early-middle Ordovician (480 - 460 Ma) back-arc basin rocks in direct or faulted contact with rocks of the Laurentian shelf and slope-rise in the southern Appalachians suggests they were built on the margin of Laurentia, beyond the edge of the continental shelf-slope break.

The Ordovician Laurentian margin probably resembled that seen in the modern Sea of Japan, with the continental mainland separated from a volcanic arc by a narrow, "marginal" seaway.