Terranes started to collide with the western edge of North America in the Mississippian age (approximately 350 million years ago), causing the Antler orogeny. During the last half of the Mesozoic Era, much of today's California, British Columbia, Oregon, and Washington were added to North America. Western North America suffered the effects of repeated collision as the Kula and Farallon plates sank beneath the continental edge. Slivers of continental crust, carried along by subducting ocean plates, were swept into the subduction zone and scraped onto North America's western edge.