division of rocks in the region of northern Minnesota and Ontario radiometrically dated to have formed about 2.6 billion years ago during Precambrian Time (the Precambrian lasted from 3.96 billion to 540 million years ago). Rocks of the Coutchiching Series appear to underlie those of the Keewatin Series, at least in some areas, and consist of mostly sedimentary rocks that have been altered to varying degrees by metamorphic processes. Some geologists consider the Coutchiching older than the Keewatin Series, but others dispute this view; much study on the stratigraphic relationships between Coutchiching and Keewatin strata remains to be done before a conclusive answer can be provided. It seems likely that much of the Coutchiching Series is interbedded with lava flows assigned to the Keewatin Series and that the two are at least partly equivalent in age.