Klippen of Precambrian mylonitic quartzite and subordinate itabirite and graphitic schist, tentatively correlated with rocks in the Pan-African age province (∼550 m.y. old), overlie a Precam-brian(?) to Cambrian(?) formation at Gibi Mountain and Liberian age province gneisses (∼2,700 m.y. old), approximately 32 km northeast of the northwest-trending Pan-African—Liberian age province boundary. Small-scale folds in the allochthonous rocks and in the underlying formation at Gibi Mountain indicate relative northeastward movement of the klippen. The age province boundary appears to be a southwestward-dipping fault zone along which the Pan-African age province was thrust over the Liberian age province in late Precambrian or early Paleozoic time. The formation at Gibi Mountain, consisting of arkose, shale, and conglomerate, is similar to units in the Rokel River Group in western Sierra Leone and appears to indicate a 225-km southeasterly extension of the late Precambrian(?) to Cambrian(?) basin proposed by Allen (1968) to have bordered the West African craton from Mauritania to Sierra Leone. Stratigraphic data in the crystalline rocks do not allow interpretation of the Pan-African–Liberian age province boundary as a cryptic suture, a zone marking the collision of two continental plates.