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