Cobbles - 1

The image of perfect beaches is of sand and shallow, turquoise water - like those at Sanna. Yet beaches like the one pictured, which runs along part of the Ormsaigbeg shore, have a different fascination: their pebbles. Strictly, most of these aren't pebbles but cobbles, which are defined as between 64 millimetres (2.5 in) and 256 millimetres (10.1 in) in length.

The joy of our local cobbles is that they are of such variety. Most were brought here around 10,000 years ago at the height of the last glaciation, and dumped, along with sand, silt and clay, to form the local soils. The sea rose as the ice melted, and eroded them out of the soil to build a beach.

Some of these cobbles have come from miles away, carried in on the river of ice. This is a granite, possibly the Strontian granite. It is formed of two main, pale-coloured minerals, felspar and quartz, and some dark minerals like biotite mica and hornblende.

This is another granite, but pinker. The colour comes from variations in the felspar, pink felspar often being the variety orthoclase. This cobble may, again, be derived from the Strontian granite but, since the glaciers eroded miles back into the Scottish highlands, it may be from much further afield.

'Granite' is very much a generic name, variations in the mineral content producing a spectrum of granite types. If plagioclase felspar is dominant over orthoclase, the granite may become a granodiorite.

This cobble is a 'granite', but the dark patches tell something of its history. Granites form from magma which wells up from deep within the crust, and then cools underground. As a result, the melt cools slowly, producing large crystals. But this granite, as it rose in the crust, picked up pieces of the surrounding rock, partially absorbing them. The dark patches are known as 'xenoliths', foreign rocks.