Friday, 16 October 2015

Gwithian Towans and some very ancient rocks

There are very few easy walks on the north coast of Cornwall, but one that does come into that category is the magnificent 3 mile stretch of sand between Godrevy and the mouth of the Hayle River. The wide sandy beach below Gwithian Towans is a haven for surfers and walkers, as are the Towans (dunes) above the beach.

Gwithian Towans and beach
Barbara and I made use of the autumn sunshine yesterday and took a 2-mile stroll along Gwithian sands from Godrevy, stopping to look at the amazing folds and faults in the rocks forming the cliffs below the dunes.

These are some of the oldest rocks in Britain, originally Devonian sediments laid down around 400 million years ago, detritus from the very ancient Calidonides mountain range which once stretched across the now border between England and Scotland. Later they were metamorphosed and tortured within the earths crust, but prior to that they lay as horizontal strata for around 150 million years. 150 million years, try to imagine that - well you can't of course, the human mind cannot get to grips with the immensity of even a million years. But around 150 million years ago the supercontinent of Pangea was beginning to break up, the Atlantic Ocean was beginning to form, as were the continents as we now know them.

But the history of these rocks takes us even further back into geological time, to the formation of Pangea itself in the late Carboniferous/early Permian period. Around 300 million years ago ancient continents were finally coming together into the single supercontinent and the unimaginable tectonic forces forged a new mountain range and partially melted the underlying mantle, which crystallised into igneous granite, which underlies all these rocks. These cliffs are the roots of the ancient Variscan mountain range, remnants manifesting themselves today as the Urals, the Pyrenees, and in SW England the high moors of Dartmoor and Bodmin.

The intense heat and pressure metamorphosed the sediments into a hard vitreous rock, known locally as killas, and as this cooled it cracked and the cracks were later filled with hot hydrothermal fluids, containing minerals, quartz in this area but a couple of miles inland the tin and copper minerals which would be mined millennia later.

Red River
A reminder of the proximity of the mines of Camborne-Redruth is the Red River which flows across the beach into the sea at Godrevy. Once the adits from the mines drained into this river, which also carried hematite-bearing tailings, from where the river gets its name. Now crystal clear I can remember seeing it in the mid 1970s, when it ran bright red from the tailings of South Crofty tin mine, as did the sea around Godrevy lighthouse. I regret that I did not take photos of this 40 years ago, but would be very interested to hear from anyone who did.

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  1. Old? I look out of my office window and can see rocks 10 times as old! And the Rocks at Gwithian aren't even half as old as those in Charnwood Forest, for instance.

    1. Not quite sure what your point is Unknown. Devonian sediments are pretty old by anyone's standards and yes, the igneous rocks of Charnwood Forest are older, being Precambrian. My fascination with the rocks in Cornwall is how these sediments have been metamorphosed and folded by the Variscan Oregeny

  2. In the 1970's I distinctly recall that South Crofty was not the only operation adding the red to The Red River; there was also Weir (Whear?) Development further up Chapel Hill, on the road from Tuckingmill to King Edward Mine (the site now occupied by Brea Granite & Slate Ltd). Weir Development used to process old mine dumps from around the area for tin (weren't they the ones who cleared the famous Wheal Gorland mine dumps around 1976) and like Crofty, discharged their tailings into the Red River. I still recall my surprise one Easter in the late 1970's at seeing the Red River flowing clear, with both operations being shut down for the holidays. Possibly one of the few times that this happened until the closure of South Crofty's mill around 1988.

    As an aside, I do wonder if this loss of tailings down the Red River has added to the loss of sand on the beaches around Hayle?

    Peter Walsh

    1. Yes I think you are right Pete, but the bulk of the discharge into the Red River was from Crofty

  3. This seems to be a global phenomena; I hope more blogs like this come out with site specific information.