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.

Bishopston Pill at Pwll Du Beach

Video

This short video clip shows two streams of water gushing from the base of the large multi-tiered shingle bank that blocks the valley at Pwll Du Bay in Gower, South Wales. The water comes from the Bishopston Pill river that flows down the valley to the shore, but which has been dammed up behind the shingle. In summer, reduced water flow means that just a trickle seeps out of the shingle base and spreads across the shore. This video was taken in October after heavy rain had increased the quantity of water in the river and subsequently the pressure of the small lake behind the pebble bank. There is a fast and steady flow and the two streams have begun to create channels through the pebbles before converging on the beach. Apparently, in winter, the build-up of water pressure behind the bank means that the river cuts its way straight through to the sea.

Shingle Banks at Pwll Du

View looking east across the water's edge at Pwll Du Bay

Long before the beach was actually visible, the thunder of the waves crashing on the shore, and the grinding of the pebbles against each other on the water’s edge, could be heard as I followed the signpost from Pwll Du Head, down the steep path through the tree-clad valley side to the shore below.

Stretched across the mouth of the valley below lies a massive bank of cobbles and pebbles, or more accurately three successive banks. Unusually for this type of beach, almost all the rocks that make up the banks are the waste product of quarrying activity. Certain privileged farming tenants up until the beginning of the 20th century were granted rights of “cliffage” that allowed them to quarry limestone from the valley sides and the eastern cliff face of Pwll Du Head. The quarry on the Head ceased operation in 1884. The workings are now mostly overgrown.

Boulders from the quarries were placed in heaps marked with wooden stakes on the beach. Ships came over from the north Devon coast to collect the stone to be burnt for lime that would fertilise the fields. Apparently, when the boats arrived, they would scupper in the shallow water at high tide next to the marker posts. Once they were in position on the bottom, the sea-cocks were closed again, with the boat remaining full of water as the tide went out. This technique meant that the bottom of the boat would not be damaged because the water cushioned the fall as the first of the quarried boulders were thrown aboard. Once the cargo was loaded, the sea-cocks were opened to drain away excess water, then closed so that the boat could float on the next high tide ready for the return trip to Devon. Small pieces of rock were too bothersome to load and remained on the shore, gradually building up over the years into the banks.

The shingle banks block the flow of the small river known as Bishopston Pill. The barrier of the shingle banks forms a dam. A small lake lies behind the banks. Beyond the pooled water in the pictures below, two white buildings are tucked into the western valley side at the back of the shingle; these were once public houses that catered for the workmen labouring in the quarries – it must have been thirsty work.

In summer when flow is reduced the water seeps gently from the base of the banks and spreads out across the shore, usually at the eastern end of the beach. When I visited in October after heavy rain, the water was emerging in two fast-flowing streams that were cutting embryo channels through the shingle at the seaward face. In winter when flow is greatest it seems that the banks are breached completely by the flow.

The pebbles nearest to the shore, where they are constantly moved against each other by the waves, tend to be the smallest and smoothest. Most of them are the local Carboniferous limestone but some “foreign” pebbles are included and these originated as jettisoned ballast from ships plying trade in the Pwll Du quarried limestone.

As you walk inland across the shingle, the nature of the stones beneath your feet changes. Whereas on the shore small rounded pebbles tend to predominate, further inland the stones increase to cobble size and they become progressively less rounded and more angular. The shingle bank is always a dynamic structure. However, for many years the innermost couple of the three terraces or ridges, storm beaches, were relatively stable. The stability partly enabled and partly resulted from vegetative colonisation.

I have read accounts of lichen-covered pebbles on the inner banks but none were visible when I visited. Although this was my first visit, I have seen earlier accounts of this locality, and I think that last winter’s storms may well have had a profound effect on the shingle banks. It seemed to me that there had been a great churning up of the stones, resulting in a greater mixing of size and shape, possibly a partial reconstruction of the ridges, destruction of some of the patches formerly stabilised by rooted plants, and removal of lichen encrustations or burial of lichen coated pebbles.

REFERENCES

Gillham, M. E. (1977) The Natural History of the Gower, South Wales, D. Brown and Sons Ltd, Cowbridge, ISBN 0 905928 00 8.

Mullard, J (2006) Gower, New Naturalist Series, Collins, ISBN 0 00 716066 6.

Pwll-Du and the quarrymen of Gower The Geological Society Website

Sea-washed Carboniferous limestone pebbles at Pwll Du Bay

 

Still Life Studies

Some still-life studies of white or cream-coloured natural objects on a black background: showing rounded white chalk pebbles from the Dorset coast in England; ribbed cockle-like bivalved seashells from Queensland in Australia; and a piece of bleached white coral skeleton washed ashore from the Great Barrier Reef.

Rocks at St. Martins, New Brunswick

The rocks at St Martins in New Brunswick, on the coast of the Bay of Fundy in Canada, are around 250 million years old and belong to the earliest and oldest part of the Triassic Period. The Triassic Period lasted from 251 to 199 million years ago. In fact some of the rocks may be so old that they extend as far back as the previous geological period – the Permian.

Two rock types outcrop in the cliffs here. The Quaco Formation is represented by light-coloured coarse boulder conglomerate while the red sandstone belongs to the Honeycomb Point Formation. It is possible to see where the two types of rock make contact in the cliff at the east end of the beach just past the water-side restaurants. Both types of rock are fairly soft and susceptible to erosion by the pounding of the waves. The sea works particularly at vulnerable areas like the bedding surfaces between the angled strata. It is here that the waves eventually form shallow sea caves by washing away the rocks. Very few fossils are found in these rocks. They are too old to have dinosaur fossils although possible reptile footprints have been found. However, Triassic rock layers like those at St Martins pass right under the waters of the Bay of Fundy and emerge on the other side in Nova Scotia near Parrsboro and Blomidon, where fossils of the oldest dinosaurs in the North American Continent have been found.

The beach at St Martins is composed of rounded pebbles with many different colours and patterns. They look their brightest and most interesting close to the water’s edge. The wet pebbles look completely different from the dull dry pebbles higher up the shore. The pebbles represent many rock types and are presumably mostly derived from two sources: the adjacent cliffs, especially the conglomerate, and the thick layer of glacial deposits that overlies much of the terrain in this part of the world.

All the information in this post was obtained from the very informative local signs on the beach. St Martins [GPS: 45.21.42N, 65.31.4.W] is one of many sites of geological interest in the region and forms part of the Stonehammer GEOPARK. Stonehammer produce a downloadable St Martins geology fact sheet and also a St Martins geology map.