Mt. Monadnock, the dominant physiographic feature of the Monadnock region, rises 2000 feet above the New England Upland in southwestern New Hampshire and is the type example of a monadnock. The region is underlain by metamorphic and plutonic rocks that range in age from Ordovician (?) to late Devonian (?).
The metamorphic rocks of the area belong to four formations. The Ordovician (?) Ammonoosuc volcanics, consisting of light-colored, fine-grained biotite gneiss and dark amphibolite, attain a maximum thickness of 600 feet in this region. The Ordovician (?) Partridge formation, chiefly mica schist, is not more than 50 feet thick in this area. The Silurian Clough quartzite here attains a maximum thickness of 100 feet. Much of the area is underlain by the lower Devonian Littleton formation which is 17,000 feet thick in the Monadnock area. The Littleton formation can be divided into several members. A lower member, 5000 feet thick, is composed of mica schist, impure quartzite, and gneiss. Above this is the rusty quartzite member which does not exceed 600 feet in thickness and is composed of pyritiferous rusty-weathering quartzite, actinolite granulite, quartz-biotite schist, and mica schist. It has proved to be an indispensable tool in unraveling the stratigraphy and structure. Above the rusty quartzite member is the middle member, 5400 feet thick, characterized by excellent bedding and composed of mica schist, sillimanite schist, sillimanite-garnet schist, and impure quartzite. Mt. Monadnock is held up by this resistant middle member of the Littleton formation. The upper member of the Littleton formation, about 6000 feet thick, is composed primarily of a massive, rusty-weathering gneiss.
Metamorphism is high grade throughout the area, although some of the rocks contain retrograde minerals typical of lower metamorphic zones.
Half of the Monadnock region is underlain by plutonic rocks. The oldest intrusive rocks are granodiorite and quartz diorite which belong to the Oliverian magma series of Devonian (?) age. The intrusives of the New Hampshire magma series—the Kinsman quartz monzonite, the Spaulding quartz diorite, and the Concord granite—are late Devonian (?). Pegmatites are associated with each of the intrusives.
The east limb of the Swanzey dome occupies the west half of the area, and the Monadnock syncline is the most prominent structural feature in the central part of the area. The rusty quartzite member zigzags across the central part of the region in a series of anticlines and synclines and is the key horizon for determining the structure and thickness of the middle part of the area. The northern part of the region is made up of gneiss; the southern part of the area is so thickly covered by glacial drift that the structure is difficult to determine.
The Oliverian magma series, exposed along the western margin of the Monadnock region, appears to be part of a large concordant sheet injected near the top of the Ammonoosuc volcanics. The Kinsman quartz monzonite, apparently concordant in other parts of New Hampshire, is cross-cutting and of batholithic proportions in the northeastern part of the Monadnock region, but elsewhere in the area occurs as small concordant and discordant bodies. The Spaulding quartz diorite forms sill-like masses as well as discordant bodies; it makes up the core of an anticline in the Stone Pond region. The Concord granite is cross-cutting.
The region was folded in middle or late Devonian time. Intrusion of the Oliverian magma series preceded the folding. The New Hampshire magma Series was intruded after the sediments were folded, but before deformation had ceased. Late faulting following the deformation and igneous intrusion is indicated by the presence of silicified zones.