The rocks in the vicinity of Otter River, in the west-central Piedmont of Virginia, consist mainly of ottrelite-graphite phyllites, micaceous schists, micaceous gneisses, hornblend gneisses, and a highly schistose zone of chlorite-tremolite schist.
The structural deformation of these rocks is shown to have occurred in three phases. The dominant foliation and its associated lineation were developed during a phase of flow folding. The foliation and lineation were subsequently deformed by a mid-phase of flexural folding which resulted in the development of a new cleavage and a crenulation lineation. A late-phase of concentric folds deformed, but did not alter appreciably the general orientation of, the pre-existing foliation and cleavage.
The gradual increase in metamorphic grade across the area is due to the progressive metamorphism of a supracrustal assemblage. This metamorphism was initiated during the phase of flow folding and its intensity is related to the level at which it took place in the earth's crust. The restriction of pegmatite bodies to a belt two miles wide is the result of preferential mobilization of the feldspathic constituents during and toward the end of the phase of flow folding. The three phases of deformation are here considered to have developed during a period of crustal disturbance affecting a supracrustal assemblage.