The Avalonia microcontinent, now a 2,500 km-long terrane on the SE margin of the Acadian–Caledonian orogen, is defined by a terminal Ediacaran–Ordovician cover sequence (cs) unconformable on a basement collage of Proterozoic blocks that accumulated on the Avalonian transform fault (Atf). Strike-slip activity along the Atf controlled deposition of a siliciclastic-dominated marine cover sequence with minor igneous activity. Stratigraphic re-evaluation assigns the Mira belt in Cape Breton Island to the inner–marginal Avalonian platform transition area, and the northern Antigonish Highlands (i.e., Bourinot belt and southernmost Bras d’Or “terrane”) to the marginal platform. A new 500.55 ± 0.10 Ma date (CA-IDTIMS) from the tuff-dominated Gregwa Formation from the Bourinot belt is bracketed by re-evaluated trilobite faunas and is a radioisotopic control point on the middle Middle Cambrian and upper Drumian Stage. The Gregwa Formation, a volcanic edifice similar to two others on the Avalonian outer platform (i.e., Northern Antigonish Highlands, N.S.; Beaver Harbour, N.B.) likely overlies the Dugald Formation with a ca. 1 Ma hiatus as Avalonian depositional sequence 8. Gregwa volcanics, part of the ca. 1 Ma-duration, trans-Avalonian Manuels River–Nant-y-Big volcanic interval (new), are ca. 8 Ma younger than the late Early Cambrian Waites Lane–Eskasoni volcanic interval (new) in Maritime Canada. These two volcanic intervals emphasize Avalonian unity from the terminal Ediacaran. They are additional evidence that areas referred to the southern margin “Ganderia” (traditional Appalachian central mobile belt), as the Bourinot belt in Cape Breton, are Avalonian.