I am increasingly aware of the number of coastal locations installing or improving sea defence structures. One of the most common protective mechanisms against erosion by the waves and sea ice is rip-rap. Rip-rap (also called riprap, shot rock, rock armour or rubble) is comprised of rock boulders or other materials like concrete. Commonly the rocks are granites or limestones. They can be local or imported rocks.
According to Wikipedia, riprap works by absorbing and deflecting the energy of waves before they reach the defended structure. The size and mass of the riprap material absorbs the impact energy of waves, while the gaps between the rocks trap and slow the flow of water, lessening its ability to erode soil or structures on the coast. The mass of riprap also provides protection against impact damage by ice or debris, which is particularly desirable for bridge supports and pilings.
Rip-rap provides a good opportunity to observe newly cut surfaces of rocks, a chance to study them up close, and to marvel at the range of rock colours, patterns, and textures. The waterfront rip-rap in Halifax includes many examples of the iron-rich meta-sedimentary bedrock that underlies the city, the newly-exposed surfaces of which are frequently coated in rust from the corroding action of air and moisture.
Most of the bedrock beneath the main waterfront area in Halifax Harbour belongs to the Cambrian to Ordovician Halifax Formation comprising slate and minor metasiltstone. The Halifax Formation has thinly bedded slate that was once mud in the deep ocean offshore of Gondwanaland 495 million years ago.
Adjacent to the Halifax Formation, to the north and southeast, lies the Late Neoproterozoic to Cambrian Goldenville Formation with metasandstone, minor metasiltstone and slate. While to the west are the Devonian South Mountain Batholith granites. All these local rock types would be ideal sources for the boulder rip-rap on the harbour’s edge.
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