Bedrock outcrop and island geology

Bedrock outcrop and island geology

This rocky outcrop of bedrock at Maugher Beach is a bit unusual as it is the only location on McNabs Island where the underlying rock is visible at the surface.

Geologists identify this rock as thin-bedded metasandstone and metasiltstone near the top of the Goldenville Group, just below the transition to overlying Halifax slate. The sediments making up this rock deposited during the Cambrian Period, roughly 500 million years ago, more than 200 million years before the dinosaurs. Sand, silt and clay from the erosion of a group of mountains located to the present southeast, washed out to sea, where they were buried, compressed into stone, and then metamorphosed through greater pressure into metasandstone, metasiltstone and slate. The rocks were also deformed by folding. The ridge of one fold (the ‘Eastern Passage anticline’) runs east-west across McNabs Island more or less in line with the tombolo of ‘Hangman’s Beach’ and Maugher Beach extending toward the lighthouse (the Horse Shoe).

McNabs Island is composed of a series of glacial drumlins, elongated hills of glacial material (called ‘till’) formed by continental ice sheets during the most recent ice ages. The oldest till was laid down sometime between 70,000 and 30,000 years ago by ice originating in Quebec and New Brunswick. It is best observed in the cliffs below Fort McNab, where its greyish colour distinguishes it from the overlying Lawrencetown Till. This later glacial deposit was formed by southeastward-flowing ice approximately 21,000 to 18,000 years ago. The reddish colour of the Lawrencetown Till reflects the source of ice flowing over the red rocks of Prince Edward Island and the Minas Basin.