Rock ... And Roll

How a glacier pushed a boulder to a place near you.

This story appears in the March 2012 issue of National Geographic magazine.

Heads bowed, hands in the soil, people are clustered around a huge rock, planting marigolds. It's an annual custom, explains Michelle Leger, a member of the extended family that congregates on this Massachusetts town common to do the honors. The Rollstone Boulder, she says, "is the pride of Fitchburg. We're lucky to be able to make it more beautiful—as it deserves."

Granted, at ten feet tall, the boulder is impressive. But do even really big rocks deserve anything? Whether they do or not, it's clear that some of them have devoted admirers. Rocks like the Rollstone Boulder are called glacial erratics. And countless numbers of them lie strewn about the planet.

Photographer Fritz Hoffmann first became obsessed with erratics while reading up on the random boulders around his property in Connecticut. And then, on a road trip in Massachusetts, he whizzed past a sign pointing the way to Plymouth Rock. "Wait! I wonder if …" He made a quick U-turn and pulled up at the Plymouth visitors center. "I just have one question," he told the women at the desk, already gathering brochures for him. "Is Plymouth Rock an erratic?"

"A glacier brought it here, if that's what you mean," came the response.

The journeys of erratics, carried by glaciers during a succession of ice ages, were long, sometimes hundreds of miles. Their name comes from the Latin errare, to wander, which makes sense because they have been carried by ice from their original locations to where we find them today, deposited over the northern U.S. On prairies the anomalous stones fracture the horizon. Early stone carvers gave them buffalo ribs and hoofprints; later fans painted them, carved initials, honored them with plaques. In forests they are enigmatic giants penned in by trees. On mountaintops they perch as though balanced by impish fingers.

How did the rocks arrive in such unlikely places? Did a primordial volcano cough them out? Did the Arctic Ocean flood so violently that it swept boulders onto mountains? Did a stutter in Earth's orbit cause uphill avalanches?

It was Louis Agassiz, a Swiss scientist at Harvard, who popularized the Ice Age theory of how such boulders came to their odd contexts.

Agassiz had explored glaciers in the Swiss Alps and observed battered rocks disgorged at their melting snouts. He had seen deposits of similar rock in the British Isles where no glacier now stood. He had a hunch that glaciers had once been widespread.

In 1871 Agassiz presented his observations on boulders scattered across Massachusetts's Berkshire mountains: A glacier must have crept down from the north, engulfing everything in its path. When the ice melted, the detritus was left on the ground. What's more, Agassiz reported, glacier-borne boulders had left scratches on rocks 11,000 feet high in the Rocky Mountains.

This means that the Rollstone Boulder, Plymouth Rock, and all their cousins were once blocks of mountain face or bedrock, possibly loosened by freeze-thaw cycles: Water seeped into fractures and then expanded into ice crystals, wedging the blocks free from their parents.

As the climate cooled and the glaciers expanded 25,000 years ago, a southbound ice sheet slid over the loose blocks like molasses over spilled sugar and dragged them along. Under perhaps a half mile or more of ice, they grated against other stones, losing their sharp edges and leaving telltale gouges. Up and over bedrock the ice carried them—and then stopped. And sometime around 21,000 years ago, the ice began melting away.

"We still use erratics," says Carrie Jennings, a geologist at the Minnesota Geological Survey, "to map glacial deposits. In some older deposits the finer grained sediments made of clay, sand, and gravel have eroded from wind and water; the boulders may be all that have survived."

If some people doubt the staying power of those boulders, they need only consult the Rollstone. Long ago, it resided at the edge of town, hovering like Humpty Dumpty on the brink of an expanding quarry. Its fan club, fearing it would tumble in, blasted it to pieces, hauled 110 tons of it to its place of honor, and put it together again. There it has stood since 1930, surrounded by traffic signs, fire hydrants, bikes, and strollers—and every May, admiring townsfolk planting a garden around it.