Core material obtained from three boreholes was studied from the Ruunaa area, Finnish northern Karelia, in order to reveal the Late Pleistocene environmental history of eastern Finland. Conventional litho-and biostratigraphical methods were used and suitable sediment samples were dated using quartz optically stimulated luminescence. The results indicate that two till units were deposited by two separate Weichselian ice advances into the area. The first advance took place prior to 52 kyr ago, most likely during the early Middle Weichselian, while the second is thought to have taken place during the Late Weichselian after 25 kyr ago. The sediment succession at Ruunaa therefore indicates a Middle Weichselian ice-free period around 50-25 kyr ago in the eastern part of Fennoscandia.

A historical review is given of the stratigraphic and chronological research of the Weichselian glaciation in Finland. Submorainic interglacial organogenic deposits have been found in Finnish Lapland and Ostrobothnia. Radiocarbon analyses give ages of over 50,000 years B.P. and the microfissil assemblages indicate climatic conditions that are more fovourable than at present. Interstadial deposits with radiocarbon ages of 42,000 to over 50,000 years B.P. contain fossil assemblages in dicating a poorer climate than at present. A tentative correlation of the Weichselian stratigraphy by various authors is presented.

Various concepts of the deglaciation of Finland are presented in the form of a historical review. The suggestions of an early (12,000–10,000 B.P) deglaciation of eastern and northern Finland are considered to be erroneous. A map depicting the ice recession as successive ice‐marginal lines is presented. According to radiocarbon dates the Finnish territory was entirely deglaciated slightly after 9000 B. P.

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