The rocks in the vicinity of Lilliwig Creek are primarily upper Paleozoic greenschist-facies metasedimentary and metavolcanic rocks that have been intruded by granitic rocks that are either related to the Taylor Mountain batholith of Triassic age to the south or the Chicken pluton of Jurassic age that is at the head of the valley to the north (Foster, 1976; Werdon and others, 2001). These units have been cut by numerous high-angle faults. In the lower part of Lilliwig Creek, Tertiary gabbro is overlain by tuffaceous material and sparse shale. Quaternary colluvium and alluvium are present in the lower valley; they consist of gravel and lesser silt and sand overlain by muck. Placer gold mining has exposed Pleistocene mammalian fossils, including mammoth, horse, caribou, and bison (Pinney, 2001).