The western part of the old supercontinent in western Canada is made up of mostly of metamorphic and granitic rocks formed between about 2400 - 1800 million years ago, although locally they are older. Although these rocks are buried beneath the sedimentary rocks covering the plains east of the mountains, and beneath the Foreland Belt of the Cordillera they are penetrated in places by wells drilled for oil and gas, so that their nature is known. A piece of this old basement, apparently detached from the continent, may be found within the Omineca Belt, along Highway 1 west of Revelstoke in the Monashee Mountains, where the rocks are about 2000 million years old.

On top of this old "basement", a roughly 20 km thick pile of deep to shallow water sandstones and shales cut by flat, sheet-like intrusions of black rock called diabase were deposited in a basin that formed within the supercontinent, about 1250 to 1450 million years ago. These rocks are called Purcell supergroup in Canada, (the Americans use the name Belt for the same rocks). They form the Purcell Mountains of southeasternmost British Columbia, which are crossed on Highway 3 between Creston and Fernie, but extend no farther north than latitude 51°N, about 50 km south of Golden.