Cartoon showing the situation prior to the Taconian Orogeny, with subduction of oceanic lithosphere on the leading edge of the ancestral North American plate beneath an overriding oceanic plate. The resulting volcanic island arc draws ever closer, with an accretionary wedge forming at the trench where subduction begins. North America's margin shows as-yet-horizontal sedimentary layers (including shallow-water carbonates) that have formed in an epeiric sea.