The post-Archaean geology of Sierra Leone is reviewed and interpreted. Late Precambrian development of fault-controlled sedimentation of glacigenic, fluviatile, and marine deposits with minor volcanics culminated in the Rokelide thermo-tectonic event at 550 Ma, resulting in a mildly metamorphosed NNW-trending synclinorium. Rokelide deposits are overlain in northern Sierra Leone by near-horizontal, shallow water sandstones and glacio-marine rhythmites of late Ordovician age. Subsequent events were associated with sea-floor spreading activity, including development of coast-parallel dykes, a confluent cone-sheet, 2 alkaline complexes and kimberlites, all controlled by lineaments reactivated by the Mesozoic rifting of Africa from North America. Late Mesozoic downwarp of new continental margin produced a sedimentary basin into which marine, estuarine and fluviatile sediments were deposited.

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