The Morton Gneiss is an Archean-age migmatitic gneiss found in the Minnesota River Valley of southwestern Minnesota. Zircon crystals from the gneiss have been dated to 3524 ± 9 Ma. Zircons from the Montevideo Gneiss from Chippewa County have an age of 3485 ± 10 Ma.
The Morton Gneiss is the oldest rock unit in the United States.
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