Morton Gneiss, Minnesota

James S. Aber
Emporia State University

Minnesota River valley

Archean crystalline basement rocks are exposed at several places along the Minnesota River valley in south-central Minnesota. These rocks are mostly gneisses of various compositions along with other igneous and metamorphic rocks.

Left: exposure of mafic gneiss at Granite Falls beside the Minnesota River. Right: closeup view of pegmatite vein within the gneiss at Granite Falls.

Among these ancient rocks, the Morton and Montevideo gneisses are the oldest whole-rock continental crust in the United States. Many attempts to date these rocks using various radiometric techniques have yielded an age ~3.5 billion years old (Goldich, Hedge and Stern 1970; Bickford Wooden and Bauer 2009).

Bedrock geology of Minnesota
Location of Morton Gneiss indicated by red asterisk
on map and legend. Map adapted from Mn/DOT.

Left: exposure of the Montevideo Gneiss which has a felsic (granitic) composition. Right: closeup view of the Morton Gneiss. Note the marked layering and contortion of light- and dark-colored minerals. A felsic (pink) vein cuts diagonally across the exposure.

Morton Gneiss

The Morton Gneiss has been quarried since the 19th century into the late 20th century for building and monument stone. The tradename rainbow granite has been applied because of the striking color bands and patterns in the rock. Over the years quarry operations have evolved with different tools and techniques.

Left: overview of quarry with large crane used for moving blocks. Right: saw house on right and cut slabs near bottom center of scene.
Left: track and tractor used to move stone into and out of the saw house. Right: wedges used to break blocks along closely spaced drilled holes.
Left: Vertical cylinders were drilled to lower wire saws for cutting flat faces in the quarry walls. Right: closeup view of mafic bodies deformed within the Morton Gneiss.
Blasting to break out large blocks. The tires arrranged to cushion the blocks as they fall from the quarry face (left). The blast proceeds in sequence across the prepared face (right).

The Morton Gneiss was widely used for buildings and monuments. It was highly valued for its swirling color patterns and great age. Because of the difficulty of extracting large, coherent blocks, it was expensive to produce compared with many other conventional granites and marbles. For these reasons, it commanded a high price in the commercial stone market.

Buildings in the city of Morton constructed with Morton Gneiss. Left: Lutheran church. Right: detail of school auditorium entrance.

Selected tombstones in Missouri and Kansas


References

Return to tectonics syllabus.
Text and images © J.S. Aber (2012).