Rhyolite

Rhyolite is a clever geological term for what you might otherwise call lava.  It’s a bit more technical of course in that it describes a sticky sort of lava with a high silica chemical composition close to that of granite.  In this case it is also bright pink and the shore at Sandway Point just North of Kingsand is a wide level platform of the stuff, whilst the beach is similarly bright pink sand.

I am here on a geology walk and we are looking at the rhyolite because it is, as far as anyone can find, the only ‘lava’ associated with the Dartmoor and Bodmin Moor granite masses.  The rhyolite is on close inspection for the most part very fine grained as would be expected from a liquid mass that cooled quickly, because, crystals take time to grow.  However, there are notable flakes of biotite, a black, glassy mineral, and those are probably good evidence for ‘fractional crystalisation’ in other words, the chemical composition of those crystals meant they formed at a higher temperature before the lava erupted and simply became frozen into the general mass.

Half the group has now gone in search of another feature (which turns out to be elusive) the remainder are chatting in the sunshine and I am going swimming.

There is still a brisk wind blowing in gusts down the Tamar across the water from the direction of Plymouth and it is chopping the surface of the water, though the chop is much less than earlier.  Otherwise the water looks clear and at first touch feels like 10degC.  Meanwhile the sun beams down from a blue sky flecked with occasional cotton wool clouds.

2016-Apr-23-Kingsand-01

Finally in water deep enough to sustain swimming I head out to the end of the point for a look back to Kingsand and Cawsand.  The two are extremely picture postcard with streets as narrow as footpaths and buildings that crowd out the light.  meanwhile there seems to be an abundance of both pubs and cafes, both out of reach for today.  Though they are not in Devon  I have elected to include them on my Wild Swimming Map for the simple reason that until 1840 they were in Devon and mostly what you can see from them is Devon.

The water is not deep and duck diving down I find the sea bed is fine, rippled sand, under no more than 10 feet at low tide as it is now.  Closer in to the shore though and there is a very sharp vertical edge to the rocks and where larger pieces have calved off thick, brown, multi-fingered kelp has got a hold, the fronds waft in the water and slither around my legs.

There are other sea weeds too: bladder wrack and bootlace weed are familiar local types but the invasive ‘Japanese wireweed’ is also here in quantity.  Back in my biology student field trip days in the mid-’80s this was unknown on the South Devon coast.  Whilst swimming I first began to see it in Torbay around the millennium but now it is just about everywhere.  On exposed coasts it struggles to grow, but in sheltered waters as here it forms thick, interlocked, tangled, worrisome masses.

I’ve been down as far as the sandy beach, about 200m, just bobbing about really, but now after 20 minutes in just shorts I am starting to feel the chill so it’s time to drift back in and sit in the sunshine to warm up.

 

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