The most common rocks in the Bellows Falls quadrangle of southwestern New Hampshire and adjacent Vermont are metamorphic, having been derived from sedimentary and volcanic formations that range in age from middle Ordovician (?) to lower Devonian. Although middle-grade metamorphism is characteristic of most of the quadrangle, a zone of high-grade metamorphism occurs along the east margin of the region, and areas of low-grade metamorphism occur in the northwest portion of the quadrangle. It is thus possible to study within one quadrangle all three grades of metamorphism. The metamorphism was dynamothermal, and there was no appreciable addition of material. The chemical composition of isomorphous minerals, such as plagioclase and biotite, depends upon both the composition of the original rock and the grade of metamorphism.
In general, the lithology of the stratified rocks is not, radically different from that to the north in previously mapped regions. But in the Devonian Littleton formation two white quartzite members and associated lime-silicate rocks appear, the latter derived from impure dolomites.
The principal folding is Acadian (late Devonian) and produced structural features that trend northeast-southwest. The Bronson Hill anticline, a major fold that can be traced 150 miles in New Hampshire, trends north-south a few miles east of the center of the quadrangle. West of this is the Walpole syncline, a broad open fold that is 6 miles across and is characterized by relatively gentle dips ranging from zero to 45 degrees. This syncline is complicated by a series of “cross folds” which trend west-northwesterly and are several miles in wave length. The northwest portion of the quadrangle is occupied by the steeply dipping west limb of the Charlestown syncline. In general, schistosity and bedding are parallel.
The Northey Hill thrust, a major fault that can be traced for at least 100 miles in western New Hampshire, separates the Walpole and Charlestown synclines from one another. It is pre-metamorphism and is now a high-angle fault along which the stratigraphic throw is approximately 10,000 feet. Four normal faults are of sufficient magnitude to show on the geological map. They trend north-south, and the stratigraphic throw along some of them exceeds 1000 feet.
As elsewhere in New Hampshire, the intrusion of thick concordant plutons was associated with the Acadian orogeny. A body of the Oliverian magma series was injected as a concordant sheet into the Ordovician (?) Ammonoosuc volcanics; probably originally horizontal, it is now exposed in the center of several north-trending anticlines developed subsequent to its intrusion. A body of Bethlehem gneiss was injected as a concordant sheet into the Devonian Littleton formation, probably during the last stages of the folding. Subsequent erosion has unroofed this body and isolated a western portion from the main pluton 6 miles to the east. Small bodies of granite intrude the schists with cross-cutting relations.