The Trout Valley Formation of Emsian–Eifelian age in Baxter State Park, Maine, consists of fluvial and coastal deposits that preserve early land plants (embryophytes). Seven facies are recognized and represent deposits of main river channels (Facies 1, 2), flood basin (Facies 4), storm-influenced nearshore shelf bars (Facies 3), a paleosol (Facies 5), and tidal flats and channels (Facies 6, 7). The majority of plant assemblages are preserved in siltstones and are allochthonous and parautochthonous, with only one autochthonous assemblage identified in the sequence above an apparent paleosol horizon. Taphonomic analysis reveals that plant material within allochthonous assemblages is highly fragmented, poorly preserved, and decayed. Plant material within parautochthonous assemblages shows evidence of minimal transport, is well preserved, and shows signs of biologic response after burial. The one autochthonous assemblage contains small root traces. Trimerophytes ( Psilophyton and Pertica quadrifaria ), rhyniophytes (cf. Taeniocrada ), and lycopods ( Drepanophycus and Kaulangiophyton ) are the most common taxa in estuarine environments. Psilophyton taxa, Pertica , cf. Taeniocrada , and Drepanophycus are found also in fluvial settings. The presence of tidal influence in deposits where parautochthonous and autochthonous assemblages occur shows that these plants occupied coastal-estuarine areas. However, the effects on the growth and colonization of plants of the physical conditions (e.g., salinity) that exist in these settings in the Early to Middle Devonian are unknown.

Metavolcanic rocks of the Winterville Formation from the prehnite-analcime subfacies of the prehnite-pumpellyite facies in north-central Aroostook County, Maine, contain an alteration assemblage including chlorite, chlorite/smectite (C/S), analcime, prehnite, and calcite. Field and laboratory study has identified areas where hydrothermal alteration has been pervasive in and around pillows. Compositional, crystal chemical, and structural variations in chlorite appear to be related to distance from this hydrothermal alteration. Samples were studied by whole-rock chemical analysis, electron microprobe analysis of individual mineral grains, X-ray powder diffraction of the clay fraction, and by computer modeling of diffraction patterns to determine the percentage of chlorite in interstratified C/S and to estimate the distribution of Fe and the size of coherent diffracting domains in pure chlorites. Whole-rock and pyroxene compositions suggest that the rocks have undergone Mg metasomatism. Modeling of X-ray diffraction data indicates that the percentage of chlorite in C/S increases to 100%, that Fe atoms become more equally distributed between octahedral sites in chlorite as it becomes more Fe-rich, and that diffracting domains grow larger with proximity to areas of more intense hydrothermal alteration. Analcime also increases near areas of hydrothermal alteration. The areal distribution of hydrothermal effects suggests that the alteration occurred as two separate events, or that two different thermal regimes were active concurrently.

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.

The Penobscot orogeny in the Appalachians, the Grampian orogeny in the British Isles, and the Finnmarkian orogeny in the Scandinavian Caledonides are Late Cambrian–Early Ordovician events that characterize terranes having similar pre- and post-deformation features. Together they record a major orogen-wide closing of the Cambrian Iapetus Ocean basin. The Penobscottian is the principal pre-Acadian deformation event in the Gander and related terranes of the Appalachians. The upper Proterozoic–Cambrian rocks deformed by it include subduction-related mélanges. In the northern Appalachians its timing is best constrained in northern Penobscot County, Maine, where polydeformed slate and sandstone (Grand Pitch Formation), dated Cambrian(?) by Oldhamia, are overlain by the singly deformed Arenig-age Shin Brook Formation. Basal conglomerate of the Shin Brook contains Grand Pitch clasts, and higher tuff contains Early Ordovician (late Arenig) Celtic province shelly fossils that indicate deposition around a volcanic island in cool waters of moderate to high latitude, remotely distant from contemporaneous warm equatorial waters of the North American (Laurentian) continental margin. The Grampian, in the Scottish–Irish orthotectonic Caledonides, deformed and metamorphosed miogeoclinal, upper Precambrian to lower Middle Cambrian Dalradian rocks that were largely derived from the Laurentian craton. An early tectonothermal phase, probably associated with subduction that produced blueschists, was followed by the main Barrovian metamorphism (510 to 480 Ma). Fossils in the Arenigian post-tectonic rocks of the South Mayo trough have strong North American affinities. Dalradian equivalents in the paratectonic Caledonides (Howth Peninsula, Ireland; Anglesey, Wales) suggest rifting of the Cambrian miogeocline, confirmed by the cool-water Celtic province Arenig shelly fauna of Anglesey. The Finnmarkian event deformed and metamorphosed rocks of the Late Proterozoic–Cambrian(?) Baltic continental-rise prism of Baltic provenance and obducted slabs of ophiolite. The minimum of its isotopic age range, 540 to 480 Ma, is confirmed by the Llanvirnian Otta serpentinite conglomerate that unconformably overlies an ophiolite remnant. Fossils from the conglomerate suggest cool-water Baltic and ocean-island affinities. The sequences deformed by these penecontemporaneous events record a complex history of the Late Proterozoic–Cambrian Iapetus Ocean, including miogeoclinal sedimentation on the margins of Laurentian, Baltic, and Armorican cratons, and the rifting and closing of intervening oceanic tracts. At the time of their deformation, these sedimentary sequences occupied the margins of a very large Early Ordovician Iapetus Ocean.