The area lies near the contact between the granodioritic Oligocene Chilliwack batholith and the Custer Gneiss complex of uncertain age.

The Iron Mountain showing consists of oxidized iron ore in a deep gorge near the summit of Custer Ridge. The hostrock is sheared granodiorite. Opencuts exposed bands of limonitic, ochreous material lying between bands of solid ore. Three short adits were driven into the iron ore which is comprised mainly of oxidized pyrite and limonite. In 1915, samples were taken from two of the adits. A sample across 4.6 metres yielded 38.8 per cent iron, 1.3 per cent sulphur and nil phosphorous. Another chip sample across 7.6 metres yielded 40.8 per cent iron, 1.5 per cent sulphur and nil phosphorous (Minister of Mines Annual Report 1915, page 304).