The Boundary Mountains terrane is defined primarily by a sialic basement consisting of a distinctive suite of diamictites, which were metamorphosed in late Precambrian time to granofels, gneiss, and schist. These rocks make up the Chain Lakes massif, exposed in the Boundary Mountains along the southwestern part of the Maine–Québec border, and large blocks of similar lithology exposed in mélange of the St. Daniel Formation, Eastern Townships of Québec. Rocks of similar lithology and age stand out as megaclasts in ophiolitic mélange near the northwest margin of the Macquereau dome, southeastern Gaspé Peninsula. The cratonal basement of the Boundary Mountains terrane may extend from central or northern New Hampshire and northeastern Vermont roughly 1,000 km to the western part of the Gulf of St. Lawrence, southeast of Gaspé. Collectively, these basement rocks are unlike those composing the Grenville tectonic province of the Laurentian Shield, and unlike high-grade gneisses exposed in the Miramichi Highlands of New Brunswick and in lithotectonic assemblages of Avalonian aspect bordering the Gulf of Maine and the Bay of Fundy. The accretionary history of the Boundary Mountains terrane is believed to have begun in Middle to Late Cambrian time. It therefore may represent one of the earliest of accretionary events in the prolonged orogenic history of the northern Appalachians.

Two parallel mélange belts, the Hurricane Mountain and St. Daniel, of the Maine and Québec portions, respectively, of the northern Appalachians, are interpreted as suture zones that define the southeast and northwest margins of the Boundary Mountains Terrane. They are named for the predominant lithotectonic units in each belt—the Hurricane Mountain Formation, in the Lobster Mountain anticlinorium of Maine, and the St. Daniel Formation, which crops out along the southeast margin of the Baie Verte–Brompton line in Québec.

The tectonic history of the Hurricane Mountain mélange belt is interpreted as expressing the amalgamation, during Late Cambrian to Early Ordovician time, of the Boundary Mountains terrane to a second terrane on its southeastern margin, probably the Gander. Sparse paleontologic and isotopic ages along the Hurricane Mountain belt indicate that suturing progressed from present-day southwest to northeast, along an ensimatic convergent plate boundary. Volcanogenic flysch deposits of the Dead River Formation, overlying the Hurricane Mountain Formation to the southeast, are believed to have formed in a forearc-basin environment. Polarity of subduction is inferred to have been toward present-day southeast. This diachronous event provides a tectonic driving mechanism, in time and space, for the Penobscottian orogeny. The Penobscottian event preceded the Taconian collision of the composite Boundary Mountains–Gander terrane to the Laurentian (North American) margin. Amalgamation of individual terranes, therefore, in this part of the northern Appalachians, did not proceed in a regular, craton-outward succession.