The Connecticut Lakes-Parmachenee area, in northernmost New Hampshire and northwesternmost Maine, is underlain principally by Lower to Middle Paleozoic clastic sediments, and felsic and mafic volcanic rocks, all of low metamorphic grade. The area includes a section of the entire northwest flank of the Boundary Mountains anticlinorium (the southeast flank of the Connecticut Valley-Gaspé synclinorium) and part of the axial region of the latter synclinorium. Approximately 23,000 feet of Ordovician, and perhaps some Cambrian, strata are represented. These include, from oldest to youngest, the Aziscohos, Albee, and Dixville formations, the last containing the coarse, clastic Magalloway Member in the Maine part of the area. These rocks were folded during the Taconic orogeny. Along the Taconic unconformity are very local, thin Upper Silurian limestones, and to the northwest lie about 20,000 feet of Devonian strata, constituting the Seboomook and overlying Frontenac formations. The Compton Formation, believed to be equivalent to the Seboomook across a synclinal axis, may appear at the northwest corner of the area.
The stratified rocks are intruded by a granodiorite stock of probable Taconic age, and by two granitic stocks and many microgranite lenses, a gabbro and a tonalite stock, many gabbro sills, and a few small serpentinites and peridotites, all of Acadian age.
Study of the minor structures shows that the Cambro-Ordovician rocks were deformed and strengthened by recrystallization at low temperatures during the Taconic orogeny, but the principal metamorphism and regional deformation were Acadian.