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 Trabeg on the Dingle Peninsula

Close-up of the Devonian conglomerate at Trabeg on the Dingle Peninsula

The sand looks black from a distance as you descend to the shore at Trá Chathail near An Trá Bheag (Short Strand) – otherwise known as Trabeg. The path cuts down deep through the stratified red rocks to get to the beach which is strewn with pebbles, mostly shades of red, maroon, green, grey, and white.

Trabeg is on the south coast of the Dingle Peninsula in Ireland, and is the “type section” of the Trabeg Conglomerate Formation which is exposed in the cliffs on the beach. This is place where that particular rock type was first described. The rock layers constitute part of the Dingle Group and were formed in the Devonian period between 345 and 395 million years ago. The conglomerates are composed of fairly well rounded pebbles of red sandstones and mudstones, with white vein quartz and chert. A few pebbles of volcanic rock and of grey limestone are also present.

The way in which the conglomerate rock has formed from the mass movement and subsequent accumulation of debris from terrestrial locations during, for example, river flood events, means that the pebbles are derived from a wide area covering many different geological types. The pebble beds or conglomerates are inter-bedded with layers of red sandstones and mudstones, the finer sediments of which were deposited normally by rivers during non-storm/flood times. The alternating layers are now tilted from the original horizontal orientation in which they were first deposited, and are clear to see dipping south at about 70 degrees.

As the cliffs at Trá Chathail are worn away by the action of waves and weathering, the pebbles contained in the conglomerate matrix are freed up and remain the shore below – an instant pebble beach. Added to these are pieces of other rock or matrix that became rounded into pebbles after they arrived on the beach. Some pebbles and rocks may have been transported by wave action from further along the coast were the geology is quite different: from the Eask Formation, West Cork Sandstone, Bulls Head Formation, and the earlier Silurian rocks of the Dunquin Group.

REFERENCE

Horne, Ralph R. (1976) Geological Guide to the Dingle Peninsula, Geological Survey of Ireland Guide Series No. 1, reprinted 1999.

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.