The area is underlain by the Pennsylvanian to Permian Nahlin ultramafic body which is a large belt of ultramafic bodies that parallel the southwestern side of the Atlin Horst. It forms two long, narrow prongs that converge in an acute angle at Nahlin Mountain. The longer axis of the body trends west-northwest from Nahlin Mountain to Peridotite Peak, paralleling the Nahlin fault.

The ultramafics consist of dark green to black peridotite with discrete crystals and crystal clusters of pyroxene ranging from 0.3 to 1.3 centimetres across. The principal variation in the body is the degree of serpentinization which is most intense along contacts and sheared or brecciated zones.

Locally, the highly serpentinized peridotite contains a filigree of fine chrysotile veinlets, usually less than 1 millimetre across. Thicker chrysotile veins, containing slip fibre up to 2 centimetres across is reported as commercial quality fibre by prospectors from the head of Yeth Creek. Generally, most of the asbestos occurrences host short, brittle fibre of little commercial value.