Garretstown Strand Rocks – Part 1

Natural patterns in rocks

These striped grey and yellow patterns in the bedrock at Garretstown Strand in County Cork are made by alternating layers of sand and mud, sometimes in rippled layers. They belong to the Cork Group of rock strata, most probably the White Strand Formation which is comprised mainly of Namurian (Upper Carboniferous) sandstones inter-bedded with brittle, commonly pyritic grey-black mudstones.

REFERENCE

Sleeman, A. G. and Pracht, M. et al (1994)  Geology of South Cork: a description of South Cork and adjoining parts of Waterford to accompany the bedrock geology 1:100,000 scale map series, Sheet 25, South Cork, Geological Survey of Ireland, White Strand Formation pp 23-24.

Natural patterns in rocks

Natural patterns in rocks

Natural patterns in rocks

Natural patterns in rocks

Natural patterns in rocks

Natural patterns in rocks

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Rhyolite rock colour, pattern & texture

Rhyolite volcanic rock with red streaks

East of Kilmurrin Cove in County Waterford, Ireland, lies the village of Bunmahon with its roadside Geological Garden and the Copper Coast Geopark Centre. The Bunmahon Geological Garden is like an open-air museum with examples of the main rock types to be found in the area with explanatory sign boards along the way. It is great to be able to compare what has been seen on the local beaches with this named reference collection; it means that  visitors can begin to put together a picture that explains the surrounding landscape through which they are travelling.

The close-up details so easily seen in these labelled rocks also helps with the identification of the rock types represented by the pebbles on the beaches. The rhyolite boulders in particular attracted my attention, providing clues to the wide range of textures and colours to be found in this local Ordovician volcanic rock. I loved the yellow to red streaks and abstract patterns in some of the boulders and the way that the surface of the massive stones has been decorated by small patches of dark grey lichen.

Rhyolite boulder in the Bunmahon Geological Garden

Close-up of rhyolite rock texture

Close-up of rhyolite rock texture

Rhyolite boulder in the Bunmahon Geological Garden

Close-up of rhyolite rock texture

Rhyolite boulder in the Bunmahon Geological Garden

Close-up of rhyolite rock

Close-up of rhyolite rock

Close-up of the texture of rhyolite rock

Boulders of rhyolite rock in Bunmahon Geological Garden

Natural pattern in a rhyolite boulder in the Bunmahon Geological Garden

Natural abstract pattern in a rhyolite boulder in Bunmahon Geological Garden

Abstract pattern in rhyolite rock

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

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

All Rights Reserved

On Garrettstown Strand

Rocks on an Irish beach

Rocks on an Irish beach

Rocks on an Irish beach

Rocks on an Irish beach

Patterns in rock on an Irish beach

Again, simply a few pictures, as a “taster” of the many photographs I have been taking, to show the incredible geology I have been witnessing on my current travels in Ireland. I will write up full descriptions and identifications with more images (as best I can) when I return home. These rock patterns and structures were captured on Garrettstown Strand, also known as White Strand or Garrylucas Beach, near Old Kinsale Head in County Cork.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Traces – Hue maps of trace fossil patterns

Just for fun, showing how nature can inspire art, the photographs of trace fossils shown in the previous post have here been colourised by the hue mapping method to produce some bright abstract images. Click a picture to enlarge it.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Patterns made by trace fossils at Lyme Regis

Visitors to Lyme Regis are always amazed by the wonderful large ammonites embedded in the beach rocks and rock pavement, and are often frantic in their search for take-away fossils as mementos for their day out…. but they frequently overlook the less spectacular but nonetheless fascinating myriads of trace fossils underfoot. Trace fossils are also known as ichnofossils and they are the evidence, preserved in stone, for the activities of organisms rather than the petrified remains of the animals themselves. They tell us about the way animals behaved and interacted with their environment in the past.

The patterns in the rocks shown in these photographs are the traces of burrows and tunnels that were made in the sediments before they became rock. They were made by marine invertebrate creatures like crabs and worms. The intriguing natural patterns are found in the Blue Lias Limestone from the Jurassic Period. The Jurassic system is exposed along the Dorset coast between Lyme Regis and Swanage with very few breaks. Melville and Freshney (1982) say that trace fossils are abundant in rocks of this area and there are four common types:

Diplocraterion with U-shaped burrows at right angles to the bedding planes, measuring 21 by 4 cm with the diameter of the tubes up to 0.7 cm. These were the permanent domicile of  worms or crustaceans that angled or swept for food suspended in water.

Chondrites with regularly branching, vertical to horizontal burrows spreading down like the roots of a tree, and 2mm or more in diameter. The work of sediment-eating animals, perhaps sipunculoid worms.

Thalassinoides Ramifying, Y-shaped branching networks of plain horizontal and vertical tubes, 1 to 5 cm in diameter. Feeding burrows of decapod crustaceans.

Rhizocorallium Horizontal to oblique U-tubes, each arm 1 cm or more in diameter with arms several centimetres apart with reworked sediment between them. Tubes short or long depending on whether the animal was in a suspension-feeding or deposit-feeding phase of activity.

The pictures show several burrow types intermingled.

REFERENCE

Melville R. V. and Freshney E, C. (1982) British Regional Geology: The Hampshire Basin and adjoining areas, Institute of Geological Sciences, NERC, HMSO, pp 7 – 8, ISBN 0 11 884203 X.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

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.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Ringstead Bay Rock Textures & Patterns – Part 2

This is a gallery of some of the rock textures and patterns found in material brought down to the seashore by land-slips on the eastern half of Ringstead Bay in Dorset, England. Some might say that the geology of this half of the bay is much more complex and interesting than the western half. In addition to a natural progression of strata from older rocks in the west to younger rocks in the east, there are faults and land-slips that result in much of this variable material appearing in mudslides at the top of the shore, and as boulders on the beach.

The pictures show details of the textures and patterns in some of the rocks and sediments that were laid down after the Ringstead Formation rocks that were illustrated in earlier posts. These later strata include those from the Jurassic Period Kimmeridge Formation (shales and clays), Portland Limestone Formation (Portland Freestone, Portland Cherty Series, Portland Sand), and the basal part of the Purbeck Formation. The layers can be seen in exposures west of Holywell House on top of the ‘cliff’; and boulders from them often end up rolling down to the beach below.

A bit further east the geologically more recent Cretaceous Period strata are exposed. These include the Gault, Greensand, and Lower, Middle, and Upper Chalk. All of these rock exposures are subject the slippage and land slide so that boulders frequently end up on the beach. I haven’t yet reached a complete understanding of the geology in this location. I have attached a description to each image that you can see if you click to enlarge the pictures. However, I cannot say with absolute certainty the identity of each of the rocks I photographed – but I am working on it with the help of the references listed below; and I’m hoping to visit the Dorset County Museum to look at their rock collections soon.

REFERENCES

West, I.M. 2013. Ringstead Bay to White Nothe: Geology of the Wessex Coast (Jurassic Coast, Dorset and East Devon World Heritage Site). Internet field guide. By Dr. Ian West, Romsey, Hampshire and Visiting Scientist at Southampton University. http://www.southampton.ac.uk/~imw/Ringstead-White-Nothe.htm. Version: 19th December 2013

British Geological Survey (2011) Geology of south Dorset and south-east Devon and its World Heritage Coast, Compiled by M. A. Woods, Special Memoir for 1:50 000 geological sheets 328 Dorchester, 341/342 West Fleet and Weymouth and 342/343 Swanage etc, NERC, ISBN 978 85272 654 9.

Melville, R. V. and Freshney, E. C. (1982) British Regional Geology: The Hampshire Basin and adjoining areas, 4th edition, Institute of Geological Sciences, NERC, HMSO, ISBN 0 11 884203 X.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved 

Ringstead Bay Rock Textures & Patterns – Part 1

These pictures show details of the colour, texture, and patterns in vertical and horizontal exposures of the rocks of the Ringstead Clay Member (also known as the Ringstead Waxy Clays) at Ringstead Bay in Dorset, England. This is in the upper part of the Corallian Formation belonging to the Jurassic Period. It comprises four different layers or strata that reach a combined thickness of up to 5 metres. The clay deposits contain bands of reddish-brown, siderite nodules (iron carbonate) that are responsible for the rusty or ferruginous colouration of the otherwise brown and grey clay. There are interesting patterns of cracks where the clay is drying out; and changes of colour tone according to the moisture content of the clay. The most obvious fossils in this particular clay group are the large, flat, almost triangular, oysters – Deltoideum (Liostrea) delta.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved