August 2010

Bretignolles-sur-Mer is but one of the many seaside resorts along the
Atlantic Coast of France, but for those interested in unusual sands, it’s a
special destination: this is one of my favourites and a classic amongst
arenophiles. As you walk on to theshore, you can’t help but notice the strange
purple-red-rust-black sand piled up at the top of the beach, against the
concrete wall. In fact, you can see this band of colour on Google Earth.

Look more closely, and you’ll see how this sand contrasts with the more
typically coloured grains of the rest of the beach, and how the two conspire to
create natural designs sculpted by waves and the receding tide.

Pick up a handful of this sand and you’ll be surprised – it’s distinctly
heavier than you would expect, and there’s a reason for this. It’s a
placer deposit, a concentration of heavier minerals winnowed by storms
and dumped at the top of the beach. I’ve written
about placers before, particularly in the context of the California Gold
Rush; natural processes of granular segregation by waves, tides, and rivers,
take heavier mineral grains originally dispersed through the sand and
concentrate them – placer deposits are commercially significant around the world
for a wide variety of industrial minerals and precious stones, not just gold.

This concentration is clear in the image at the head of this post – whereas
the normal beach sand at Bretignolles is dominated, as is so often the case, by
clear quartz grains, there are few of these to be seen in the placer sand.
Instead, we see a dazzling variety of red, orange, pink, and black grains. So
what are these minerals?

Any beach sand represents a recipe of local ingredients: sand formed from the
weathering and erosion of rocks exposed along the coast, mixed with the
generally even greater volumes of material sampled from the geology of the
interior by rivers, flushed down to the coast and then relentlessly distributed
by waves and currents along the shore. Bretignolles, although culturally lying
in the Department of the Vendée, samples its sand grains from the southernmost
outcrops of Brittany. And Brittany has a long and torrid geological story to
tell. Without worrying about the details, a glance at the geological map of this
part of France reveals the complexity of this recipe:

It’s a tangled fabric of colours, each one a different rock type, apparently
stirred together and then shot through with faults on all scales. This is the
Armorican Massif, named after the old area of Gaul, its geological – and
cultural and linguistic - history linked to parts of Cornwall. The tangled
fabric is testament to not just one drama of colliding tectonic plates and
mountain-building, but two: first, the Cadomian Orogeny (mountain-building
event) around 600 million years ago, then the Hercynian or Variscan orogeny.
This series of chaotic and complex tectonic events lasted more than 100 million
years, creating the Ouachitas and the Appalachians together with much of old
Europe, and ended up welding the supercontinent of Pangaea together 280 million
years ago.

What we see today in the geology of Brittany are the roots of these old
mountain belts – the younger and shallower rocks have been stripped off to
reveal the foundations forged in the extreme pressures and temperatures deep
in the crust. And it’s these baked and tortured, metamorphic, rocks that the
sands of Bretignolles come from.

The red and pinkish colours are grains of garnet, a mineral name that covers
a wide variety of compositions – all silicate minerals, but with varying
proportions of calcium, magnesium, aluminium, iron, and chromium; those
proportions determine the colour. The black grains are iron oxides of various
types, among them magnetite (hence much of the heaviness of the sand).

There are also grains in this sand that I really can’t identify – metamorphic
rocks contain exotic and diverse ingredients. But the “treasure” can be seen in
the centre of the image above: bright, sparking blue, sapphire. This is the
aluminium oxide mineral, corundum, which also comes in a range of colours, and
thus some of these varieties are gemstones: if it’s red, it’s a ruby, blue is a
 sapphire.

Of course these sapphire grains (e.g., below, close-up and personal) are not things you would put in a ring that
would be appreciated as a gift, but I hope you would agree that they are
treasures – and beautiful examples of the stories that sand can tell.

[I was directed to Bretignolles by Carla, a Dutch sand collector to whom I am
grateful for this and many things.] SIGNATURE

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2010/08/index.html