The Twin Sisters dunite body of northern Washington, an alpine-type ultramafic body, is upper-mantle material tectonically emplaced in the crust. Gravity and magnetic anomalies indicate that the body has a base at shallow depth. It is a flat, serpentine-sheathed pod of peridotite, 16 km by 6 km and 2 km thick, thickened and more highly serpentinized near its western margin. The pod is elongate parallel to a thrust fault, and a branch of the magnetic anomaly continues northwest along the thrust; this suggests emplacement by thrusting. Serpentine is more abundant at depth than in outcrop; this might explain late movement along near-vertical contacts if a pistonlike uplift accompanied expansion during serpentinization.