Rb-Sr whole rock dating indicates that the oldest rocks in the Needle Falls area are the western granitic rocks, with the possible exception of some inclusions of mafic metamorphic rocks. The western grantitic rocks (~2300 m.y. old or older) are mainly quartz monzonite and appear to be of igneous origin. The available evidence suggests that they are unconformably overlain by two groups of metamorphic rocks, the "older metamorphic rocks" and the "cordierite-garnet rocks." The "older metamorphic rocks" are unconformably overlain by the Meyers Lake Group. The relationship of the "older metamorphic rocks" to the "cordierite-garnet rocks" is uncertain. Migmatitic and metasomatised rocks derived from the metamorphic rocks are of widespread occurrence. Younge intrusive rocks include epidiorite, hornblende quartz diorite, the eastern granitic rocks (mainly biotite quartz diorite and granodiorite), pegmatite, and quartz veins. No consolidated rocks in the area are known to be younger than the Hudsonian orogeny, which it dated at ~1750 m.y. by K-Ar dating of micas.

The "older metamorphic rocks" were probably deposited in an intracratonic basin or geosyncline. They consist of metamorphosed arkosic wacke, greywacke, and basic to acidic volcanic (?) rocks. The Meyers Lake Group consists of a basal quartz-pebble meta-conglomerate, overlain by metamorphosed quartz arenite, feldspathic arenite, and pelites. This group was deposited during a period of relative stability in the area. The "cordierite-garnet rocks" consist of metamorphosed pelites, arkose, and minor calc-silicate rocks within the thesis area.

Most of the metamorphic rocks belong to the andalusite-sillimanite series, amphibolite facies, of Miyashiro (1961). Rocks belonging to the granulite facies, hornblende granulite subfacies, occur as inclusions in granitic rocks. A minimum temperature of about 520°C and maximum pressure of about 8 kilobars are suggested for rocks belonging to mineral zone C of the amphibolite facies. A maximum depth of burial of about 28 kilometers and a minimum geothermal gradient of 22°C/km have been calculated.

The metamorphic rocks occur in two main fold belts. In the eastern fold belt the fold axes plunge gently to the north-northeast and south-southwest. The limbs are nearly vertical and the folds are isoclinal or nearly isoclinal. There may be some cross-folding. The folds in the western fold belt are probably similar in style and orientation. Northeasterly longitudinal and north-northwesterly transverse faults occur.

The "cordierite-garnet rocks" can be traced north-northeast for some 400 miles, and the "older metamorphic rocks" are traceable north-northeast for at least 75 miles, a feature of interest in regional correlation.

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