Decided to visit Economy River Falls yesterday for some exercise, a nice drive, and 750 Ma Geology…
The Canadian Shield is a massive piece of the earths crust that has been geologically stable for at least 1000 million years. However, the margins of this plate have been the location of collisions, faults, and separations (rifts) with adjoining plates, resulting in hills and valleys as well as forming mountains and seas. With the passing of a few tens of million years, mountains would erode and fill in sea basins, resulting in more rocks which could then be deformed and faulted with the further movement of the plates.
The Appalachian (geologic) Province forms a very wide band of rocks immediately southeast of the Canadian Shield and the majority of the eastern seaboard of North America, from Texas all the way to Newfoundland! Geologists are fond of these rocks because they record direct evidence of at least three major plate collisions resulting in three major mountain-forming events (orogenies). This is where the rocks of Economy River Falls come in.
The rocks creating the falls were originally part of a mountain chain that formed a microcontinent in the seas between two plates that rifted apart around 900 million years ago. The plates then reversed their direction, closing the sea, and sandwiched the mountain chain some 730 million years ago. Geologists call the mountain chain Avalonia (after the Avalon peninsula in Newfoundland) and call the collision event the Acadian Orogeny. Today the rocks show deformation characteristic of major crustal convergence (folding, faulting, and stretching) resulting in gneisses and mylonites (pictured below).
The most amazing part of the story is that Avalonia rocks can be found on three continents! The original plates that squashed Avalonia have drifted apart and are now separated by the Atlantic Ocean. They carried pieces of Avalonia with them and can be found in Europe (Great Britain, Germany, and Spain) and in Africa (Morocco).
To learn more about the geology of Nova Scotia (including the Economy River Falls), pick up a copy of Geology of Nova Scotia at your local bookstore!