When the pebbles on this beach are dry, they have the nuanced pastel shades of expensive dragees, but when they are wet they assume the brighter hues of the cheaper varieties of sugared almonds. The pebbles have developed over thousands of years from the rocks behind the shore and underlying the sand. The two basic types of sedimentary rocks are Carboniferous Limestone (CL) and Old Red Sandstone (ORS) which have markedly different compositions. The layers which comprise these rocks are not uniform. Sequenced strata have different characteristics and have their own names, colours and textures.

The Devonian Old Red Sandstone is older than the Carboniferous Limestone. The Lower Old Red Limestone is composed of undivided red mudstone. Brownstones were laid down on top of the LORS, with brown sandstone, thin mudstones, and conglomerates. Above this the concluding deposits of Upper Old Red Sandstone contain undivided red and white quartz conglomerates and sandstones.

The series of younger Carboniferous Limestone rocks in this area, that were laid down above the ORS, are made up of the Lower Limestone Shales (Avon Group) of mudstones and limestones which can be a bit greenish, and the dark grey Black Rock Limestone Group.

The whole area at the tip of the Gower Peninsula has undergone many transformations over time with folding and faulting that has changed the orientation of the rocks several times so that they no longer lie in a series of beds lying horizontally with the oldest ones lower most and the youngest uppermost. As you walk from the south of Rhossili beach northwards to the midway point, you are walking back over in time. The sand beneath your feet conceals first the Black Rock Limestone, and Avon Group (formerly called the Lower Limestone Shales) from the Carboniferous Limestone; followed by the Upper Old Red Sandstone, ORS brownstones, and then the Lower ORS.  When you reach the halfway stage of the walk, about where the solifluction terrace at the foot of Rhossili Down is replaced by the dunes of Hillend Burrows, the geological sequence is reversed until you get to Burry Holms and Spaniard Rocks where it is Black Rock Limestone again.

The varied solid bedrock geology has given rise to the wide variety of pebbles. Much of the solid rock was fractured and broken into smaller pieces by weathering, especially repeated freezing and thawing during periods of glaciation. The pieces rolled down the slope from the hills towards the sea, forming at one time long ago an extensive platform of loosely consolidated material (a solifluction terrace) that extended far out into the bay as we know it today. Gradually the material has been eroded back by the sea to the remnant we see today at the foot of Rhossili Down where the Old Rectory stands. The angular pieces of rock have been worn smooth by rolling around with the tides and knocking against each other. There are also some pebbles of  other rock types that have come from further afield, either transported along the coast by sea, or overland by ice, adding to the mix.

7 Replies to “Dry Pebbles (RB08.18)”

  1. Thanks for the details. I enjoy poking through rocks on the beach and especially like finding stones with white stripes through them (though I’m sure that’s not the technical term for them). I appreciate your keen eye and extensive knowledge.

  2. A potpourri. I’m always fascinated by the folding of layers- we see them in cuts for highways- and how different the rocks can be from each other inside the same hillside.

  3. Thank you, Jean. I am pleased you enjoyed the post. I think the white lines in stones are actually veins or sheets of contrasting mineral material that seeped into the spaces opened up by fractures in the rock.

  4. Thank you, Claudia. Road cuts are a great way for amateur and professional geologists alike to get a good look at the succession of different layers in the rock. Each stratum representing a change in conditions in deep geological time.

  5. I am not certain about that. It may apply. Looking at an earth science dictionary, an intrusion is usually given to the emplacement of igneous (?molten lava) rock within pre-existing rocks. A vein can be like this. A vein is said to be a tabular deposit of minerals occupying a fracture, in which particles may grow away from the walls towards the centre – and it can result from magmatic activity (molten rock) or be a deposit from circulating groundwater. So it the white line in the pebble is quartz, then it might be the result of magmatic activity (an intrusion). If the white line is made of calcite, it could have precipitated from groundwater.

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