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The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile...

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