The Rusty Mountain and Bonnet Plume Pass areas are underlain by sedimentary strata of the Paleoproterozoic Gillespie Lake Group, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Gabbro and diorite of the ca. 1.38 Ga Hart River sills intrude the Gillespie Lake Group. A swarm of east-west striking, mafic dikes intrude strata as young as the Tonian Hematite Creek Group. The main structures in the area include a steeply dipping axial-planar cleavage and upright, gently plunging folds. The northwestern part of the map area is dominated by northeast-southwest oriented structures that affected the ca. 1 Ga Hematite Creek Group, but predate deposition of the ca. 780 Ma Callison Lake Formation (Windermere Supergroup). In the southern and northeastern parts of the map area, the structures are parallel to and likely related to younger Mesozoic–Cenozoic structures.