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 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.
There were no other visitors on the dull day that we turned off the Cabot Trail to look at the Corney Brook shore in the western Cape Breton Highlands. There was very low cloud cover, and it began to rain after a while, but there were treasures to be found – at least if you are like me and are fascinated by beach stones. Three main rock types are found at Corney Brook. The oldest are Neoproterozoic-Ordovician granitic pluton rocks of the Bras D’Or Terrane. Ordovician-Silurian metasedimentary rocks of the Aspy Terrane are slightly younger. And red sandstones and conglomerates belonging to the Horton Group come from the Devonian to Carboniferous Period.
The stones on the beach include all three types and probably a lot more due to the glaciation of the area. I wish I could identify and tell you the exact composition of each photographed pebble, stone or boulder – but that is tough for an amateur to determine. There is a great variety of colour, pattern, and texture to the stones which look dull when dry but amazing when wet. They include igneous and metamorphosed rocks like granite, gneiss, schist as well as sedimentary rocks like sandstone. It is possible to see just how difficult it is to not only understand the texts but also to convert into straight forward language for the non-specialist reader from the following detailed description that I discovered about the Corney Brook schist by Jamieson et al. (1987).
Comey Brook schist (unit 3d)
Medium- to high-grade pelitic and semi-pelitic schists, with minor marble and psammite, occurring on the Cheticamp River, Corney Brook, the northeastern end of Jumping Brook, and the central highlands near Calumruadh and Coinneach brooks, are referred to here as the Corney Brook schist. This unit is equivalent to the “medium grade belt” of Craw (1984). Pelitic and semi-pelitic members of the unit characteristically contain coarse staurolite, biotite, and garnet porphyroblasts, with kyanite at the highest grade, in a phyllitic to schistose matrix. Medium- to high-grade marbles, quartzites, albite schists and hornblendite recognized in the Corney Brook area (Plint et al., 1986) have not yet been identified south of the Cheticamp River. Centimetre- to metre-scale compositional layering, folded by tight to isoclinal folds, is interpreted as transposed bedding. Based on bulk compositions and rare relict primary textures, the Corney Brook schist is interpreted to have formed as a suite of clastic sediments interlayerd with felsic tuffs and minor basic flows – that is, it appears to represent the higher grade equivalents of units 3a-3c.
The softer sandstone cliffs are being eroded back by the sea. This has implications for the ground higher up and the roadway further back from the shore. For this reason a sea defence structure has been emplaced to protect the base of the cliffs. This is a gabion made of wire cages full of large beach stones and boulders that are stacked up into a wall, positioned at the most vulnerable part of the shore.