GEOLOGY

Northern Tutchone elders still tell the story of the explosion of the mountain called Nelruna (Volcano Mountain). While scientists have not been able to date the oldest of the volcanic eruptions and lava flows, the youngest of these would appear to be at least 4,200 years old (Jackson & Stevens, 1992). This is a recent event geologically but a very old story to have carried forward in oral tradition.

The Fort Selkirk area features a series of volcanic centres. Nelruna is the youngest of these although there are the remains of an older volcano very nearby. Geologists have determined that there were two or three eruptions that sent lava flowing south from this mountain. These flows formed into basaltic rock, ranging in composition among alkaline olivine basalt, olivine nephelinite, and basanite. This porous, igneous rock forms the basalt cliffs seen across the river from the townsite. Robert Campbell and his men hauled chunks of this basalt across the river to build the chimneys of the original Fort Selkirk.

On at least three occasions, glaciers covered this area to a depth of over 900 metres. The most recent glaciation in this part of the Yukon ended some 11,000 years ago and was known as the McConnell Glaciation. It did not reach quite as far as Fort Selkirk. During one of the three earlier glaciations Ne Ch’e Ddhawa, the Selkirk Cinder Cone, erupted. The entire process took place under the ice. The evidence of this is the unique rock formation known as basaltic pillows that only form when lava is in constant contact with water during its emergence and cooling period. Ne Ch’e Ddhawa lies 7 km upstream from Fort Selkirk, on the opposite bank.

The lava flows from Nelruna interrupted the flow of the ancient Yukon River and caused it to change its course. The basalt flows to the south of Fort Selkirk also formed a dam. Eventually, the river managed to cut through the basalt and change its course yet again. Fort Selkirk sits on soils made by the glaciers and overlain by sands, silts and gravels deposited by the river when it flowed over this spot.

There has been little prospecting or rockhounding in the Fort Selkirk area as basalts are not a fruitful source of commercial minerals and gemstones. Some geodes can be found as well as small peridots, a gem quality olivine.

Further Reading:

Dawson, George. Report on an Exploration in the Yukon District, Northwest Territories and Adjacent Northern Portion of British Columbia. Whitehorse: Yukon Historical & Museum Association, 1987 (reprint of 1888 edition)

Jackson, Lionel. E. Jr. Pleistocene Subglacial Volcanism Near Fort Selkirk, Yukon Territory. Geological Survey of Canada, September 1988.

Jackson, Lionel. E. Jr and Brent Ward, et al. The Last Cordilleran Ice Sheet in Southern Yukon Territory. Géographie physique et Quatemaire, 1991, Vol 45. No. 3.

Jackson, Lionel. E. Jr and Wayne Stevens. "A recent eruptive history of Volcano Mountain, Yukon Territory." in Current Research, Part A; Geological Survey of Canada, Paper 92-1A.