The Magnificent Morton Gneiss, Part 1: Detail of St. Mark's Catholic Church Auditorium & School, Kenosha, Wisconsin, USA (1968)
(Last updated on April 6, 2025)
Facing northward and looking at some of the polished cladding panels affixed to this building's southern elevation.
This is as good a place as any to begin our exploration of the one most striking and also most ancient rock type used in American architecture. Known to geologists as the Morton Gneiss, it has been given a number of trade names that range from the merely misleading to the overtly silly—including "Rainbow Granite" and "Oriental Granite." But even its official scientific moniker is not completely accurate because this stone is not merely a gneiss; it's a migmatite.
Migmatites are unusual in being not just one rock type, but a blend of two or more. It includes both an original igneous variety that was subsequently metamorphosed, and younger constituents that were later intruded into it. They, too, may become metamorphosed. In any event, migmatite often show signs of having endured multiple episodes of tectonic activity and severe deformation. That's certainly true of the Morton, as its dizzying swirls, folds, and varying zones of different colors suggest.
Quarried in the southwestern-Minnesota hamlet of Morton, this chaotically patterned stone has been used as an architectural and monumental selection, and can be found adorning some of America's greatest Art Deco skyscrapers. But it has also done duty in many other modes—for example, as strip-mall and coffee-shop cladding, and in countless cemeteries as an enduring choice for both high-end mausolea and humbler gravestones.
- - - - - - -
Here's a handy starter's guide if you'd like to become an aficionado of the ever-fascinating and wildly beautiful Morton Gneiss migmatite. Its different ingredients are, in order of oldest to youngest:
1. Black-and-gray striped gneiss derived from two granitoid rocks, granodiorite and tonalite. This gneiss has been assigned a remarkably precise and staggeringly old isotopic date of about 3.524 Ga (or 3524 Ma, if you prefer).
2. Black amphibolite, which in this case is metamorphosed tholeiitic basalt and komatiite. It has not been directly dated, but is thought to be as old or even older than the gneiss just cited. The amphibolite is most recognizable as discrete clasts or "rafts," sometimes partially melted or intruded by veins of lighter material, that float amidst the rest of the petrological mayhem.
3. Other, pink or gray gneisses added during the early to late Archean eon. They were derived from granodiorite, pegmatite (very coarse-grained granite), and other rock types. These have been dated from 3.37 Ga (3370 Ma) to 2.619 Ga (2619 Ma). The easiest way to spot these gneisses is to look for the pink areas and huge white feldspar crystals derived from the pegmatite.
4. Cross-cutting dikes of whitish aplite, a type of fine-grained granite. This youngest Morton rock component—itself still older than most other building stones—is the only one that remains igneous and undeformed. Its radiometric date has been listed as approximately 2.59 Ga (2590 Ma). While I have seen the aplite in bedrock exposures and at one other architectural site, I haven't spotted it at St. Mark's.
- - - - - - - -
It's a staggering thought to realize the Morton began its journey in the Paleoarchean era, when our planet and solar system were only one-quarter as old as they are now.
In the following posts of this series, we'll explore other aspects of this amazing stone, and even see it in outcrop in the ancient slice of crust called the Minnesota River Valley Terrane.
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.
The Magnificent Morton Gneiss, Part 1: Detail of St. Mark's Catholic Church Auditorium & School, Kenosha, Wisconsin, USA (1968)
(Last updated on April 6, 2025)
Facing northward and looking at some of the polished cladding panels affixed to this building's southern elevation.
This is as good a place as any to begin our exploration of the one most striking and also most ancient rock type used in American architecture. Known to geologists as the Morton Gneiss, it has been given a number of trade names that range from the merely misleading to the overtly silly—including "Rainbow Granite" and "Oriental Granite." But even its official scientific moniker is not completely accurate because this stone is not merely a gneiss; it's a migmatite.
Migmatites are unusual in being not just one rock type, but a blend of two or more. It includes both an original igneous variety that was subsequently metamorphosed, and younger constituents that were later intruded into it. They, too, may become metamorphosed. In any event, migmatite often show signs of having endured multiple episodes of tectonic activity and severe deformation. That's certainly true of the Morton, as its dizzying swirls, folds, and varying zones of different colors suggest.
Quarried in the southwestern-Minnesota hamlet of Morton, this chaotically patterned stone has been used as an architectural and monumental selection, and can be found adorning some of America's greatest Art Deco skyscrapers. But it has also done duty in many other modes—for example, as strip-mall and coffee-shop cladding, and in countless cemeteries as an enduring choice for both high-end mausolea and humbler gravestones.
- - - - - - -
Here's a handy starter's guide if you'd like to become an aficionado of the ever-fascinating and wildly beautiful Morton Gneiss migmatite. Its different ingredients are, in order of oldest to youngest:
1. Black-and-gray striped gneiss derived from two granitoid rocks, granodiorite and tonalite. This gneiss has been assigned a remarkably precise and staggeringly old isotopic date of about 3.524 Ga (or 3524 Ma, if you prefer).
2. Black amphibolite, which in this case is metamorphosed tholeiitic basalt and komatiite. It has not been directly dated, but is thought to be as old or even older than the gneiss just cited. The amphibolite is most recognizable as discrete clasts or "rafts," sometimes partially melted or intruded by veins of lighter material, that float amidst the rest of the petrological mayhem.
3. Other, pink or gray gneisses added during the early to late Archean eon. They were derived from granodiorite, pegmatite (very coarse-grained granite), and other rock types. These have been dated from 3.37 Ga (3370 Ma) to 2.619 Ga (2619 Ma). The easiest way to spot these gneisses is to look for the pink areas and huge white feldspar crystals derived from the pegmatite.
4. Cross-cutting dikes of whitish aplite, a type of fine-grained granite. This youngest Morton rock component—itself still older than most other building stones—is the only one that remains igneous and undeformed. Its radiometric date has been listed as approximately 2.59 Ga (2590 Ma). While I have seen the aplite in bedrock exposures and at one other architectural site, I haven't spotted it at St. Mark's.
- - - - - - - -
It's a staggering thought to realize the Morton began its journey in the Paleoarchean era, when our planet and solar system were only one-quarter as old as they are now.
In the following posts of this series, we'll explore other aspects of this amazing stone, and even see it in outcrop in the ancient slice of crust called the Minnesota River Valley Terrane.
To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.