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Saint John: Reversing Rapids and Harbour Passage

Cambrian rocks of the Avalon Terrane can be seen on the downstream side of the bridges at the Reversing Rapids and along Harbour Passage. At the Reversing Rapids the St. John River rushes through the gorge, crosses the Precambrian-Cambrian terrane contact (Brookville-Caledonia terranes), and then turns east to follow the folded structure of the Cambrian rocks.

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A terrane is a fragment formed on one piece of the Earth’s crust (or tectonic plate) that breaks off and then becomes attached or welded to another plate. The terrane preserves its own distinctive geologic history, which is different from the crust to which it is welded. The place where a terrane attaches to another crust is often marked by a fault, along a line of weakness. The Caledonia Fault crosses the St. John River beneath the road bridge, separating the terranes. The two terranes have a very different fossil record. The Cambrian rocks have trilobites, sponges, brachiopods and small shelly fossils. The Precambrian rocks preserve stromatolites, fossil structures created by cyanobacteria.

Geologists have walked these rock outcrops and studied the geology here since the 1830s. Charles Lyell, a founder of the science of geology, visited this place in 1852 and saw the graphite mines while in operation below the bridge.

Harbour Passage trail follows a section of the harbour known as the ‘Straight Shore’. In the 1800s George Matthew and members of the Natural History Society of New Brunswick studied the geology along the shore and on Navy Island in the harbour. Layered Cambrian rocks along the ‘Straight Shore’ can be seen tilted almost vertically.