The Acadian orogeny refers to the second of three major periods of geological uplift that created the Appalachian mountain chain. The active period began around 375 million years ago during the middle to late Devonian, and lasted 50 million years.
During the Middle Devonian Period, what would become the east coast of North America existed as a chain of islands separated by stretches of shallow sea. These islands had been uplifted during the earlier Taconic orogeny of the middle Ordovician Period. As North America collided with an ancient microcontinent known as Avalonia, the weight of the newly captured landmass folded the rock of eastern North America as if it were a viscous mass. This folding raised what would become the primordial Appalachian chain to a height that rivaled some of the tallest modern mountains. In the 325 million years since the uplift, much of the height of the Appalachians has eroded away, leaving the modern, much lower chain.
The collision between North America and Avalonia took place very slowly even by geological standards. The estimated rate of collision was approximately 5 kilometers per million years, which is only one-half to one-quarter the observed modern rate of drift in the North Atlantic region.