In the Columbia Valley north of Trail, the Carboniferous Mount Roberts Formation is underlain by a mass of gneiss. This, the Trail Gneiss, consists of amphibolite, grey biotite gneiss with minor mica schist, and marble, cut by hornblende and biotite quartz diorite orthogneiss, all laced by abundant pegmatite and aplite. Siltstone, slate, and greywacke with chert pebble conglomerate and limestone lenses of the Mount Roberts Formation face away from the Trail Gneiss, separated by a recrystallized mylonite zone, which may represent an unconformity between basement and cover, tectonized during décollement of the cover. The Trail Gneiss likely is a partially reworked and possibly allochthonous piece of the Precambrian North American craton. An upper Paleozoic to Jurassic sequence of 'arc-type' clastic, andesitic volcanic, and volcaniclastic rocks accumulated on this basement and was probably over-thrust by it in the Jurassic.The Trail Gneiss and its cover pass northward through a zone of increasing metamorphic grade into augen gneiss and pegmatite incorporating slabs and sheets of Trail Gneiss. This younger complex, the Castlegar Gneiss, closely resembles the upper sheet of mixed gneiss in Valhalla Dome and is nearly continuous with it. The Trail and Castlegar Gneisses outline a broad antiform continuous with Valhalla Dome.