The Acadian orogeny is a middle Paleozoic mountain building event (orogeny), especially in the northern Appalachians, between New York and Newfoundland. The Acadian orogeny most greatly affected the Northern Appalachian region (New England northeastward into the Gaspé region of Canada). The Acadian orogeny should not be regarded as a single tectonic event, but rather as an orogenic era. It spanned a period of about 50 million years, from 375 to 325 million years ago. In Gaspé and adjacent areas, its climax is dated as early in the Late Devonian, but deformational, plutonic, and metamorphic events extended into Early Mississippian time. During the course of the orogeny, older rocks were deformed and metamorphosed, and new faults formed and older faults were reactivated.
It was roughly contemporaneous with the Bretonic phase of the Variscan orogeny of Europe, with metamorphic events in southwestern Texas and northern Mexico, and with the Antler orogeny of the Great Basin.
The cause of this great period of deformation is a result of the plate-docking of a small continental landmass called Avalonia (named after the Avalon Peninsula of Newfoundland). The docking of Avalonia onto the composite margin of Ganderia and Laurentia resulted in the closing of a portion of the Rheic Ocean. 
Avalonia was gradually torn apart as plate tectonic forces accreted the landmass onto the edge of the larger North American continent. Today, portions of the ancient Avalonia landmass occur in scattered outcrop belts along the eastern margin of North America. One belt occurs in Newfoundland, another forms the bedrock of much of the coastal region of New England from eastern Connecticut to northern Maine.
A period of lithospheric thinning that followed the Acadian orogeny created volcanoes, such as the large Mount Pleasant Caldera in southwestern New Brunswick, Canada.
* Orogeny - 'mountain building'
* Dictionary of Geological Terms, 3rd. Edition,1984, Robert L. Bates and Julia A. Jackson, Eds., prepared by The American Geological Institute