One of the most noteworthy aspects of Newfoundland geology is a result of the constant movement of tectonic plates. Approximately 500 million years ago the action of these plates forced parts of the oceanic crust beneath the Iapetus Ocean up and over the eastern margin of the North American plate. Sections of oceanic crust which overlie continental crust are known as ophiolites. Gros Morne National Park was designated a UNESCO World Heritage Site because it is one of the best places in the world to see the effects of plate tectonics and one of the few places where rocks formed at the Mohorovicic Discontinuity between the crust and the upper mantle of the Earth can be seen.