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

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Ringstead Bay Rock Textures & Patterns – Part 3

Another gallery of close-up photographs showing natural colour, textures and patterns in an assortment of rocks on the beach at Ringstead in Dorset, England, illustrating the great variety in the geology of this stretch of World Heritage Coast.

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Pebbles at Saints Rest Beach

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

The pebble beach at Saints Rest extends for one kilometre between the mainland and the peninsula that is covered by Irving Nature Park near Saint John, New Brunswick, Canada. The gravel and sand of which it is composed are precariously held together by the roots of marram grass. The beach forms a barrier between the sea and the salt marshes and river valley beyond.

The shore is covered by an amazing variety of pebbles of different geological origins, exhibiting a wide range of natural colours and patterns. Many or most of the smooth pebbles are derived from igneous rocks like granite found in the deposits of glacial moraine that covers much of the surface area in the region – rocks that were plucked up some distance away when the massive ice sheets passed over the native bedrocks, and then were eventually released in the thick superficial layers of sediment as the ice sheets melted.

Some of the other pebbles on the beach must have originated from local bedrocks that lie beneath the loosely compacted glacial moraine deposits covering the river valley, Irving Nature Park and Sheldon Point. The solid bedrock here belongs to the Taylor Island Formation, composed of Precambrian volcanic rocks (ZCTIvs). This underlying solid geology is visible in outcrops around the coastline of the nature park as well as on the headlands to the east and the west of Saints Rest Beach.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

Natural patterns and colours in pebbles of mostly igneous rocks like granite on the shore at Saints Rest Beach, Irving Nature Park, New Brunswick, Canada.

View of Saints Rest Beach, New Brunswick, looking west.

COPYRIGHT JESSICA WINDER 2013

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Fault Gully Rocks at Mewslade Bay

Fault breccia with calcite matrix and Old Red Sandstone

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.

Fault breccia either side the gully to the bay

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.

Fault gully entrance to Mewslade Bay

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.

Fault breccia compared with normal rock layers at Mewslade bay.

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.

Fault breccia or fault gouge in a fault gully

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.

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

View of fault gully from Mewslade beach.

COPYRIGHT JESSICA WINDER 2013

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Pebbles on Chesil Beach 4

Pebbles on the beach

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.

Pebble with fossils in it.

Pebbles on beach

Orange and red pebbles on the beach

Different pebble types on Chesil Beach

Grey pebble with a bright red mark

Coloured and patterned pebbles on the beach

Micro-crystalline granite pebble on the beach

Pebbles with patterns on the beach

Dark grey glassy pebble with red and white lines

Coloured and patterned pebbles on the beach

Pale grey pebble with red veins

Pebbles on Chesil Beach

Patterned pebble on the beach

Pebbles on Chesil Beach

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Rusty Pebbles at Whiteford

Rusty Pebbles at Whiteford (1) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

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.

Rusty Pebbles at Whiteford (2) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (3) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (4) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (5) - View looking across towards Whiteford Lighthouse showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer. Ancient waterlogged wood from the submerged forest is also visible.

Rusty Pebbles at Whiteford (6) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (7) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (8) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

Rusty Pebbles at Whiteford (9) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

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Triangular Beach Stones

Triangular Beach Stone (1) - Natural triangular shape of a pebble or beach stone with a natural geometrical pattern.

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

Triangular Beach Stone (2) - Natural triangular shape of a pebble or beach stone with a natural geometrical pattern.

Triangular Beach Stone (3) - Natural triangular shape of a pebble or beach stone.

Triangular Beach Stone (4) - Natural triangular shape of a pebble or beach stone.

Triangular Beach Stone (5) - Natural triangular shape of two pebbles or beach stones with an oyster shell on the beach.

COPYRIGHT JESSICA WINDER 2013

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