U-Pb analyses of zircons from gneissic granite in the Sifton and Deserters Ranges flanking the northern Rocky Mountain Trench in north-central British Columbia indicate crystallization ages of 1.85 b.y. and 728 m.y., respectively. Augen gneiss in the Sifton Ranges appears to be basement rock associated with a sequence of amphibolite, quartzite, schist, and carbonate thrust southerly over upper Proterozoic strata of the Ingenika Group. Gneissic granite in the Deserters Range is nonconformably overlain by pebble conglomerate and pure quartzite, in turn overlain by a thick sequence of amphibolite, schist, phyllite, gritty clastic rocks, carbonate, quartzite, and diamictite assigned to the upper Proterozoic Misinchinka Group. The Deserters data establish a probable maximum age for rocks of the Windermere Supergroup in the northern Rocky Mountains. The age and structural setting of these gneisses are similar to other gneisses of the Omineca Crystalline Belt of the Canadian Cordillera.