For reasons that are not clear to geologists, tectonic plates have moved in different directions over the history of the Earth, sometimes completely reversing their directions. The eastern edge of the North American plate has collided with other plates at least four times during the past billion years leading to mountain building events called orogenies. The Grenville mountains formed during the Grenville orogeny that took place approximately 1 billion years ago. Subsequent orogenies that influenced southern Appalachian geology were the Ordivician-Silurian Taconic, Devonian Acadian and Permian Alleganian. There were "quiet" times between the mountain building events when of erosion of the mountains and deposition in basins took place. The central and southern Appalachian Mountains were formed during the Alleganian Orogeny. During the Jurassic, the final break between the North America plate and others took place to form what has become the Atlantic Ocean.