The stony west bank of Pennard Pill

Sea Lavender on lichen covered stones of the river bank at Three Cliffs Bay Just before the Pennard Pill watercourse takes a dramatic swing to loop around the giant sand dune to get to the shore at Threecliff (Three Cliffs) Bay on the Gower Peninsula, the right hand or west bank is composed of rough stones and then transitions into a salt marsh. [The area lies on the opposite side of the river to a more substantial and higher shingle bank that can be reached via a set of concrete stepping stones].

The low shingle surface of the right bank is relatively stable. Despite regular tidal inundations of brackish water, life clings to the limestone. Last August it was particularly attractive, covered with bright patches of yellow and black lichens, and ground-hugging clumps of partially red-stemmed plants with clusters of small pink flowers. I will have to find out what these plants are the next time I visit. I didn’t take close-ups. I had thought they might be Sea Heath (Frankenia laevis) but apparently that does not grow in this area – although it likes the same kind of habitat.  I think Sea Sandwort was also present. However, the numerous flowering stems of Sea Lavender I did recognise; and these plants were found equally spread in stony ground and on the wetter salt marsh area.

Pebbles at Albecq

Colourful and patterned pebbles of igneous rocks

The colours and patterns of the pebbles on the beach at Albecq on the north coast of Guernsey in the Channel Islands reflect the varied geology of the north of the island. The northern part of Guernsey is made up of igneous rocks such as granite, diorite and gabbro and formed about 400 – 500 million years ago (de Pomerai & Robinson 1994 pp 5 & 7). Most of the pebbles in these photographs would have originated in nearby bedrock that would include Cobo Granite, Icart Gneiss, Perelle Gneiss Group, and Bordeaux Diorite Complex. These rocks are constituents of what is known as the Northern Igneous Complex. The patterns in the rocks are due to the coarse mineral crystal composition. Potassium-rich orthoclase feldspar is responsible for the recurring pink-orange colour, with light grey plagioclase feldspar; quartz grains are transparent; and biotite mica is seen as small black shiny crystals (de Pomerai & Robinson 1994 p 49).

REFERENCE

de Pomerai, M. and Robinson, A (1994) The Rocks and Scenery of Guernsey. La Societe Guernesiaise, St Peter Port, Guernsey. ISBN 0 9518075 2 8.

 

Beach Stones with Iron at Rhossili

A beach stone made of iron on the Gower Peninsula in South Wales

When you visit a place often enough, you think you have seen all there is to see. For all the photographs of rocks and pebbles that I have taken over the years, I don’t think I had ever noticed so many stones made of iron as I did on my last trip to Rhossili on the Gower Peninsula, South Wales. Mostly they were lying in the narrow strip of pebbles, cobbles, and boulders that is exposed on the upper reaches of the sandy shore, right at the base of Rhossili Down.

Some of these iron stones appear to be made entirely from iron (maybe iron carbonate) while others were sedimentary rock with a solid iron centre, or with groups or bands of small iron nodules. One stone was stratified with black and rust layers. All of them were very heavy and many exhibited concretionary layers. I know very little about these stones. Their forms of iron are distinctly different from the haematite form that I have previously shown in other blog posts about Gower geology, and from the iron pyrites nodules I have featured from Dorset’s Jurassic rocks. Those stones with groups or layers of smaller nodules look like the siderite form of iron that I photographed at Joggins in Nova Scotia. Iron is recorded in Lower Carboniferous rock strata and also in the Coal Measures higher up in the geological sequence, so iron stones on this Gower beach make general sense but they may have been transported there from another location. Clearly more investigation is required.

Pebbles at Whiteford (4)

Pebbles, shells, and a feather on the beach near Whiteford Point

A sand bar spreads southeast from Whiteford Point in Gower, South Wales. At low tide in the Burry Estuary, it is part of a very extensive sandy area over which cockle and mussel fisherman can traverse in vehicles from places further along the north Gower coast. The sand depth is variable and mostly envelops a spit of pebbles. Sometimes the pebbles are entirely hidden. Sometimes they are partially exposed. Intermixed with the pebbles are seashells – cockles, mussels, whelks, and oysters are the most commonly occurring. There is a wide range of colours and textures in the pebbles and they are particularly interesting because of the range of rock types they represent.

As you take a 360 degree scan of the horizon from this isolated expanse of sand and pebbles, there is not a single rocky outcrop in sight. So where have these beach stones come from? The collection includes sedimentary rocks from the locally occurring Carboniferous limestone and Devonian sandstones, mudstones, and conglomerates – like the bedrock exposed at Rhossili and at Broughton Bay. It also includes samples from higher up in the Carboniferous strata such as the Millstone Grits, sandstones, and shales, and Coal Measure layers. These strata underlie the Burry Estuary into which this spit extends, east Gower, and the Swansea district and way beyond. There are many rock types with which I am not familiar but I notice that some are metamorphic and igneous in nature. So how have all these rocks ended up on this spit, far from their place of origin?

Part of the answer is undoubtedly the effect of sea drift, currents, and storms carrying weathered and broken stones along the shores of Carmarthen Bay and into the estuary or inlet – but a significant proportion of the stones are thought to have been brought to the area from considerable distances away by glaciation, and deposited by the melting of an ice sheet, possibly in the late Devensian era about 24,000 years ago. Most of these stones lie hidden in a mass beneath the Whiteford Dunes but some are exposed high on the shore at the foot of the dunes, and beneath the disused iron lighthouse on Whiteford Point. Over time the waves have dislodged the often frost-shattered stones from the surface of the deposit, and washed them further along the beach around the Point to form pebble spits and banks, in the process smoothing and rounding them into the pebbles visible today.

Click here for more posts about Whiteford Sands, Whiteford Point, and Whiteford Burrows.

Pebbles at Manorbier

Brightly coloured pebbles on the beach at Manorbier in South Pembrokeshire, Wales, with patterns of reflected light under clear sparkling water in a gently flowing stream.

Coloured pebbles under water on a sunny day

Coloured pebbles under water on a sunny day

Coloured pebbles under water on a sunny day

Coloured pebbles under water on a sunny day

Coloured pebbles under water on a sunny day

Pebbles with Lichen at Chesil

Bright orange lichen encrusting a beach pebble in a sheltered zone

On the eastern flank of the great pebble bank of Chesil Beach in Dorset, on that part adjacent to the Fleet Lagoon where fishermen’s huts are strewn along the water’s edge, the pebbles were sufficiently undisturbed in the spring of 2011 for vegetation to get a grip. A few scattered plants had established themselves amongst the rounded stones; and many pebbles in the same area were coated with slow-growing black or bright orange lichens.

This type of colonisation of beach shingle is a fairly uncommon phenomenon because, in general, the constant movement and abrasion of the pebbles does not allow plants to establish a root system or lichens to encrust the surface of the pebbles themselves. However, in this particular location on the leeward sheltered side of the pebble bank, there had been a period of years of relative stability that enabled vegetation to start growing.

During the winter of 2013 to 2014 the Chesil Bank sustained enormous damage from the storms. The wave action resulted in massive movements of the pebbles. I haven’t revisited the site since the storms but I strongly suspect that the plants and lichens will have suffered and may no longer exist. I must go and check it out. Whatever the outcome of last winter’s weather and subsequent maintenance work on the pebble bank, the slow colonisation process will surely begin again but will take time to restore the habitat to its former situation.

Pebbles with patches of black lichen on the Chesil Bank in Dorset

Pebbles with patches of black lichen on the Chesil Bank in Dorset

Pebbles with patches of black lichen on the Chesil Bank in Dorset

Plant growing on pebbles at Chesil Beach

Plants and bright orange lichen growing on beach pebbles

Pebbles with patches of black lichen on the Chesil Bank in Dorset

Makeshift boardwalk up the Chesil Bank in Dorset

Groynes on Rosslare Strand

On Rosslare Strand in Ireland, a series of groynes transects the beach to prevent loss of sediments from the shore. Most of these sea defence groynes are constructed as a row of wooden posts embedded deep in the sand. Over time, the posts have been weathered and whittled down to varying degrees, dependent on their position and exposure to wave action. Some rows still stand knee-high, festooned with seaweed and fishing lines, but others have been worn down to mere stumps. The eroding posts reveal intricate wood-grain patterns, and have sometimes become narrow and tapered with wear, thus opening up gaps in the line that become traps for wave-driven pebbles.