The area is underlain by the Mississippian to Permian Nahlin ultramafic body, part of the Cache Creek Complex. The Nahlin body is 100 kilometres long and up to 8 kilometres wide and is the largest alpine-type ultramafic in the Canadian Cordillera. On the southwest it is faulted against Lower Jurassic sedimentary rocks (Inklin Formation, Laberge Group, and Upper Triassic Stuhini Group volcanic rocks) and on the northeast against Upper Paleozoic rocks of the Cache Creek Complex. This ultramafic body consists of oceanic crustal ultramafic rocks consisting of peridotite, dunite, pyroxenite (in general serpentinized), and locally includes pods of nephrite jade and small bodies of listwanite, rodingite and talc. The rocks weather uniformly reddish brown and are generally devoid of vegetation.

The ultramafic body is largely composed of hard dark green to black peridotite consisting of fine-grained, partly serpentinized olivine, orthopyroxene, augite, and chrome spinel. The pyroxene forms discrete crystals and crystal clusters ranging from 0.3 to 1.3 centimetres across. The principal variation in the body is the degree of serpentinization. It is most intense along contacts and sheared or brecciated zones. Normally the unfoliated peridotite has a platy fabric, accentuated by streaks of serpentine, lenses of magnetite and slickensides along fractures. Serpentinization of the unsheared peridotite, containing pseudomorphs of olivine and pyroxene (bastite), has occurred in zones adjacent to sheared serpentinite, and in lenses surrounded by highly sheared serpentinite. Locally, the highly serpentinized rock contains a filigree of fine chrysotile veinlets usually less than 1 millimetre across. Thicker chrysotile veins contain brittle slip-fibre that are deemed as of no commercial value.

Serpentinization of unsheared peridotite can be observed in all stages, from the initial formation of platy antigorite to complete replacement of both olivine and pyroxene. Where serpentinization is complete, the original grain boundaries are preserved as veinlets of reticulated antigorite blades with disseminated magnetite grains or are converted to clear amorphous serpentine. Where shearing has accompanied serpentinization the rock is converted to a mass of feathery antigorite crystals with lensoid streaks of magnetite.