Fourchu Head Rocks Part 5

Part 5 of a series of photographs taken at Fourchu Head on Cape Breton Island in Nova Scotia, Canada, showing details of rocky outcrops and beach stones composed of very ancient Neoproterozoic volcanic rock. They are all made from volcanic ash that was spewed from the volcanoes together with shattered pieces of rock that broke away from the bedrock with the explosive force of the eruption. The rusty coloured streaks in some of the rocks are due to oxidising iron minerals. It is possible that rocks brought to the area from much further afield by ice sheets lie among the loose stones on the shore.

Fourchu Head Rocks Part 4

Part 4 of a series of photographs taken at Fourchu Head on Cape Breton Island in Nova Scotia, Canada, showing details of rocky outcrops and boulders composed of very ancient Neoproterozoic volcanic rock. They are all made from volcanic ash that was spewed from the volcanoes together with shattered pieces of rock that broke away from the bedrock with the explosive force of the explosion.

Fourchu Head Rocks Part 3

Part 3 of a series of photographs taken at Fourchu Head on Cape Breton Island in Nova Scotia, Canada, showing details of rocky outcrops and boulders composed of very ancient Neoproterozoic volcanic rock.

Fourchu Head Rocks Part 2

Part 2 of a series of photographs taken at Fourchu Head on Cape Breton Island in Nova Scotia, Canada, showing details of rocky outcrops, boulders, beach stones, and pebbles composed of very ancient Neoproterozoic volcanic rock.

Fourchu Head Rocks Part 1

Volcanic rocks belonging to the Fourchu and Main-à-Dieu Sequences can easily be seen on the coastline to the north and south of Louisbourg in Cape Breton Island, Nova Scotia, Canada. They are around 570 million years old and are part of a complex of Late Precambrian to Early Ordovician rocks largely resulting from the activities of a series of volcanoes that erupted on the Avalonia terrane at the margin of Gondwana.

The volcanic rocks at Fourchu Head are similar in age and type to those at Kennington Cove and Rochefort Point, including pyroclastic rocks composed of fine fragments and particles like ash and crystal tuffs. Additionally, the tuffs contain some very large pyroclastic blocks violently ejected from vents during one of the eruptions. The pebbles derived from the rocks at this site demonstrate an amazing range of colours in every shade of purple, red and green which are particularly evident on wet days.

This is the first of several posts featuring the fascinating array of colours, patterns, and textures in these rocky outcrops, boulders, and pebbles at Fourchu Head.

[We stayed at the most excellent Louisbourg Harbour Inn while we explored this part of Cape Breton Island.]

USEFUL REFERENCES

Atlantic Geoscience Society (2001) The Last Billion Years – A Geological History of the Maritime Provinces of Canada, Nimbus Publishing, Halifax, Nova Scotia, ISBN 1-55109-351-0.

Barr, S. M. (1993) Geochemistry and tectonic setting of late Precambrian volcanic and plutonic rocks in southeastern Cape Breton Island, Nova Scotia. Canadian Journal of Earth Sciences, Volume 30, 1147-1154.

Canadian Federation of Earth Sciences (2014) Four Billion Years and Counting – Canada’s Geological Heritage, Nimbus Publishing and Canadian Federation of Earth Sciences, ISBN 978-1-55109-996-5.

Donohoe, H. V. Jnr, White, C. E., Raeside, R. P. and Fisher, B. E, (2005) Geological Highway Map of Nova Scotia, 3rd 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. ISBN978-1-927099-43-8

Keppie, J. D., Dostal, J., and Murphy, J. B. (1979) Petrology of the Late Precambrian Fourchu Group in the Louisbourg Area, Cape Breton Island, Paper 79-1, Nova Scotia Department of Mines & Energy.

Rocks at Dunquin on the Dingle Peninsula

The Blasket Islands are deserted now but at one time the small pier at Dunquin was a lifeline for the small island community. It is hard to imagine the hardships of their existence and the way they would have navigated in all weathers across the short stretch of water to the mainland of the Dingle peninsula in tarred canvas-covered open boats called curraghs. For the islanders wanting to buy or sell goods, needing a doctor, having to attend church, confession, christenings, weddings, or funerals, or to visit mainland friends and family, Dunquin was an important landing place. The very basic, even primitive, life of the islanders is movingly and simply told in The Islandman by Tomás O’Crohan  who lived and died on Great Blasket Island (1856 to 1937). Nowadays, it seems to be mostly small boats that launch from the pier to ferry tourists to the uninhabited islands .

Dunquin harbour not only has this important historical association but it is also a noteworthy geological location. Walking down the steep, zig-zag path from the stone-walled green fields above to the beach and pier below, there are great views of the cliffs to the north and south of the harbour. It is a transition zone between two major geological periods – where a predominantly marine environment changed to a mainly terrestrial one due to vascillating sea level relative to the land. It is the location where yellow marine siltstones belonging to the Drom Point Formation of the Silurian Dunquin Group lie next to the reddish, purplish, and greenish sandstone strata of the Silurian/Devonian Dingle Group which are terrestrial in origin.

The cliff faces seem to be striped in contrasting subtle hues. The rock layers are steeply angled now following earth movements over the many millions of years since they were originally laid down in a horizontal position. Odd circular or spherical formations can be seen in some layers. The bedding plane of one outcrop next to the pier has a roughly polygonal pattern of drying mud cracks preserved in the stone. Curving veins of quartz cut across the strata to the south of the harbour.

Altogether a very good place for rock enthusiasts and well worth a return visit. Next time I would like to take a boat trip to the Blasket Islands where (on Inishvickillane at least) the rocks are mainly volcanic tuffs and lavas.

REFERENCES

O’Crohan, Tomás,  1937 The Islandman, Oxford University Press, ISBN 978-0-19-281233-9, re-issued 2000.

Horne, Ralph R., 1976, Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, Minister for Industry and Energy, Geological Survey Office. Reprinted 1999.

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.