Absaroka Sequence - A sequence of Permian-Pennsylvanian sediments bounded by both above and below by a regional unconformity and recording an episode of marine transgression over an eroded surface, full flood level of inundation, and regression from the craton.

Acadian Orogeny - An episode of mountain building in the northern Appalachians during the Devonian Period.

Acanthodians - The earliest known vertebrates(fishes) with a movable, well-developed lower jaw, or mandible; hence, the first jawed fishes.

Accretionary terrane - A block of continetal crust having fault boundaries that is geologically distict from surrounding terranes.