartmoor area. For instance, Gerrard (1988) and Ballantyne and Harris (1994) illustrated a series of curving cryoplanation terraces carved into a hillside. Iannicelli (2000) emphasized that these curving landforms were formed by barchanoid-ridge snow dunes. So, snow dunes can be categorized as sand dunes (domal; transverse; barchanoid-ridge; barchan) (Iannicelli, 2000). I guarantee that the future of geomorphology will emphasize that the nival world always carved its own regime of landforms ahead of the continental ice sheet. Perhaps I will also contemplate writing a paper in the future showing that all of the mesas of the American southwest are in actuality, periglacial inselbergs like the “crown” plateaus in the Driftless Area.
        References:
        Ballantyne, CK and Harris, C (1988). The Periglaciation of Great Britain. Cambridge University Press, UK, 330 pp.
        Gerrard, AJ (1988). Periglacial modification of the Cox Tor-Staples Tors area of western Dartmoor, England. Physical Geography, vol. 9, pp. 280 – 300.
        Iannicelli, M. (2000). Snow dune erosion and landforms. Northeastern Geology and Environmental Sciences, vol. 22, pp. 324 – 335.
        Iannicelli, M. (2003). Devon Island’s oriented landforms as an analogy to Illinois-type paha. Polar Geography, vol. 27, pp. 339 -350.
        Iannicelli, M. (2010). Evolution of the Driftless Area and contiguous regions of Midwestern USA through Pleistocene periglacial processes. The Open Geology Journal. Vol. 4, pp. 35 – 54.