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Alberta lay close to the equator during the Devonian Epoch and
was mostly covered by alternately expanding and receding seas. The
resulting rocks were largely limestones with some shales. In the Williston
Basin (see map below) extensive reefs were developed. Arid conditions and
restricted circulation in reef-enclosed basins resulted in deposition of
impressive thicknesses of gypsum and salt. The reefs provided permeable
structures into which petroleum migrated, resulting in the rich oil deposits in
what we now call the Western Sedimentary Basin.
