In the Ohio River Valley, large mounds and earthen walls that reach more than three meters (12 feet) high are remnants of a people who resided in the region from 200 B.C.E. to 500 C.E. Perhaps more impressive than the mounds and earthworks of the Hopewell tradition—a culture that included various Native American tribes—are a collection of artifacts suggesting they had regular contact with cultures thousands of kilometers away.

Among the items found near the earthworks in the U.S. state of Ohio are fossilized shark’s teeth that either came from the Gulf Coast or a southern part of the East Coast. Copper and silver used in jewelry was mined in the northern Great Lakes region. The collection includes mirrors made from mica, a mineral commonly found in the Appalachian Mountains. Spear points found in the region were made of obsidian, a volcanic glass that has been traced to what is today Yellowstone National Park in Montana, more than 2,400 kilometers (1,500 miles) away.

Brad Lepper, curator of archaeology at the Ohio Historical Society in Columbus, says there is reason to believe the Hopewell did not acquire the obsidian through trade.

“The obsidian is really interesting, because if it were from trade from village to village, you would expect there to be a trail of obsidian from Montana to Ohio,” he says. “And there really isn’t.”

Nor is there evidence of the region being a trade hub, where goods are collected and redistributed, he says.

The artifacts were amassed somehow, however, and archaeologists have come up with a theory.

Hopewell Interaction Sphere

Rather than calling this ancient system a trade network, archaeologists who study the relationship between the Hopewell people and other far-flung cultures of the time call it the Hopewell Interaction Sphere.

Bret Ruby, an archaeologist at Hopewell Culture National Historic Park in Chillicothe, Ohio, says the term describes the long-distance sharing of certain artifact styles and raw materials, including copper, mica, and marine shells.

“These things are moving over the whole eastern U.S., but it is not necessarily a trade network,” he says. “In fact, in a number of cases, we can show it wasn’t trade.”

So how did all of these exotic objects end up with the Hopewell? There are two theories.

“A lot of what people had traditionally called trade is probably direct procurement,” Ruby says. “It’s people going out from Ohio and bringing these exotic things back.”

The other theory has to do with the mounds and walls that dot the Ohio River Valley. The monumental structures were arranged in various shapes and in some cases covered hundreds of acres.

“Another thing that could be going on is that these big earthwork centers in Ohio were probably widely known,” Ruby says. “They were built over a period of hundreds of years. They are awesome to see, so people knew about them. It’s also likely people went as pilgrims from distant places to Ohio to visit these great religious centers,” bringing the objects with them.

Lepper agrees with the second theory.

“I think that during the Hopewell era in the Middle Woodland period, Ohio was this nexus, this cultural center for much of eastern North America,” he says.

Lepper notes that there have been small amounts of flint from Flint Ridge in Newark, Ohio, found in Pinson Mounds, Tennessee. He believes this is further evidence of the Ohio earthworks being pilgrimage sites.

“For me, the interpretation of that is that these massive offerings of thanksgiving or supplication are being brought to Newark, and what people are taking away are pilgrim’s tokens,” he says.

Lepper even has some ideas on how the Hopewell or pilgrims visiting them might have traveled.

“In terms of evidence, I don’t think we have identified any definitive Hopewell roads,” Lepper says. “But we have identified sources of raw material and endpoints. They could have either followed a direct overland route, which is unlikely. Or they could have followed streams. You could just jump on the Missouri River and go to the Mississippi River then go up the Ohio River and Scioto River, and you’d be there in less time and less effort.”

Ruby says the existence of the Hopewell Interaction Sphere might correct a misconception about ancient cultures like the Hopewell.

“I think it’s good for people to know that there were these continent-spanning journeys happening, that people weren’t isolated,” he says. “We have this picture that people were isolated in little villages, not in contact. In fact, there is all this evidence that for thousands of years, people were moving and in contact with one another.”