Geological Pilgrimage: The Tors of Dartmoor

First climb the slope of Fox Tor to the tomb of poor Childe the Hunter, who died on that very spot in a Dartmoor blizzard.  From there, do keep close track of your way through the tors, for Vixana waits to conjure up her mists to confuse you.  If you aren’t quick to flee, you will find yourself in her boggy lair, and when all is done you will be turned to stone, like Bowerman the hunter who looks out over Dartmoor to this day.  His loyal stone hounds stand watch nearby.

Left:  Bowerman's Nose, Dartmoor.  Courtesy Don Coldwell via Wikimedia Commons.  Below, Bowerman's hounds nearby at the Hound Tor.  Courtesy PhillipC by way of Wikimedia Commons.

I am fascinated by tors ... by their fantasical shapes and mysterious origins.  I spent a lot of time reading and thinking about tors last winter, when I had it in mind to write a post about these “creatures of stone”.  That post had turned into three by the time I was done.  The story of tors is neither simple nor clear, and I find them even more intriguing now.

My neighbors, the tors at Blair in southeast Wyoming.

When Denise Tang at life as a geologist announced the topic for Accretionary Wedge #45 would be Geological Pilgrimage, Dartmoor in the South West of England was the first destination that came to mind.  I considered others, but Dartmoor kept popping into my head.  It was there that some of the earliest studies of tors were done, and Dartmoor remains an active area of research in geomorphology.

Dartmoor is underlain almost entirely by granite -- the largest area of exposed granite in southern Britain.  Dartmoor granite was intruded into sedimentary rocks and lavas during late Carboniferous (Pennsylvanian) - early Permian time, ca 280 Ma.  It is one of several exposed domes making up a single large batholith.

Maps right and below from Geology and Landforms Fact Sheet from the Dartmoor National Park Authority.

Three major types of Dartmoor granite have been recognized, including one associated specifically with tors -- the tor granite, which contains large megacrysts.  Like many tors throughout the world, Dartmoor tors are thought to have developed via a two-stage process:  underground weathering followed erosion of the overlying and surrounding megolith (diagram below, click to view).  However, the contribution of underground weathering vs. post-exposure above-ground mechanisms such as freeze-thaw continues to be hotly debated.

Below:  a Dartmoor tor with clitter (fallen blocks) on slopes below.   From "A Book of Dartmoor" by Sabine Baring-Gould, 1900 (public domain).

Much of Dartmore has protected status as part of Dartmore National Park, covering 954 sq km (368 sq mi).  The National Park Authority has put together a Geology and Landforms Factsheet (2005), a good overview of the geology of the area.

And beware the Devil, he too haunts the tors!  Once when my master and I were returning home we came upon a stranger who generously offered us bread and cheese.  But just as my master was about to break the bread, I saw a cloven hoof peeking beneath the long coat of the stranger.  I knocked the food from my master’s hands and it immediately turned to stone.  We quickly overcame our shock, and ran for home.

Branscombe's Loaf of Dartmoor; lithified bread and cheese dropped by Walter Branscombe, the Bishop of Exeter.   © Copyright Richard Knights; licensed for reuse under Creative Commons Licence