Seventy per cent of the bedrock in the Mt. Cube area consists of metamorphic rocks of sedimentary and volcanic origin, ranging from middle Ordovician (?) to early Devonian. The total thickness is about 16,000 feet. Two of the oldest stratigraphic units, the Orfordville formation and the Piermont member of the Albee formation, are described in detail because this is the type locality.
The Orfordville formation, presumably middle Ordovician, is 5000 feet thick, and consists of interbedded black schist and volcanic rocks. The schist represents carbonaceous marine mud and sand. The volcanics represent metamorphosed basalt flows and pyroclastics (now amphibolite) with some rhyolite, quartz latite, and dacite (now biotite gneiss). The Piermont member, 1000 feet thick at maximum and locally absent, comprises most of the lower part of the Albee formation, and consists of aluminous, carbonaceous schist interbedded with noncarbonaceous quartzmica schist, quartzite, and fine conglomerate.
About 30 per cent of the bedrock consists of gneissic igneous rocks, ranging from granite to quartz diorite. Most of them belong to the late Devonian Oliverian and New Hampshire magma series.
The stratified rocks were severely folded, broken by thrust faults, and metamorphosed during the late Devonian. Thrusting and overturning of folds is toward the east-southeast, except in the southeastern part of the area, where it is toward the west. The concordant, domical intrusives of the Oliverian magma series were active during the later stages of the folding, and represent the tops of syntectonic phacoliths, rather than pretectonic laccoliths.
The metamorphic and tectonic relationships of minerals in the middle-grade zone of metamorphism are briefly discussed, and an area of wholesale retrograde metamorphism associated with a major thrust fault is described.