Geochronological units have been established in West Greenland partly on the basis of 34 new K–Ar age determinations, of which 32 were made on biotites.The central part of West Greenland belongs to a single basement gneiss unit more than 2700 m.y. old. Blocks of basement rocks are traversed by rectilinear shear zones tens of kilometers long and several kilometers wide. In these tectonic belts relic slices of supracrustal rocks occur within reworked basement gneisses. The latter give K–Ar ages of 2500–2700 m.y. Ages close to 1800 m.y. are found locally. North and south of the central region of old basement younger orogenic rocks are found: the Nagssutôqidian fold belt in northern West Greenland dated at approximately 1700 to 1750 m.y. and the Ketilidian fold belt in South Greenland of which the late- to post-kinematic granites are about 1500 to 1600 m.y. old. The Gardar non-orogenic igneous activity, 1000 to 1300 m.y., is found only in South Greenland.