Rocks at Presqu’ile

Presqu’ile means “almost an island” and it refers to a narrow stretch of coastline just off the Cabot Trail in Cape Breton Island, Nova Scotia in Canada. It is nearly separated from the mainland by a long narrow lake. The road passes first along the eastern lake shore before crossing to the western shore; and just on the bend is where a track cuts down to the of the shore of Presqu’ile.  Parallel fault lines run along each side of the lake and one of these extends along the beach between the sea stack Pillar Rock and the mainland, where it has been responsible for interesting changes to the rocks.

Three different rock types originating in different geological periods lie incongruously side by side where they have been brought together by major faulting. Most noticeable is the phyllite rock that forms expansive, pale, gleaming surfaces beneath the highway and extending seawards. This is a metamorphic rock that started life as muddy sediment accumulating late in the Ediacaran or early in the Cambrian period (about 550 to 509 million years ago) on the margin of the ancient micro-continent of Ganderia. It was subsequently converted to shale and, when Ganderia collided with Laurentia in the Silurian period (443 to 418 million years ago), was buried by earth movements at a depth of about 8 kilometres and baked by temperatures as high as 300 degrees centigrade. This resulted in its deformation into phyllite by a realignment of the crystals. It was deformed again when Avalonia collided with Ganderia in the Devonian period (418 to 360 million years ago). Veins of quartz and calcite are common in the phyllite.

The black basalt of the sea stack Pillar Rock, lying just off shore from the phyllite cliffs and separated from them by a fault line, was extruded by volcanic activity in the Devonian period. Looking north-east along the shore, the cliffs are composed of sandstones from the Carboniferous period (360 to 300 mya). This odd juxtaposition of rocks from different periods is (I think) due to thrust faulting.

The weakened area of the fault line is reinforced against erosion by wave action by massive rip-rap boulders of granite obtained from Neil’s Harbour further along the Cabot Trail. There were road maintenance works going on during May, and the activities of heavy plant being used to arrange the boulders on the beach prevented access to the site on my first attempt. The digger had gone when I revisited a few days later and the light proved much more favourable for taking photographs.

REFERENCES

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 84-89.

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.