The Western Canada sedimentary basin is floored almost completely with igneous and high-grade metamorphic rocks which crystallized during the Hudsonian orogeny, dated at 1,800 m. y. Since that time most movements in the basement have been epeirogenic, in response to isostatic adjustments following long periods of erosion. Five areas of the basin floor, defined on the basis of geophysical anomalies, inferred faults, and positive or negative tectonic history, show distinct variability in percentage of rock types. Basement areas of persistent negative tendency contain higher than average percentages of sedimentary, volcanic, and basic intrusive rocks. The isostatically positive arches are composed dominantly of granitic gneisses.

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