Roving Boulders
What are those strange rocks, and where did they come from?
By Marc Hequet
Some would fit in the back of a pickup. Others would barely fit in your garage. But none of them fits in where it is.
A Dakota medicine man claimed to have seen one move across the prairie and come to rest on a Mississippi River bank. Another one looks so out of place on the Chippewa River flood plain near Montevideo that pioneers concluded it fell from the sky.
Wherever they came from, there they are-for now. Someday, they may move on.
The sojourners are boulders. Minnesota is strewn with mighty rocks that came from distant geological formations, sometimes hundreds of miles away.
How did they get where they are? The Dakota might call it wakan, the power of the Great Spirit. European settlers who marveled at the out-of-place greenstone boulder in the Chippewa Valley also looked on high for its origin: They dubbed it the Montevideo Meteorite.
The key to the puzzle of these roving boulders does indeed come from the sky. Every winter, in fact. It's snow.
Minnesota spent much of the past 2 million years under piles of densely packed snow-glaciers-up to about a half-mile thick. The climate changed repeatedly, from warm to cold and back again. When it was cold, snow fell winter after winter, and in some years didn't melt before more snow fell. As the overlay piled up, the crushed snow at the bottom turned to ice and then to water, and the whole mass began to move across the ground.
From ice accumulations in Canada, -glaciers generally moved south. Driven by their own sheer mass, the glaciers flowed over boulders, melting and refreezing, prying the leviathans from bedrock, and finally lifting and carrying them along. The rocks dropped as the ice melted. Sometimes the next glacier, hundred or thousands of years later, picked up the rocks and carried them farther. In this way, some rocks traveled hundreds of miles. Today Minnesota is littered with these brawny glacial trail markers, known as glacial erratics.
The Big Drag
Swiss peasants knew before the 19th century that glaciers moved boulders. They found the stony nomads at the base of mountain glaciers, covered with scratches from their travels. Geologists call such scratches striae. Striations are unmistakable evidence of a moving experience. In the case of glacial erratics, travel was a drag-literally.
Many glacial boulders didn't get far. Terrence Boerboom of the Minnesota Geological Survey has found glacial boulders in the Boundary Waters typically -within 10 miles of their source bedrock.
But some rocks moved hundreds of miles. Sioux quartzite boulders from southwestern Minnesota now reside in northeastern Kansas, 400 miles south.
Near the town of Jasper in far southwestern Minnesota, Minnesota Geo-logical Survey geologist Carrie Jennings found an omar, a distinctive kind of graywacke-dark gray, fine-grained sandstone with concretions of calcium carbonate. Omars range in dimension from the size of your fist to the size of your skull. Only one source is known: the Omarolluk Formation of the Belcher Islands in eastern Hudson Bay-more than 1,000 miles away. That omar might have made the trip in stages, Jennings says, plowed along by successive glaciations.
Moving ice can "bulldoze" boulders ahead of it, Jennings says. Or rocks might slide up onto the ice for a ride. The ice might "pluck" or "quarry" chunks from a rock formation, breaking them loose through the forces exerted by cycles of freezing and thawing. More often, a boulder is sitting around encased in weathered, rotten bedrock waiting for a glacier to knock it off its foundation. Some stalwarts put up a fight-and win. Minnesota River Valley boulders near Redwood Falls once thought to be erratics are actually still holding on-although ice and meltwater have scoured around their bases, leaving them "mushroom-shaped, almost," says Jennings.
Riding on a glacier can be a grinding experience. What survives as a glacial erratic depends on the hardness of the rock. Northeastern Minnesota's sturdy granite traveled well. In northwestern Minnesota and North Dakota, however, the bedrock is softer, and the ice broke it up and eventually dropped it as much smaller pieces of rock.
Finding where erratics came from is important for geologists who want to trace the direction the glaciers flowed. Mining geologists look for boulders bearing kimberlite, which contains distinctive minerals, sometimes including diamonds. Mining geologists who find a kimberlite erratic and can trace the rock back to its origin within the Canadian Shield just might strike the mother lode.
Rolling Stones
Possibly the state's best-known rock is the Kensington Runestone, a slab of graywacke that bears an old inscription still in dispute. Who wrote it? Beleaguered Norse -explorers in 1362? Or a 19th--century imposter?
Whatever the answer, the rock's geology is clear: A glacier plucked the 31-inch-by-16-inch, 202-pound slab from its snug bedrock somewhere in northeastern Minnesota or Canada and hauled it to west-central Minnesota, says Scott Wolter, a St. Paul geologist who studied it for the Runestone Museum in Alexandria. You can still see the scratches on its back from being dragged.
Other erratics are much larger. In southwestern Minnesota, Pipestone National Monument has the famous Three Maidens, considered sacred by American Indians. The name notwithstanding, there are six huge boulders, each of which would fit into a two-car garage-barely. The pink-and-gray granite boulders contrast with the red Sioux quartzite bedrock that pokes through the prairie where they rest. The nearest granite outcrop is 90 miles away at Ortonville, says Jennings, but she thinks the Maidens probably came from even farther north and might be pieces of a single monster that broke up in the ice.
More petite but also regarded as sacred by Indians is the 4-foot, oblong Red Rock in Newport, near St. Paul. "Certain boulders became regular places where people would pray," says Kevin Callahan, a University of Minnesota anthropologist who studies sacred rocks. Dakota Indians probably painted the granite boulder at least twice a year. Red Rock still has faint traces of red paint. An 1851 book by W.G. LeDuc, a settler, reported that Me-ah-du-ta, a great medicine man, said he saw the rock move for miles of its own accord down to the bank of the great water. His vision eerily reflects the journey geologists describe-sans glacier.
Moved first by a glacier, Red Rock has since been moved again-to protect it. The 2-ton boulder now rests in front of Newport United Methodist Church.
Many other erratics pepper the state. A mysterious low stone wall in Blue Mounds State Park near Luverne is built of -erratics. The 1,200-foot-long, 2- to 3-foot-tall wall might have been a barrier that prehistoric Indians erected to help drive bison over a nearby precipice. Or it might have been a boundary marker built on the prairie by early settlers who could find no other fencing material. Mark Hollabaugh, an astronomer from Normandale Community College, studied the site and found no evidence for a theory that the wall was a solar observatory, pointing to sunrise at a particular time of year-though it does run east-west.
Erratics and You
Erratics matter to many homeowners. In southern Minnesota near the Iowa border, landscaper Todd Ihrke collects glacial boulders and sells them for $85 or more per ton. He pays from $5 to $200 a ton for fieldstones. The rocks "bring a sense of nature and beauty to any landscape," says Ihrke.
Landscaper Jon Feldman grows corn and soybeans east of Northfield. His sideline is collecting rocks-big ones. His 4-acre rock lot bulges with 4-foot boulders.
Feldman's champion was a field boulder about 10 feet in diameter that got in the way of his plowing. He dug a deep hole next to it, planning to tip it into the hole and cover it up. But excavation showed the behemoth had broken into pieces. Bewitched by the swirls of pink, gray, and purple, Feldman found the massive rocks too pretty to bury, so he used them to ornament his rock lot.
Feldman and his brother Matt place fieldstone in their customers' yards. "My whole plan," he says, "is to make it look like it's supposed to be there."
But will the big stones stay there? Glaciers come and go. And come again. So don't take any roving boulder for granted.
Marc Hequet is a veteran St. Paul journalist and a stringer for Time magazine.
