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.

6 Replies to “Fourchu Head Rocks Part 1”

  1. Thank you, Adrian. I don’t know exactly what minerals or rock compositions the colours in the pebbles represent. Some of the stones could be derived from the large rock fragments embedded in the solidified volcanic ash. They could also have originated in rocks brought to the area from much further afield by the passage of ice sheets.


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