Onverwacht series, division of Archean rocks (the Archean Eon lasted from 3.96 to 2.5 billion years ago) in the Swaziland region of southern Africa. The Onverwacht series is well known from exposures in the Komati valley in the eastern Transvaal region, South Africa, where Onverwacht rocks consist of dark, andesitic lavas, dolomitic limestones, cherts, and jaspers, as well as serpentines, gabbros, and peridotites. Many of the rocks have been intensely deformed by metamorphic processes; the results of these processes are observed as locally abundant schists and marbles. Rocks of the Onverwacht series underlie those of the Fig Tree series and form the basement rocks in the region of their occurrence.

Radiometric dating techniques have established the age of the Onverwacht series at about 3.7 billion years. Tiny cell-like forms have been discovered and studied in the Onverwacht rocks of sedimentary origin; studies of the hydrocarbon compounds in the rocks have also been carried out. The Onverwacht series may contain some of the earliest known traces of living organisms on Earth. Indeed, it is possible that the transition from nonliving to living material is recorded within the Onverwacht. Further study, it is hoped, will elucidate the actual steps in this transition.