Morton Gneiss

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.

Its type locality is in Morton, Minnesota where it is also known as "Rainbow Gneiss", and is quarried as a facing stone for buildings.

The Morton Gneiss is the oldest rock unit in the United States.