Friday, April 26, 2013

Dartmoor Tors


The high ground of England's southwest peninsula, from Devon, through Cornwall down to the Scilly Isles, is dominated by a huge subterranean geological structure (a batholith) that outcrops as a series of moorlands. Dartmoor, Bodmin moor and the moors of West Penwith are all underlain by connecting granite, outcropping as "tors" at the crest of each boggy hill. The granite has sat here for 300 million years or more, formed during a great mountain-building event: the "Variscan" (or Hercynian/Amorican) orogeny during the Devonian and through the late Carboniferous. Similar mountain-building processes that had formed the Appalachians in eastern USA formed the granites here in Devon and Cornwall, the result of the collision of the ancient continents of Gondwana (to the south) with Euramerica/Laurussia to the north. These collisions resulted in a single supercontinent, Pangea, which spanned from pole to pole.

In their more recent geological history, a mere 50 million years ago or so, the southwest granites were weathered. Their large feldspar crystals weathered down to clays, and china clay pits are worked today in Cornwall and Devon, extracting kaolin and other clays from the rotted granite. Weathering removed rocks above, and decompression and cooling has resulted in a pattern of joints or cracks in the granite tors, that are exploited in freeze-thaw weathering today to produce a characteristic pattern of outcrops.

Dartmoor was home to humans through the late Neolithic and early Bronze ages, as evidenced by the ubiquitous stone circles, rows of standing stones ("menhirs"), and ancient hut circles that can be found in the remote stretches of this National Park today. It is a fantastic space for recreation and exploration. But be sure to take a map and compass whenever you tread out onto the moors, for fear of being "pixie led". It is said evil spirits bring down the mists on the moors and lead the unwary astray. It is certainly true that the weather can close in, and each tor looks much like any other with rolling slopes, which makes navigation a challenge without technical assistance.


Image: Staple Tor, Dartmoor, courtesy of Matt Whorlow © www.mattwhorlowphotography.com

Links:

http://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Variscan-Orogeny

http://www.dartmoor-npa.gov.uk/lookingafter/laf-naturalenv/laf-ecologywildlife/laf-geology

http://www.legendarydartmoor.co.uk/piskie_led.htm

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