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Somerset Street, Saint John

Anyone who has travelled from Highway 1 to the Saint John Regional Hospital or the University of New Brunswick Saint John campus along Somerset Avenue has crossed a major geological boundary. The street passes through the Precambrian-Cambrian boundary in the oldest rocks of the Saint John Group. The Cambrian Period is an exciting time in the geologic history of the earth because it is the time when the ancestors of most animals first appear in the fossil record. So many fossils appear for the first time that it is sometimes called the ‘Cambrian Explosion’ of life. Few fossils are found in the Cambrian rocks along Somerset Street. In other areas these rocks have trace fossils, winding trails left by snails, trilobites and brachiopods.

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Driving backwards (north) though time you pass the Hanford Brook Formation with the oldest trilobites, an ancient beach called the Glen Falls Formation, the red and grey rocks of the Ratcliffe Brook Formation with its trace fossils and reddish-purple rocks of the Seely Beach Formation where you cross into the Precambrian. Rocks from the late Precambrian McBrien Lake Formation are found all the way to the top of the hill. These are rough and blocky green volcanic rocks called dacite formed from molten magma near the end of the Precambrian, about 554 million years ago. At Magazine Street you have left the Caledonia Terrane and are now on the Brookville Terrane.