Rhossili pebbles (1)

The pebbles on Rhossili beach in Gower are not only attractive objects worthy of attention and interest because of their wonderful variety of colours, shapes, patterns and textures – but also because they are important clues to the geology of the region. I have been fascinated by them for years but I am only slowly getting to understand what they represent. I’m actively challenged in trying to identify the different rock types and origins whilst still admiring and photographing them for their aesthetic qualities.

The pebbles in this post were all photographed on the strip of pebbles exposed at the top of the sandy beach on Rhossili Bay – at the foot of the Rhossili Down solifluction bench to the south, and at the base of the Llangennith dunes to the north.

Their grainy texture makes me think they are all composed of sandstone of some type (with the possible exception of Pebble 4 which is much finer grained like a limestone). The colour is grey or beige. They all have patterns of lines and concentric circles in a darker rusty colour which probably represents staining by iron. The pebble patterns indicate that the rock from which they are formed is made up of layers or strata (sedimentary rock). The way the broken pieces of rock have been rolled around the beach by the waves over thousands of years has worn and weathered them to a smooth, rounded shape at the same time as revealing the delightful patterns.

Working out exactly which bedrock series the pebbles came from is complicated on Rhossili beach. The pebbles may not only have originated in the immediately adjacent land forms – like the headlands of Burry Holms, Worm’s Head, and Rhossili cliffs which are made of Carboniferous limestone; and Rhossili Down which is made of Old Red Devonian sandstone – but also may have been brought around the coast from other parts of the Gower Peninsula where Millstone Grit or Coal Measures are the main bed rocks. In addition to this, ice sheets periodically covered much of Gower in the past (though not as far as Rhossili beach itself) and the ice brought with it rocks from far afield, which were deposited when the ice melted and the ice sheet retreated. So some stones on the beach may be completely foreign to the area and brought around the coast by the tides.

Furthermore, within each of the main locally occurring rock types, there is a series of sub-types which would have been laid down in differing environmental conditions. For example, The Old Red Sandstone in West Gower includes quartz conglomerates, brown-stones, and red marls – which vary enormously in composition and appearance. Millstone Grit apparently consists of a varied and variable group of sandstones, grits and shales from very coarse to very fine in texture.

My best guess, at the moment, is that the pebbles shown in this post are derived from the Millstone Grit sequence…but I am very open to correction if any reader could advise me.

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