Geology of Bird Islands

Nova Scotia’s abundant coastlines and rich geological history provide many places with significant geology. Therefore, we can see interesting geology in the places where we live, work, and play.

Lobster fishermen working around the Bird Islands in northern Cape Breton are able to see some unique geology as well as a diversity of sea birds.

Bird Islands

I had the opportunity to see the geology of the Bird Islands during a recent trip with a lobster fisherman, Kevin Squires. However, similar views are available with boat tours that operate out of Big Bras d’Or, Cape Breton, Nova Scotia.

The Bird Islands are famous for the opportunity to photograph sea birds, including Great Cormorants, Razorbills, Puffins, Eiders and many others.

Puffins on Hertford Island, June 2023.

The Bird Islands are approximately 4 km off the coast of Cape Dauphin. The two islands, Hertford and Ciboux, are each about 1 km long, with high cliffs where birds establish their nests. The birds are safe from predators and have easy access to bountiful fishing along the shore.

Sandstone Nesting Sites

The light brown bedrock that makes up the islands is sandstone from the Late Carboniferous approximately 320 million years ago. You can see similar rocks along other parts of the coastline near Sydney Mines.

It is interesting that Razorbills preferred to roost along the tall cliffs of the western side of Hertford Island. From the boat, the geology layers are clearly visible. You can see climbing ripples, and lenses of clay that preferentially erode to form small cavities. These pockets made by erosion are perfect for nesting.

Razorbills nesting in a cavity of the sandstone.

Geology for the Birds

When you look at a larger photo of the site where the Razorbills have established their roost, you can see the layers of sedimentary rocks.

Detail A – Below the roost, is a layer that has larger grains. This represents faster flowing water that could move larger sizes of sediment.

Detail B – The layers where the Razorbills have their roost has large oval shaped features, with larger grains but irregular boundaries of the layers. There appear to be large clasts of clay, which erode easily and form the initial voids that become small caves.

Detail C – Most of the layers in the image are horizontal. Some show ripple surfaces at the bottom edge. Others look like u-shaped depressions that would form in shallow channel bodies. Other areas with linear, ripple bedding show climbing ripples and dune cross-beds.

Future Work

The geological maps of Nova Scotia identify the geology of the Bird Islands as belonging to the Morien Group, Carboniferous age, 323 to 298 million years ago.

A more detailed analysis of the stratigraphic layers of the Bird Islands might narrow down within the Morien Group and provide interesting information for understanding the faults and geology of the region.

It may also be interesting to consider the unique features of the geology layers that might relate to the success of the islands as bird nesting sites. How do mud clasts in the sedimentary layers contribute to the development of cavities that are used by nesting birds?






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