The Morton Gneiss is an Archean

Archean

The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

-age migmatitic

Migmatite

Migmatite is a rock at the frontier between igneous and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks. Migmatites are not crystallized from a totally...

 gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

 found in southwestern Minnesota

Minnesota

Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

. It has been dated to 3.6 billion years ago, making it one of the oldest rock units on the planet. It is believed to have originally been a granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 before it was metamorphosed.

Its type locality is in Morton, Minnesota

Morton, Minnesota

Morton is a city in Renville County, Minnesota, United States. The population was 411 at the 2010 census.-Geography:According to the United States Census Bureau, the city has a total area of , of which, of it is land and of it is water.U.S...

.

It is also known as "Rainbow Gneiss", and is quarried as a facing stone for buildings.