November 2010

What a difference a few kilometres make. Local ingredients are, more often than not, the key to a beach’s composition and, as the geology changes along a coast, so does the sand. The images above are from two beaches around twenty-five kilometres apart on the west coast of Sardinia, and, given the varied geology that is packed into that relatively small island, it’s no surprise that they are so completely different. On the left is sand from the wonderfully named Putzu Idu, made almost entirely of biogenic and limestone fragments – shells, broken shells, sea-urchin bits and pieces, and whatever that is in the centre of the image. Yes, there are a few quartz and volcanic rock grains, but this is definitely a carbonate sand. Further up the coast, the beach at Porto Alabe is totally different. Glittering quartz grains, angular and young, volcanic rock fragments and shiny, polished, orange-brown limestone grains.

Look at the geology and it’s clear why these sands are so different. Porto Alabe is backed by hills of explosive volcanic rocks, formed as the western Mediterranean fractured and fragments, like Sardinia, of the old continents moved into their current positions. Putzu Idu is backed by younger sedimentary rocks, river and coastal deposits, and limestones, apparent in the photograph, some of them travertine, chemically formed from springs.

I wasn’t completely sure that the polished orange-brown grains from Porto Alabe were, indeed, limestone, and so, not having any hydrochloric acid to hand, I poured a little vinegar on them (appropriately, a chardonnay vinegar from Italy). The telltale bubbles soon appeared, some of course from the bits and pieces of shells, but others clearly originating from the grains in question. I’d never looked at such things down a microscope before – it’s really quite captivating. And yes, I have to get a life….

[Many thanks to my brother-in-law, Hans, for collecting these sands for me. Beach photos from Panoramio and Travel webshots.] SIGNATURE

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2010/11/index.html