Gneiss to Meet You

Exploring a Rare Prairie Growing Upon Some of the World’s Oldest Rock

Story and Photos by Addeline Theis

July 25, 2023

Pulling up at the edge of a gas station parking lot in the small town of Morton, Minnesota, I had to double check if this was the correct way to enter the prairie. A small foot path leading up through hip–height sumac bushes was visible from my car.  It was a sign that others before me have taken this way to view the historical outcroppings. Obviously, this was my first ever visit to Morton Outcrops Scientific Natural Area (SNA). Unknowingly, I scrambled up some boulders to find myself above the landscape, overlooking the amazing river valley. Below my hiking boots were dark slabs of granite that were covered in encroaching lichen. Finding a lichen-less portion of the rock, I noticed distinctive swirling of the pink and black bands. This was the 3.6-billion-year-old Morton Gneiss (pronounced “nice”) dry prairie that the site was preserved around.

This site is one of Minnesota’s largest and highest quality examples of Crystalline Bedrock Outcrop Prairie, defined as a dry, open, lichen-dominated plant communities on areas of exposed bedrock. Woody vegetation is sparse within this prairie type, and vascular plants are restricted to crevices, shallow soil deposits and rainwater pools. Shallow soils found on the margins of the bedrock exposures host plant species that are adapted to this drought-prone microhabitats. This prairie system is quite different from the wet prairie potholes that developed all around this river valley and the species found at this dry prairie showcase the difference. As a botanist, finding the native Brittle Prickly Pear Cactus (Opuntia fragilis) thriving out of the knobby exposure was exhilarating. The idea of such an arid plant flourishing in this site is one of the driving forces that led me to investigate the natural history of this site further.

 

For geologists, the glacial history of southwestern Minnesota tallgrass prairie region usually does not draw up much interest. The last glacial period left this area covered in thick glacial till, covering up any bedrock for one to investigate. For soil scientists, this thick glacial till was the original parent material for developing the thick rich Mollisols soils that the prairies are known for.   

What draws geologists into this area is the events that happened after the glaciers melted. After the Des Moines lobe of the Laurentide Ice sheet retreated from southern Minnesota and the global temperature began to rise, meltwater began to collect in large inland lakes throughout southern Canada and the northern Midwest. The largest of these lakes was ancient Lake Agassiz. Varying water levels led to the lake overflowing a moraine ice dam near present day Browns Valley, creating an outlet river. Called Glacial River Warren, this outlet river was a prehistoric river that drained Lake Agassiz between about 13,500 and 10,650 BP (Before Present) years ago. The strength and power of Glacier River Warren carved out the valley that is now known as the Minnesota River Valley.

This violent history impacted the landscape of southern Minnesota as well as affected how plant communities would development within the valley. The tremendous power of Glacial River Warren cut through layers of glacial till and clay-rich deposits of weathered bedrock all the way down to scour some of the oldest bedrock in North America. Some of the bedrock exposed on the valley floor is as old as 3.6 billion years old. It is called Morton Gneiss (pronounced “nice”), and it formed deep in the earth’s crust, where extreme heat and pressure changed, or metamorphosed, an earlier kind of rock. The beautiful and distinctive banding of colorful minerals within the Morton Gneiss makes it an attractive building stone, which is used around the world. There are just a few similar exposures of Morton Gneiss in Minnesota, all in the Morton Area.  
This place is also an amazing story of private landowner conservation, Carl Colwell was instrumental in the site’s preservation. After completing a career in the military, Carl returned home to Morton, Minnesota where he served as Renville County Historical Society director for six years. He purchased 10 acres as a private citizen in 2009.  

Today the land is protected as a Scientific Natural Area (SNA) managed by the Minnesota Department of Natural Resources. Colwell reflects:  

When I was a boy, the Dakota story I heard was that the rock was the keeper of all knowledge. It had seen everything since the beginning of time, and if someone would ask in the correct way, they would learn the answer. It was almost like a library. I, of course, didn’t believe that story at the time. Once I started escorting geologists, anthropologists, and “ists” of all sorts through the site, I started to hear the same story repeatedly. The rock has seen everything since the earth cooled. If we look carefully and study hard, we will learn the answer to our questions. 

Before returning to my vehicle, I took a few final moments on the ancient rocks, remembering that these rocks perhaps do have the answers to our questions. But only if we ask the right questions. I can only be thankful for the efforts of those that have contributed to the conservation and preservation of this site so that future generations can witness this library of knowledge within the Minnesota River Valley. 

If you’re interested in dry prairies, visit one of our sites, like Muralt Bluff Prairie.