British Columbia. Mountain
Headwaters of Geike Creek
52°42’00” N 118°22’00” W — Map 83D/9 — Google — GeoHack
Earliest known reference to this name is 1921 (Boundary Survey).
Name officially adopted in 1951. Official in BC — Canada
Headwaters of Geike Creek
52°42’00” N 118°22’00” W — Map 83D/9 — Google — GeoHack
Earliest known reference to this name is 1921 (Boundary Survey).
Name officially adopted in 1951. Official in BC — Canada

Mt. Casemate (left) and Mt. Postern (right) looking across the Geikie Valley from Drawbridge
Cyril G. Wates, 1926
Canadian Alpine Journal 1927
Postern means placed at the back. The feature was named by Wheeler and Cautley’s interprovincial boundary survey in 1921.
References:
- Wheeler, Arthur O., and Cautley, R.W. Report of the Commission appointed to delimit the boundary between the Provinces of Alberta and British Columbia. Part II. 1917 to 1921. From Kicking Horse Pass to Yellowhead Pass.. Ottawa: Office of the Surveyor General, 1924
- Wates, Cyril G. [1883–1946]. “The Ramparts in 1927.” Canadian Alpine Journal, Vol. 16 (1927):85-95. Alpine Club of Canada