The evidence for a continental margin on the southeastern side of the Appalachians, analogous to that recognized on the northwestern side, and the nature of the margin are described. Remnants of such a margin are probably best preserved in Newfoundland as a thick wedge of complexly deformed dominantly metasedimentary rocks (Gander Group) resting on a gneissic continental basement. These rocks separate the Avalon zone to the southeast from an oceanic terrane to the northwest, now represented by a Lower Ordovician ophiolite suite and overlying volcanic and sedimentary rocks. The Gander Group suffered orogenic deformation, plutonism, and metamorphism, probably in late Precambrian time. Unconformable relationships in the Avalon zone can also be interpreted as the result of this orogenic episode, termed the Ganderian orogeny. Probable equivalents of the Gander Group are locally preserved in northeastern Cape Breton Island and central New Brunswick, but farther southwest they have not been recognized, although continental basement is preserved. Late Precambrian granitic rocks may represent the Ganderian orogen in southeastern Connecticut and eastern Massachusetts, and possible equivalents of the Gander Group are present in Rhode Island and in the British Isles. These occurrences suggest a widespread erogenic episode or episodes of late Precambrian age on the southeastern side of the Caledonian-Appalachian belt. This orogenic activity is interpreted to have developed at and within a continental margin on the northwestern side of a continental mass and is distinct from later orogenic episodes within the system. The ocean, on one of whose margins this orogenic belt was developed, must have opened considerably before Cambrian time, in contrast to the present northwestern margin of the Appalachians.