Budleigh Salterton Pebbles Part 4

Posted on April 16, 2013 by winderjssc

Click here for more pictures and Posts on Pebbles at Budleigh Salterton in Jessica’s Nature Blog.

Fault Gully Rocks at Mewslade Bay

Posted on April 5, 2013 by winderjssc

As you arrive at the narrow gully that forms the entrance to Mewslade Bay, you cannot help but notice just how different the rocks are from those that outcrop on the hills around. Up on Thurba Head to the left and to the high cliffs and outcrops on the right, thick grey layers of High Tor Limestone from the Carboniferous period are clearly visible. However, down in the gully, there is nothing but a jumble of assorted broken rocks of all kinds of origin, which in some places have been cemented together with white crystalline calcite.

The reason for the unusual geology at this point is an ancient rupturing and tearing of the rocks – faulting. The steep-sided dry valley that leads from Pitton village down to Mewslade Bay follows the line of a fault. In fact, two faults converge here. The fault line itself extends inland and North-north-east across first the High Tor Limestone which appears as cliffs along the South Gower shore in this locality, then passes across Hunts Bay Oolite (the next in the succession of Carboniferous Limestone strata), and possibly other rocks in the series such as Gully Oolite. Further north, around and beyond Pitton and Middleton, superficial drift deposits of glacial till, like gravelly clay, mostly cover and obscure the solid geology below. Beneath this layer of till deposit lies the solid Old Red Devonian Sandstone rock.

Faulting occurs as brittle rocks crack when they can no longer bend and fold to accommodate earth movements. Once the rocks break, there can be major movements of the strata on each side of the crack. One side may move up or down in relation to the other. The two sides may slide and tear against each other horizontally. The friction of the rocks grinding one side on the other often breaks up the rock into smaller pieces. Compared with the solid rock strata, the area of smaller fragments is very water permeable.

It is thought that the entrance to Mewslade Bay is a fault gully, which has probably been eroded out by glacial meltwater during a period of permafrost when an ice sheet covered most of the Gower Peninsula. The various rock types represented in the gully structure would have been derived from both the Carboniferous Limestone and the Old Red Devonian Sandstone geology fractured by the fault and carried down towards the sea by water. The name for this type of deposit is Fault Breccia when the pieces are large, and Fault Gouge when they are much smaller. Secondary minerals such as calcite can precipitate from groundwater circulating around the loose fragments, filling the empty spaces or voids and cementing rock together.

Low down, nearest the ground, the fault breccia looks like tutti-frutti ice cream with rock fragments coloured pink, red, brown, yellow, and grey embedded in a crystalline calcite. Higher up, the fragments are consolidated – but without the all-encompassing white matrix.

Gallery of fault breccia rocks in the fault gully at Mewslade Bay

Calcite occurs in different forms. There are neatly formed crystal clusters as veins or in pockets. Most are basically white but some are stained orange with iron. Superficially, the crystals are often coated with green or brown algal bio-films. In some places, especially in tunnels and low down in the deposit, where the the rocks are washed by the sea and abraded by debris at high tide, the calcite is worn down to a smooth surface with just an outline pattern of the crystals and cleavage.

This slideshow requires JavaScript.

Slide show of crystal calcite from the fault breccia at Mewslade Bay

Pebbles on Chesil Beach 4

Posted on March 29, 2013 by winderjssc

I love Chesil Beach. It’s so exhilarating. I love the pebbles, millions and millions of pebbles, different sizes of pebbles, different types of pebbles, the way they glisten wet after pounding by the waves, bank upon bank, terrace above terrace, the deafening roar of the pebbles as they are jostled and dragged on the water’s edge. I relive the experience every time I look back at my photographs. I’d like to share with you these pebble pictures from my last visit.

Click here for more Chesil Beach posts – or posts about Pebbles in Jessica’s Nature Blog.

Rusty Pebbles at Whiteford

Posted on February 9, 2013 by winderjssc

Vast swathes of pebbles on the beach at Whiteford in Gower are coloured orange – or at least they were the last time I looked. (The beach sediments there are very mobile so it cannot be guaranteed that you will see exactly the same thing on each visit). These coloured pebbles are found in a band stretching from the base of the sand dunes at the eastern end of the beach towards the disused Victorian Whiteford Lighthouse.

The pebbles seem to be coated in rust rather than rusty because of their intrinsic composition. I guess the first couple of times that I noticed the orange pebbles I vaguely thought that they were stained by rust emanating from the decomposing remains of the old iron causeway that linked the lighthouse to the shore. You can often find pieces of the iron framework of the walkway – sometimes supports still in situ and other times single pieces of the structure lying free.

However, lately, I have been discovering more and more about the Quaternary geology of the Gower – a relatively recent geological period dating from about 2.5 million years ago to the present. This includes the Pleistocene with a variety of glacial, peri-glacial and inter-glacial deposits; and the recent Holocene (from 11,800 years ago) with peat and submerged forests, marsh, dune, beach and alluvial deposits. As I read more, I am gradually reaching something of an understanding about some of the natural phenomena that I observe and photograph on Gower beaches. So I now tentatively consider that the rusty pebbles are not related to the dilapidation of the old lighthouse but are the result of a much older natural geological process.

I have already mentioned in Jessica’s Nature Blog the remains of the submerged forest at Broughton Bay which lies to the west and adjacent to Whiteford Sands. These ancient tree trunks are embedded in peat deposits. While I was reading George (2008), I learnt that the peat decomposes to form a hard ferruginous layer called an iron-pan or hardpan. This has led me to wonder if the iron compounds that coat the pebbles at Whiteford are derived from an iron pan layer.

Supporting evidence for this idea comes from the presence of ancient tree trunks emerging from black peat deposits close to the rusty pebbles – similar to those stumps found at Broughton. The old waterlogged wood is also stained with rust – as you will see from the photographs below. Additionally, slightly higher on the beach, closer to the dunes, the shore is strewn with pebbles around which orange-coloured watery ‘tears’ rise to the surface and weep across the surface of the sand – making me think they might originate from a concealed ferruginous hardpan below.

Then again, I suppose the rust could come from buried decomposing munitions as the beach was used for firing practice in the Second World War!

Reference:

George, Gareth T. (2008) The Geology of South Wales – A Field Guide, G.T.George at gareth@geoserve.co.uk , ISBN 978-0-9559371-0-1, p 70.

Triangular Beach Stones

Posted on January 26, 2013 by winderjssc

Just a selection of naturally triangular-shaped pebbles or beach stones that I have found on the seashore at various times. Some of them have interesting natural patterns too. They remind me of the triangular boulders and patterns of cracks and crevices that I’ve seen in rock outcrops like the ledges at Kimmeridge in Dorset, and Spaniard Rocks at Rhossili on Gower.

See Kimmeridge Bay triangles

Also Limestone & lichens at Spaniard Rocks

Underwater Pebbles

Posted on December 18, 2012 by winderjssc

Pebbles viewed through shallow river water, with patterns of reflected sunlight – photographed on the beach in Neptune State Park, south of Yachats, Oregon, U.S.A.

Trinity Beach-combings

Image

An assortment of seashells and pebbles from the strand-line at Trinity Beach, Queensland, Australia

Sea ripples & reflections off Portland (2)

Posted on July 24, 2012 by winderjssc

More natural colours and abstract patterns of reflection on the gently moving water surface of waves lapping ashore over pebbles on the Isle of Portland in Dorset, UK.

Wealden Beds at Lulworth Cove

Posted on July 3, 2012 by winderjssc

There are five main types of rock strata visible at Lulworth Cove: Portland Stone, Purbeck Beds, Wealden Beds, Greensand and Chalk. The Wealden Beds are the most colourful and some of the rocks have wonderful patterns as well.

The colours range from yellow and orange through to deep red, brown and black. The strata are very variable with interesting textures and compositions.

Although this World Heritage Site is called the Jurassic Coast, it includes earlier Cretaceous rocks as well. The Greensand, Chalk and Wealden Beds all belong to the Cretaceous period. The photograph above shows the soft sandstone layers at the base of the cliff that are being worn back and undercut by the sea.

The photograph below shows a general view of part of the cliff containing the Wealden Beds.

For more information about the geology of Lulworth Cove look at the Lulworth Web Site.