Rock Texture & Pattern at Dog’s Bay

The rocks at Dog’s Bay in Connemara, Ireland, are part of the Galway Batholith. In particular they are composed of the Errisbeg Townland Granite riven by faults and many dikes containing other younger intrusive igneous rocks. The juxtaposition of the different rock types is a phenomenon marked by contrasting, colours, textures, and patterns. The whole rocky terrain has been levelled off and smoothed by ice sheets and reflects many glaciation features.

Feely, M. Leake, B.E., Baxter, S. Hunt, J. and Mohr, P. A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland.Geological Survey of Ireland, 2006.

Rock Texture & Pattern at Black Brook Cove

Patterns of dykes in granite in the cliffs at Black Brook Cove

Black Brook Cove along the Cabot Trail in Cape Breton Island, Nova Scotia, gets its name from the dark colour of the river water which flows into it. On the southern edge of the cove, the upper banks of the estuary are piled high with large bleached driftwood lying on a bed of boulders and pebbles. Curving banks of pebbles on the main body of the beach give way to smooth waterworn rock outcrops; and spectacular jagged cliffs surmounted by pines form the northern arm of the cove.

The rocks at Black Brook Cove are part of the Devonian Black Brook Granitic Suite formed about 375 million years ago. They are igneous plutonic rocks. The magma from which they formed was created by the melting and recrystallization of meta-sedimentary rocks that were sub-ducted during the collision of the ancient land masses called Ganderia and Avalonia.

The remarkable feature of the rocky outcrops at Black Brook Cove, and at Green Cove just a little further south, is the number of criss-crossing dykes or veins of contrasting colour that create abstract angular patterns on the rock surfaces. These patterns and colours are accentuated when the rock is wet. The whole beachscape is captivating on a bright sunny afternoon but the area must look its best after a heavy downpour of rain.

The main rock is a grey granite with small black flakes of biotite. Earth movements and increased pressures on numerous occasions subsequent to its emplacement have cracked the rock and opened up fissures into which certain minerals that were squeezed out of the mother rock have entered and recrystallized. Mostly the veins formed in this way are composed of aplite or pegmatite. Both are pink-orange in colour Aplite is made of quartz and feldspar and is fine-grained with a smooth sugary texture. Pegmatite is darker and coarser with large visible individual crystals of quartz, feldspar and mica in both the black biotite and clear muscovite forms.

REFERENCES

Anoiyothin, W.Y. and Barr, S.M. (1991) Petrology of the Black Brook Granitic Suite, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol. 29, pp. 499-515.

Barr, S.M. and Pride, C.R. (1986) Petrogenesis of two contrasting Devonian Granitic Plutons, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol.. 24, pp. 137-146.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp. 94-97.

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Rock Texture & Pattern at White Point

Without a four-wheel drive vehicle we couldn’t make it as far as the promontory called White Point along the Cabot Trail in Cape Breton Island. We stopped instead at the harbour in the village. Here the walls and sea defences were made of large stacked boulders of the same local rock that outcrops at the Point so it was possible to get a really good look at the composition of them. It was amazing. It was a sunny day and everything sparkled. The rocks themselves actually sparkled. I couldn’t believe my eyes. Even the sand and the waves sparkled. I tried desperately to capture the magic of it but it was difficult because the camera mostly over compensated for the bright points of light.

Close-up it was possible to see that the rocks had large plate-like crystals of transparent mica minerals (muscovite) which were acting as small mirrors. The sand had an abundance of these shiny crystals weathered out of the rocks and catching the light. The waves agitated the crystals and further  increased the sparkling effect.

The rocks are a mixture of pink granitic gneiss and silvery black biotite schist. They originated as intrusive molten magmas beneath the earth’s crust in Devonian times approximately 375 million years ago, forming a large mass called a pluton – specifically  the Black Brook Granitic Suite. The igneous rock which has  been metamorphosed to black biotite schist was the first to be deposited and compressed into rough layers or foliations with a parallel alignment of the crystals. The igneous rock which is now mainly granitic gneiss intruded into the schist preferentially along the lines made by the foliations. There are also veins of aplite and pegmatite. The alternation of these two rock types is a wonder to behold on White Point itself. My photographs have focussed on details of the patterns and textures as revealed by the rip-rap boulders in the nearby harbour.

REFERENCES

Anoiyothin, W.Y. and Barr, S.M. (1991) Petrology of the Black Brook Granitic Suite, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol. 29, pp. 499-515.

Barr, S.M. and Pride, C.R. (1986) Petrogenesis of two contrasting Devonian Granitic Plutons, Cape Breton Island, Nova Scotia. Canadian Minerologist, Vol.. 24, pp. 137-146.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, Third Edition. Atlantic Geoscience Society Special Publication #1.

Hickman Hild, M. and Barr, S. M. (2015) Geology of Nova Scotia, A Field Guide, Touring through time at 48 scenic sites, Boulder Publications, Portugal Cove-St. Philip’s, Newfoundland and Labrador. ISBN 978-1-927099-43-8, pp. 94-97.

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Atlantic Geoscience Society Special Publication No. 15, Nimbus Publishing, ISBN 1-55109-351-0.

Rocks at Cobo Bay

Quarried surface of Cobo Granite

The beautiful and popular beach of Cobo Bay on the north coast of the Channel Island of Guernsey marks a transition between two igneous rock types: the Cobo Granite and the Bordeaux Diorite Complex. The character of the rocks that punctuate the stretches of clean sand and clear blue water changes as you walk from one end of the bay to the other.

In the southeast, near Le Guet Quarry and  Albecq the orange-pink Cobo Granite is at its most even-textured and pure with coarse grained crystals of pink potassium-rich orthoclase feldspar, light grey plagioclase feldspar, glassy quartz grains, and small black shiny crystals of biotite mica. The colours are best seen in freshly broken rock but are often obscured or muted by weathering and encrustations (lichens on dry land; algae and barnacles between the tides). The pebbles at this end of the beach are mostly brightly coloured water-worn remnants of the Cobo Granite.

The Bordeaux Diorite Complex rocks are superficially grey, composed of mostly black and white crystals  with grey plagioclase feldspar, black biotite, with minor minerals such as hornblende, pyroxene, and quartz. The Cobo Granite is younger than the Bordeaux Diorite. Deep beneath the earth’s crust, the molten granite intruded into the diorite before it was fully solidified. This led to a mixing of the two types of magma, and also a breaking-off of pieces of semi-solidified diorite that became enfolded in the granite magma before cooling. Small dark grey pieces of diorite (xenoliths) can be seen in the granite to the west below the Le Guet quarry. The numbers of xenoliths increase as you walk north east. Veins of pink aplite also run through the rocky outcrops. Whole areas of rock on the beach below the Rockmount Hotel are greyish in colour where the two rock types have melded together.

Walking northeast in the direction of Port Soif, large patches of greenish-grey inclusion rock can be seen ever more frequently embedded in the granite. I think these kinds of rocks are called granite-diorite marginal facies. By the time you reach the end of the beach, the grey diorite is more evident in outcrops and boulders. An attractive stone slipway demonstrates the contrasting colours and textures of the two rock types, the dark grey of the diorite and the orange of the granite. The angular beach stones and rounded pebbles at this point  also show the two rock types but with the grey diorite dominating, in contrast to the mostly orange granite pebbles at the other end of the beach.

This is just a very simple description of the geology at Cobo Bay and is intended only as a general guide to the rock features. My apologies for any inaccuracies. The expert explanation is much more complex and can be found by consulting the references given below.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pp 48-51.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 66-70.

L’Eree Granite

Close-up of L'Eree Granite with pink megacrysts of feldspar

The special thing about the L’Erée Granite is the presence of distinct large pink/orange crystals. I mean big. Some of them are a up to 4 centimetres across. They are magacrysts of feldspar which are thought to have grown very slowly deep within the earth’s crust when the bulk of the granite had already crystallised (Pomerai & Robinson 1994). The action of hot gases fed these feldspar crystals that sometimes have concentric rings of dark inclusions (as you can see in some of the photographs) marking stages in their growth between 646 + 25 million years ago in the early Cadomian Age.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, pages 30 – 32.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, . Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pages 11 – 12, & 75 – 78.

All sorts of colourful & interesting rocks

This gallery displays a selection of the most colourful and interesting rocks that have been featured in posts here at Jessica’s Nature Blog over the past couple of years. While I am out walking on beaches, I am always drawn to the colours of the rocks, sometimes bright and other times more subtle, and the many different patterns and textures. Initially it is the way that the rocks look that is so appealing. So much of what I see seems like amazing natural abstract art. I try to frame the composition so that it stands alone as an attractive image in its own right. But then I get curious and lots of questions come into my mind. I always want to know what kind of rock is it? What is it called? How old is it? What is it made of? How did it get to look like that? What happened while the rock was buried? What are the elements doing to it now that it is exposed?

As an amateur with a keen interest in geology, I start by looking at maps. I try to pinpoint the exact location where I photographed the rock. Then I try to get hold of the correct geology map. Geology maps have a lot of information about the age of the rock, the type, the period in which it was laid down or developed, as well as the distribution of the different rock types in the locality. Often there are references to special papers, memoirs and so forth that discuss the geology of the area. Sometimes these publications are available on-line. I do a lot of Googling. Sometimes a visit to the library is needed. Libraries and the internet don’t always have the information I am seeking so I buy books too. Sometimes books about a specific place, and sometimes more general textbooks. I need those too because it is quite difficult to understand everything. Geology is a complex subject with a great deal of specialist terminology.

Once I am fairly certain what the rocks are, I try to write a bit about them in a straightforward way so that anyone else who is truly interested will be able to understand. It is fascinating. Slowly I learn more about the rocks and can fit the pieces together into the bigger picture. Walking along shorelines becomes a whole new experience when you are able to visualise the former environments in which the bedrock originated, or the drift geology was created, when you begin to understand what has happened to the strata over the millions of years since they came into being, and when you first begin to grasp what processes are affecting them once they are exposed to air. I love it when I can recognise strata belonging to the same geological period in different parts of the world, and see their differences and similarities, whether in situ or in buildings, walls and other structures. I begin to feel an enormous sense of wonder and awe, as well as an enormous feeling of humility, at this hugely significant part of the natural environment, a part on which everything else in nature depends or by which it is affected.

Rocks at Port Douglas, Queensland

These are some personal observations, just thinking aloud, and part of my learning process and fascination with geology. I have a lot of questions to ask about the rocks at Port Douglas on the Queensland coast in Australia! They are marked by a GeoCache site which says they are a batholith. A batholith is formed deep under the earth’s crust where molten magma from superheated, melted rocks, cools slowly and forms granite made up of fairly large crystals. In eastern Australia the batholith formations, known collectively as the Kennedy Province, were created between 330 and 255 million years ago  – following the earlier formation of the Hodgkinson Province which was created between 440 and 360 million years ago, and into which the batholith magma eventually intruded.

I have seen a batholith before – at Peggy’s Cove in Nova Scotia – and the the rocks I photographed at Port Douglas are very different from the rocks exposed at Peggy’s Cove – at least the ones around the edges of the formation in Port Douglas, notwithstanding that the two outcrops of bedrock are in two separate continents and have been subjected to very different erosional and weathering processes. Peggy’s Cove rocks have been smoothed and polished by ice sheets (glaciated) and their surfaces remain clean in a temperate climate. At Port Douglas, on the other hand, the rocks have eroded out and weathered in the wet tropics climate which has led to different erosional characteristics and a surface obscured in many parts by black bio-film.

Batholiths are made of granite. I think I can see granite in some areas of the Port Douglas outcrop on which the Lookout stands. A lot of what I believe to be granite is covered in black biofilm (possilbly cyanobacteria and lichen) and is colonised by organisms like barnacles so that the details are obscured. However, most of the detailed close-up shots I took of the rocks, particularly those around the edges of the feature, including loose boulder lying on the waters’ edge, did not seem at all like granite to me. There are various colours, textures and features as shown in the photographs in this post. I have been wondering to myself, speculating, whether these rocks may represent the junction between the granite of the Kennedy Province batholith and the Hodgkinson Province rocks into which they intruded, showing further changes to the earlier overlying (and already much altered, stretched, compressed and vulcanised) metasedimentary rocks.

The geological map of the area describes the outcrop of bedrock in the Port Douglas environs as Larramore Metabasalt Member which is part of the Hodgkinson Province rocks. I suspect that some of my photographs may be showing these metabasaltic rocks, or metasedimentary rocks with evidence for explosive volcanic activity and volcanic intrusions. I have read Rocks, Landscapes and Resources of the Wet Tropics by Berndt Lottermoser et al (2008) published by the Geological Society of Australia, Queensland Division, and have found it very useful. However, a more relevant account of the port Douglas geology might be given in another book which I have been trying to track down: Rocks and Landscapes of the Cairns District by W. F. Willmott and P. J. Stephenson (1989) published by The Queensland Department of Mines and Energy but it is out of print. I think it might be useful in helping me answer some of the questions I’m posing.

Granite at Peggy’s Cove

Glaciated granite landscape at Peggy's Cove, Nova Scotia, Canada

Peggy’s Cove is one of the most popular, and most photographed, places in Nova Scotia. The iconic red and white lighthouse perches on a spectacular glaciated granite landscape. From the photographs below showing people (1, 3, 15 & 19), it is possible to see the vast scale of these outcrops over which visitors like to climb.

The rocks are the crystallised remains of an enormous underground chamber of molten magma that formed 10 miles below the surface of the Earth’s crust 385 million years ago by a combination of tremendous pressure and heat during the movement of crustal plates and subsequent mountain building. See the earlier post Close-ups of granite from Peggy’s Cove, N. S. for more details.

The criss-crossing patterns of fractures, cracks and crevices are evidence of later pressures to which the granite was subjected. During the process of crystallisation of the molten rock, a great deal of water built up, and become pressurised and created cracks as the magma slowly cooled into coarse-grained granite. The pressurised water was forced into the cracks carrying with it some of the magma, which then cooled at a faster rate, making very small light-coloured crystals and forming fine-grained aplite rock. This aplite can be seen as thin sheets of uniformly creamy-coloured sugary rock extruding from granite layers and as veins in many of the images in the gallery below (for example, photos 13, 14, 11, & 7).

The impact of the collision of the two micro-continents of Meguma and Avalon 400 million years ago caused stresses that lasted for millions of years, pulling, pushing, and tearing rocks in the Earth’s crust, including the newly-formed granite in its subterranean chamber, the rock now partly exposed at the Peggy’s Cove location. The whole structure in Nova Scotia is known as the South Mountain Batholith. The natural fracture patterns in the rocks are almost grid-like when viewed from above, with cracks apparent on different scale levels. In images 9 and 10 below, the rocks seem almost to be in the process of twisting or tearing.

Since the granite first started to form hundreds of millions of years ago, the mountains and rocks above the batholith have been steadily but inexorably worn away by many natural processes including glaciation. Relatively recent periods of glaciation (20,000 years ago) have resulted in the smoothly sculpted appearance of the Peggy’s Cove outcrops.

Pre-existing cracks in the rocks were also widened out by the passage of the one kilometre thick ice sheet over them (see for example pictures 15, 12, & 2 in the gallery below). The rock masses are themselves part of a feature known as roche moutonéés. In these photographs it is mostly the gentle sloping land-ward faces that are shown. The steep, seaward facing sides are over the ridges and out of sight. Hundreds of erratic boulders are spread over the sparsely vegetated countryside around Peggy’s Cove.

Altogether a fascinating and beautiful place!

Click here for more general interest photographs of the quaint fishing village of Peggy’s Cove.

COPYRIGHT JESSICA WINDER 2013

All rights reserved

Pebbles from Chesil Cove, Portland

Chesil Beach pebbles: Dry pebbles from Chesil Cove, Portland, Dorset, UK - part of the Jurassic Coast

Dry pebbles from Chesil Cove, Portland, Dorset, UK – part of the Jurassic Coast World Heritage Site (1)

Chesil Beach is a great natural phenomenon – a huge shingle bank extending from Chesil Cove near Portland, Dorset, UK,  at its eastern end to Bridport in the west. Millions of tons of pebbles but it is strictly forbidden to remove any pebbles from the beach as they are now thought to be a basically unrenewable resource.   

Chesil Beach: View looking westwards along Chesil Bank from just below the promenade at Chesil Cove, Portland, Dorset, UK - part of the Jurassic Coast (2)

View looking westwards along Chesil Bank from just below the promenade at Chesil Cove, Portland, Dorset, UK – part of the Jurassic Coast World Heritage Site (2)

It is difficult to capture in a photograph the sheer scale of this spectacular feature which extends for as far as the eye can see.

At first sight, the pebbles all look more or less the same, especially when they are dry and the surfaces are dull. However, once the stones become wet, their true colour, beauty and variety is revealed. So it is well worth going right down to the water’s edge to look at them more closely.  

Pebbles on Chesil Beach: Large wet pebbles from the easternmost end of Chesil Cove, Portland, Dorset, UK - part of the Jurassic Coast (3)

Large wet pebbles from the easternmost end of Chesil Cove, Portland, Dorset, UK – part of the Jurassic Coast (3)

The pebbles of of the great Chesil Beach have been size sorted by natural processes so that the largest ones occur at the easternmost end of the shingle bank in Chesil Cove below the promenade. In the picture below you can see the size of the pebbles at this point relative to my walking pole.    

Chesil Beach pebbles: Larger sized pebbles from the eastern end of Chesil Beach at Chesil Cove, Portland, Dorset, UK - part of the Jurassic Coast (4)

Larger sized pebbles from the eastern end of Chesil Beach at Chesil Cove, Portland, Dorset, UK – part of the Jurassic Coast (4) 

However, a mile further on as you walk along the top of the shingle ridge from Chesil Cove to Ferry Bridge on the causeway, the size of the pebbles is considerably smaller as you can see in the picture below. By the time you get to Cogden Beach and Burton Bradstock, the pebbles on the western tail of the shingle bank are just about pea size.  

Pebbles on Chesil Beach: Medium sized pebbles from the top of the shingle bank of Chesil Beach, one mile west of Chesil Cove, Portland, Dorset, UK - part of the Jurassic Coast (5)

Medium sized pebbles from the top of the shingle bank of Chesil Beach, one mile west of Chesil Cove, Portland, Dorset, UK (5)

Looking at the wet pebbles, the different varieties can clearly be seen. The difficult part is identifying what sort of mineral or stone gives rise to the different colours and shapes.  However, 98% of them will be either flint or chert. I have compiled a gallery (below) of a selection of the pebble types that I saw washed by the surf at Chesil Cove.

As a very rough guide to recognising the some of the various sorts of pebbles, here is a list of the possibilities for each kind of colour – but remember that there is a greater chance than 9 out of 10 that a pebble will be just flint or chert:

White – could be quartzite from Triassic pebble beds at Budleigh Salterton in Devon; white outer layer – flint; white to grey Upper Greensand Chert; or bluish white chalcedony in Upper Greensand Chert.

Yellowish or yellow-brown  – could be quartzite containing iron oxides from the Triassic pebble beds at Budleigh Salterton – and this includes strange orange patterns on a lighter background.

Slightly brownish – could be flint from Upper Cretaceous chalk with a brownish outer layer.

Medium grey – could be flint from the Upper Cretaceous chalk.

Grey to brownish – could be flint from Cretaceous chalk.

Light bluish grey – could be chert from Upper Greensand

Black – could be chert from Portland Chert Series, chert with fossils from the Portland Roach; chert from the basal Purbeck formation; or tourmalinised rocks.

Pink or blue tinted translucent or clear – could be Lower Cretaceous, Upper Greensand chert.

Reddish or purple or liver-coloured – could be quartzite from Triassic pebble beds at Budleigh Salterton in Devon; porphyry from Permian breccia of Dawlish in Devon; or red chert from Cornish Palaeozoic rocks.

Mixed colours – could be vein quartz from Cornish Palaeozoic rocks; porphyry which has a combination of crystals and matrix with white, cream, greyish brown, and pink/purple parts; or granite with coarse grains of pink or white feldspar, grey quartz and black minerals such as mica.

Fossiliferous – could be chert from Portland series with fossil shells; flints with fossil sea urchins or cavities left by fossil sponges; or silicified shelly limestone from the Purbeck Beds.

If you are keen to learn more about the pebbles of Chesil Bank, you can do no better than consult Ian West’s web pages on Chesil Beach – Pebbles.

[I thank Dr West to whose work I have referred in order to try and identify the types of pebble and make the information more accessible to the average reader of the posting – but any mistakes or misunderstandings that may appear in what I have written are mine alone].

Revision of a post first published 20 May 2009

COPYRIGHT JESSICA WINDER 2011

All Rights Reserved