More pebbles at Rhossili after storms

More pebbles are visible on the beach at Rhossili on the Gower Peninsula since last winter’s storms. Of course, they were always there but not so many on show. Lots were hidden beneath the sand. The greater number of pebbles is noticeable as soon as you arrive at the beach via the path through the dunes from Hillend campsite. Looking north towards Burry Holms, and south towards Worms Head, a wider band of pebbles than usual is immediately noticeable at the top of the shore.

As you walk towards Diles Lake, which is the stream that flows across the shore from Llangennith Marshes, the pebble layer deepens into a wide bank that more or less dams the stream. This place is subject to frequent change as a result of changes in weather, tides, and currents. It can sometimes be difficult to cross, especially in winter. Right now though, crossing is easy because the stream mostly runs beneath the pebbles, with only a narrow watercourse visible on the surface. It is clear that not only are more pebbles around because sand has been washed away from them but also the pebbles have been pushed up the shore with great force.

Pebbles at Kilmurrin Cove – Part 2

Wet beach stones of volcanic rock

Here are some more examples of the pebbles at Kilmurrin Cove on the Copper Coast in County Waterford, Southern Ireland (see the previous post). Again, most of them seem to be volcanic rocks of Ordovician age, derived from the underlying bedrock of the beach and the outcrops in the surrounding cliffs, and including rhyolite in its solidified lava form and also as consolidated volcanic ash and breccias. Some stones may be of different geological types and origins, having been brought from much further afield and deposited by melting ice sheets.

This time most of the beach stones were photographed at the western end of the beach where  the stream or small river, that is dammed-up behind the pebble bank, surfaces through the stones and makes a break for the sea via jagged outcropping bedrock. The water is stained tea-colour by the peat through which it has flowed down the glaciated valley.

Click on the pictures to enlarge them and see the description.

Pebbles underwater in a beach stream

Dry beach stones with frond of seaweed

Wet beach stones and pebbles on the beach

Wet beach stones in a cove rimmed by cliffs of Ordovician rock topped by glacial deposits

Kilmurrin Cove pebbles on the Copper Coast in Ireland

Dry beach stones with washed up kelp

Pebbles with spots and stripes at Kimurrin Cove

Wet beach stones of mainly Ordovician volcanic origin

Pebbles underwater in a beach stream

Wet beach stones at Kilmurrin Cove

Wet beach stones at Kilmurrin Cove

Wet pebbles with sea foam bubbles at Kilmurrin Cove

COPYRIGHT JESSICA WINDER 2014

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Pebbles at Kilmurrin Cove – Part 1

Spotty rhyolite pebble with phenocrysts

Continuing with my Irish pebble and rock theme, trying to understand the phenomena I encountered on my travels, I noticed lots of patterned pebbles on the next shore along the Copper Coast at Kilmurrin Cove in County Waterford. Many of the beach stones had natural patterns based on either spots or stripes, or a combination of the two. As far as I can make out, the spot pattern is due to a phenomenon where thick viscous lava from a volcano is extruded and cools quickly trapping many gas bubbles. The shape of the bubbles is preserved in the solidified lava. Over time, the gas is replaced by minerals such as clear quartz, salmon-coloured K-feldspar, off-white sodium-rich plagioclase, or natural glass, which crystallise in the spaces initially created by the gases. Sometimes the bubble shapes have merged in the lava giving odd shaped spaces for the new minerals to fill.

Rhyolite is one of the rocks in which this happens.  It has the same chemical composition as granite but because it is an extrusive rock and cooled quickly, the crystals of the matrix of the rock cannot be distinguished. [Whereas granite is an intrusive rock that cooled slowly giving rise to a rock composed entirely of large visible crystals]. The crystals that make up the solidified lava form of rhyolite are so small that they cannot be seen with the naked eye or even with a hand lens. However, the minerals that have percolated into the bubble spaces are macroscopic, they can easily be seen. I think the types of crystals like those shown in image 1.1 are called spherulites; these  spherulites are rounded bodies, often coalescing, comprising radial aggregates of needles, usually of quartz or feldspar. Spherulites are generally less than 0.5 cm in diameter, but they may reach a metre or more across – though not in this part of the world as far as I know. These relatively regular structures in the rock can be compared with isolated large crystal inclusions that are known as phenocrysts.  Rhyolite with phenocrysts is called porphyritic rhyolite.

A number of the pebbles have parallel lines or swirling layers defined by varying colour or granularity – maybe with spherulites as well. These rhyolite pebbles may be showing flow banding that appears like linear or striped patterns when seen in cross-sections of the rock. The lines have been described as being similar to tree rings. This type of rock is called banded rhyolite and forms from slow flowing lava in which bubble- and crystal-rich layers form on the cooling surface. Multiple flows build up one on top of the other to create the multiple lines. At least that is what I think is shown in these striped pebbles. I am open, as always, to correction. I suppose I can’t rule out that some of the lines I noticed might be Liesegang rings.

Not all rhyolite rocks are solid forms of lava. Rhyolites are mostly tuffs and breccias rather than lavas. Rhyolitic Tuffs are rocks consisting of consolidated volcanic ash ejected from vents during a volcanic explosion, while rhyolitic breccias are composed of larger angular fragments thrown out by the explosions. I’ll talk more about this subject later when I write about my visit to Bunmahon Geological Garden further along the Copper Coast.

As usual, click on the pictures to enlarge them and see the description for the image.

Natural patterns, shapes and textures in pebbles on the beach

Beach stone with stripes and spots

View of a pebble beach on a Copper Coast in Southern Ireland

Pebble bank on the shore at Kilmurrin Cove

Dry beach stones on the Copper Coast in Ireland

Natural patterns, shapes and textures in pebbles on the beach

Natural patterns, shapes and textures in pebbles on the beach

Natural pattern of banding in a rhyolite pebble

View of the west end of Kilmurrin Cove showing river dammed by large pebble bank.

View of the west end of Kilmurrin Cove with stream crossing exposed Ordovician rocks

Natural patterns, shapes and textures in pebbles on the beach

Dry beach stones on the Copper Coast in Ireland

COPYRIGHT JESSICA WINDER 2014

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The Sea Wall at Annestown

Pebble bank on the seashore retained by a seawall

The pebble bank on the seashore at Annestown is retained by a low sea wall that curves from one end of the bay to the other. (You can see it on the right of the picture above). It stops incursions by the sea in rough weather, and also prevents the movement of the pebbles inland. The car park is mostly tucked behind the sea wall but where an opening allows access to the beach, recent winter storms and wave-borne beach stones have pounded and ripped-up the tarmac.

A footpath follows the landward side of the wall from east to west. The wall is high enough to fulfill its function and low enough to allow walkers to enjoy the view at all times. The construction of this sea defence is interesting, seemingly made up from individual blocks of small pebbles in a cement matrix, and the blocks then cemented together to form the wall. It looks as though the wall is using local materials in an attempt to blend better with the surroundings. However, the rough surface is colonised extensively by black, yellow, and white lichens whose distribution varies, presumably according to the degree of exposure to prevailing sun and wind and rain, making the wall stand out as a feature rather than merge with the landscape – although there is a certain resonance with the dark cliffs and headlands beyond.

The pebbles on the seashore at Annestown, banked up against a sea wall

View of the sea wall at Annestown looking west

View of the sea wall at Annestown looking east

Close-up of lichens on a sea wall

Close-up of lichens on a sea wall

Close-up of lichens on a sea wall

COPYRIGHT JESSICA WINDER 2014

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Pebbles at Annestown

Pebbles on a Copper Coast beach

The first beach that I visited on my recent trip to Ireland was near the small village of Annestown in County Waterford. It is part of the Copper Coast Geopark, and I wish I had known at the time that the Geopark website offers informative trail guides and an audio podcast to guide visitors on a walk around this particular area, starting at that very beach.

I was immediately struck by how different the rocks in the cliff are from anything I have seen before, and the pebbles on the windy and surf-washed shore have their own unique character. A sign-board in the car-park explains that the rocks in this location are extremely old, mostly dating from the Ordovician Period, resulting from ocean-bed volcanic eruptions at a time when the land which is now Ireland was formed near the South Pole between 460 and 450 million years ago. Movements of the earth’s crustal plates over vast eons of time have caused the land to gradually migrate northwards to its current position.

In amongst the pebbles of volcanic origin and Ordovician age are others from sources further along the coast and also, no doubt, pebbles derived from the deposits of clay, boulders, and sand that were dumped over the land surface at the end of the Ice Age 12,000 years ago as the ice melted, and which can be seen today as a yellow-brownish layer on top of the cliffs.

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

Pebbles on a Copper Coast beach

COPYRIGHT JESSICA WINDER 2014

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Pebbles from St. Martins, New Brunswick

Red, green and patterned pebbles in a dish

St Martins in New Brunswick, on the Bay of Fundy coast in Canada, is famous for its red Triassic cliffs. These have been eroded by the sea to form large caves in which seals like to rest. The adjacent long beach is composed of millions of pebbles or beach stones from not only the nearby Triassic strata but also brought down from far and wide by the massive ice sheets that once covered the land mass. There is an amazing variety of rock types represented by the pebbles, with red and green colours most noticeable, and many textures and patterns exhibited.

An assortment of patterned and coloured pebbles

Red, green and patterned pebbles in a dish

COPYRIGHT JESSICA WINDER 2014

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Raised Beach Deposits on the Oregon Coast

Sea cliff exposing raised beach deposits at Neptune State Park in Oregon, USA

Sixty miles due west of Eugene the Suislaw River hits the Pacific. From here, where the town of Florence straddles the mouth of the river, the U.S. Highway 101 follows the shoreline south to San Francisco, and northwards all the way to Washington State. We were headed north as far as Yachats.

The views along the coast road were stunning. At first, sand dunes on both sides. Thereafter, accompanied all the way by the seashore and pounding ocean to our left, and forested slopes of the Cascade Range to our right. The shore was unlike anything I had previously encountered at home in Great Britain, because the Oregon shore had been greatly influenced by relatively recent volcanic activity. However, one element was familiar: an ancient wave-cut platform and accompanying raised beach or terrace deposits.

The terrace deposits were once extensive along the Oregon Coast but are now present only as segments in more protected areas of the shore. They were formed during the Late Pleistocene period when the sea level was higher than it is today. The sea at that time cut away or levelled the basalt bedrocks on the beach and created a layer of sediment including rounded pebbles and sand which remained in an elevated position, marking the site of the Late Pleistocene beach, when the sea level subsequently lowered.

The raised beach or “terrace sediments range in thickness from a few feet to tens of feet and have been weakly consolidated into sandstone and conglomerate capable of maintaining a vertical sea cliff on the seaward side” (Lund 1971). The pictures below illustrate the appearance of segments of terrace and sea cliff at Neptune State Park on the central Oregon Coast.

Earlier posts in Jessica’s Nature Blog have described aspects of the rocky shores in the region of Yachats, just a few miles north of Neptune State Park. Most of the town of Yachats (population 650) is built on a segment of Late Pleistocene raised beach or terrace. Raised beach deposits occurring on the Gower Peninsula in South Wales have also been described in a previous article in this blog. Raised beach deposits containing abundant shells and pebbles with holes made by sea creatures will be discussed in a following post.

REFERENCE

Coastal landforms between Florence and Yachats, Oregon by Ernest H. Lund, February 1971, The ORE BIN, Volume 33, No. 2, pp 21-44.

COPYRIGHT JESSICA WINDER 2014

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