Three Sisters formation is named for the prominent Three Sisters Peaks lying east of the headwaters of Sheep Creek, where the formation is well exposed. With the Quartzite Range formation it forms the highest peaks in the area along the summit of the Nelson (Quartzite) Range. Varies slightly in thickness along the strike, but on the whole is remarkably constant in thickness and composition. Lower 2000 ft is composed of massive, greenish gray grit or fine conglomerate, exhibits little evidence of bedding, and breaks down on the high summits into huge, angular blocks. Thin bed of slate occurs at about the middle of this part of the formation. Massive grits are succeeded by 450 ft of alternating beds of grit and white quartzite, and these by about 1700 ft of gray, gritty quartzites. An argillaceous bed occurs about the middle of the 1700 ft of gritty quartzites. A greenish gray boulder conglomerate, averaging about 100 ft in thickness, succeeds the gritty quartzites. It is a distinctive member easily traced across the map-area. The boulders are well rounded and fine grits overlie the conglomerate and form the uppermost member of the formation. Age is late Precambrian.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Three Sisters Formation is adopted following the usage of Walker (1934) in British Columbia. Name is derived from the Three Sisters Peaks, located south of Nelson, Kooteney district, British Columbia. Composed of quartzite, conglomeratic quartzite, conglomerate, phyllitic quartzite, and phyllite. Maximum thickness equals about 2100 m in its Sullivan Lake sequence. In its Abercrombie Mountain sequence, unit is conformably overlain by the Gypsy Quartzite (Cambrian) and underlain by the Monk Formation (Late Proterozoic); locally, contacts in other areas are faults. Unit is geographically extended into the report area at the expense of part of the Gypsy Quartzite. Age is considered to be Late Proterozoic on the basis of regional stratigraphic relations.

Source: GNU records (USGS DDS-6; Menlo GNULEX).