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 Guernsaise, 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.

The Landscape of Inishowen

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The Landscape of Inishowen

The beautiful countryside around Inishowen is featured in many articles and fantastic images by Aidymcglynn: Landscape, hillwalking and nature photography around Ireland. Adrian’s photographs of the hills, beaches, and seascapes most effectively capture and reflect the incredibly interesting geology of the region. It is the shores around Inishowen that are the source of the wonderful beach stones collected and photographed by Noel Tweedie which were mentioned in the earlier posting.

Irish Beach Stones

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Irish Beach Stones

If you are as fascinated by beach stones as I am, you will definitely enjoy looking at the new web site by Noel Tweedie at The 365 Beach Stone Exhibition where he has amassed a great collection of photographs and artwork showing amazing beach stones from the Inishowen area in the north of Ireland. His images reflect the incredible geology of the area.

Rocks at Presqu’ile

Phyllite rock face on the Cabot Trail in Cape Breton Island

The Cabot Trail road leading to Presqu'ile and Pillar Rock in Cape Breton Island, Nova Scotia, Canada.Presqu’ile means “almost an island” and it refers to a narrow stretch of coastline just off the Cabot Trail in Cape Breton Island, Nova Scotia in Canada. It is nearly separated from the mainland by a long narrow lake. The road passes first along the eastern lake shore before crossing to the western shore; and just on the bend is where a track cuts down to the of the shore of Presqu’ile.  Parallel fault lines run along each side of the lake and one of these extends along the beach between the sea stack Pillar Rock and the mainland, where it has been responsible for interesting changes to the rocks.

Three different rock types originating in different geological periods lie incongruously side by side where they have been brought together by major faulting. Most noticeable is the phyllite rock that forms expansive, pale, gleaming surfaces beneath the highway and extending seawards. This is a metamorphic rock that started life as muddy sediment accumulating late in the Ediacaran or early in the Cambrian period (about 550 to 509 million years ago) on the margin of the ancient micro-continent of Ganderia. It was subsequently converted to shale and, when Ganderia collided with Laurentia in the Silurian period (443 to 418 million years ago), was buried by earth movements at a depth of about 8 kilometres and baked by temperatures as high as 300 degrees centigrade. This resulted in its deformation into phyllite by a realignment of the crystals. It was deformed again when Avalonia collided with Ganderia in the Devonian period (418 to 360 million years ago). Veins of quartz and calcite are common in the phyllite.

The black basalt of the sea stack Pillar Rock, lying just off shore from the phyllite cliffs and separated from them by a fault line, was extruded by volcanic activity in the Devonian period. Looking north-east along the shore, the cliffs are composed of sandstones from the Carboniferous period (360 to 300 mya). This odd juxtaposition of rocks from different periods is (I think) due to thrust faulting.

Mechanical digger moving granite boulders for coastal rip-rap sea defence at Presqu'ileThe weakened area of the fault line is reinforced against erosion by wave action by massive rip-rap boulders of granite obtained from Neil’s Harbour further along the Cabot Trail. There were road maintenance works going on during May, and the activities of heavy plant being used to arrange the boulders on the beach prevented access to the site on my first attempt. The digger had gone when I revisited a few days later and the light proved much more favourable for taking photographs.

REFERENCES

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 84-89.

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.

Gabbro at Spur Bay

The rocks at Spur Bay in Guernsey, Channel Islands, are composed of igneous St Peter Port gabbro which was formed from 500 – 550 million years ago in the Cadomian phase of activity. This dark grey rock has characteristic large hornblende crystals that often occur in layers alternating with feldspar at this particular location. The distinct dark crystals also give this rock the common name of  “bird’s eye” gabbro. In fact the gabbro has quite a variable composition, texture and patterning within a relatively small area. Some of the natural patterns have been caused by the infill of later-forming cracks and fractures by different molten rocks such as aplite, and the inclusion of rock fragments within the aplite.

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 Guernsaise, ISBN 0 9518075 2 8, pp 40-42.

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 45-52.

Silurian Rocks at Arisaig, NS

Silurian rocks from Arisaig, Nova Scotia, Canada.

The Northumberland Strait shoreline of Arisaig Provincial Park in Nova Scotia, Canada, is described as one of the best sections of Silurian rock in the world. The strata are shales, sandstones, and siltstones from the Arisaig Group which was deposited in the early Silurian Period dating from about 443 to 424 million years ago.

I was fascinated by the way that some of the rocks were made up hundreds of extremely fine layers that were breaking up very easily. As far as I understand it, these darker shale layers were the result of deposits created in the coastal waters of the time by storm events rather than by tides or currents; and they are known as tempestites.

Hickman Hild and Barr (2015) say that the uninterrupted accumulation of fine-grained sediment during the Silurian Period, exposed here along a continuous 5 kilometre stretch, suggests that the area was tectonically quiet for at least 20 million years.

REFERENCES

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 50-53