The Presvecokarelian (Archean) basement of Finland consists mainly of granitoids and granite gneisses. Locally, there also occur some greenstone provinces, where komatiites occur as minor but characteristic rocks. The Karelian formations (Upper Archean-Lower Proterozoic) lie discordantly over the Presvecokarelian basement and are at least 2500 Ma old. The Karelian formations are divisible into Sariolian, Jatulian and Kalevian groups and the former two further into Lower, Middle and Upper subgroups. The boundary between the Sariolian and the Jatulian group is estimated to be ca. 2300 Ma old and the one between the Jatulian and the Kalevian ca. 2000 Ma old. Significant key horizons of the Karelian formations are Upper Jatulian dolomites, black schísts and volcanites, with an age of ca. 2100–2000 Ma, and a thick orthoquartzite with overlying and underlying volcanites. The synorogenic granitoids cutting the Kalevian metasediments are ca. 1900 Ma old and cogenetic with the metavolcanites overlying Svecofennian phyllites and mica schísts.