Rock Texture & Pattern at Black Brook Cove

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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.

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