2,900,000,000 - 2,800,000,000 years ago
(2.8 - 2.9 billion years ago)
Wisconsin’s Oldest Rocks
During this time Wisconsin and North America did not exist. Instead, a series of islands or small plates called terranes were scattered over the globe. Remnants of these terranes have been dated in Wisconsin to about 2.8-2.9 billion years old. In Minnesota some are over 3 billion years old. The shape, size and location of these terranes are not known. Much of the original Archean terranes have been destroyed by weathering or are buried below younger rocks. The small amount of Archean rock that survives today has been severely altered and metamorphosed to gneiss and schist making the original rock unidentifiable.
At least two different Archean terranes have been identified in Wisconsin. In the northern part of the state the remains of one Archean terrane is exposed in Iron, Ashland, Bayfield and Sawyer counties. In the central part of the state the remains of a second, different, Archean terrane is exposed in Wood, Jackson and Clark counties. Whether or not any Archean terranes occur south of this is unknown due to the thick layer of Paleozoic sedimentary rock covering the southern part of the state.