A look beneath the surface Research reveals details about interior of linear mounds
Luther College student research assistant Ian Gonzales conducts a resistivity survey of a linear mound. It’s one of two techniques students and their professor, Colin Betts, used to look inside the mounds and get an idea of how they were built based on the earth’s physical structure. (Photo submitted)
From the exterior, each of the linear mounds Luther College professor Collin Betts and his students researched appeared to be uniform—largely unbroken and continuous, a seamless whole. However, the geophysical survey revealed that wasn’t the case. Varying levels of resistivity, and often a range of magnetic values, showed the linear mounds were discontinuous or segmented inside. (Photo submitted)
By Audrey Posten
Research at Effigy Mounds National Monument and several other northeast Iowa sites continues to shed light on the interior makeup of linear mounds.
Colin Betts, a professor of anthropology at Luther College who conducted the research with help from his students, spoke about the process and what he learned during a presentation at the Effigy Mounds Visitor Center last month.
Betts’s work was guided by an advisory committee of the Iowa State Archeologist’s Office as well as the Indian Advisory Council for Effigy Mounds.
“Doing work on mounds, obviously, is a little bit different from other kinds of sites because of the sacred nature of them. In this case, all the research proposals were first put forward to those councils to see if they were appropriate to do. The work we did couldn’t have happened without their permission,” he said.
From 2016 to 2022, Betts and his students investigated seven linear mounds at four sites—three in Allamakee County and one in Clayton—including on private and state land. The work at Effigy Mounds was at the Marching Bear Group. Both public and private institutions helped fund the research and purchase of equipment.
According to Betts, the area had a long tradition of mound building for over 2,000 years, with a focus on what’s called the Late Woodland Period from 1400 to 750 B.P. (before present). The mound builders are sometimes called “the people,” he said, or “the ancestors—those who came before—and encompasses a range of different tribal groups. It avoids that need to get super specific and say, well, this particular mound was built by this tribe, because that’s not really realistic to do, both because of the nature of how people interacted way back in time, but also the nature of what we have.”
Among the modern descendants are the Iowa tribe, the Ho-Chunk or Winnebago and Dakota.
“The people” built a variety of mounds: large, round mounds and smaller conical ones, as well as long, cylindrical linear mounds and compound mounds that are a combination of round mounds connected by linears. Then, of course, there are effigy mounds in shapes like birds and bears.
Betts focused on linear mounds, which he said often get “second shrift” because they’re not as evocative as the effigy mounds, for example. But these long banks of dirt come in a range of sizes, averaging around 36 meters in length, and are widespread in the region.
“There’s more linears and they occur at more sites,” Betts explained. “Going through the records and finding every piece of evidence for linear mounds in northeast Iowa, there are at least 78 sites, and on those 78 sites, we have at least 176. We find them scattered throughout a really wide distribution.”
The project utilized remote sensing, or geophysical techniques, to non-invasively look inside the mounds and get an idea of how they were built based on the earth’s physical structure.
“We look for ways in which the natural soil structure has been changed by people. We can see differences. We know what it should look like, and what we’re looking for is places where it doesn’t look like it should, if you want to think of it that way,” Betts said. “When people move dirt around and they bring stuff in, when they create fires, all of those make differences we can potentially see.”
The techniques resistivity and magnetic radiometry yielded the best results.
In resistivity, metal spikes are put onto the ground, and a current goes between them to measure how easily a current can pass through the ground from one spike to the other. If the ground has been altered, that will change how easily the current can pass through, according to Betts.
“If you dug a hole and filled it back in, it’s going to hold more moisture. That current can pass through easier because of the moisture. If you piled a bunch of rocks, that current can’t go through as easily. If you pack the soil down really hard, with less water, the current can’t get through as easily,” said Betts. “It allows us to see differences in how soil was compacted, the texture of it, sand as opposed to clay, other things that are all going to cause differences.”
Through magnetic radiometry, two sensors can detect minor differences in the earth’s magnetic field.
“What’s cool about that is, when people start moving dirt around, it changes the earth’s magnetic field in that spot. Topsoil is more magnetic than the soil underneath it. So if you mix it around, it now becomes less magnetic than the soil around it. If you create a fire, it’s more magnetic. If you bring in rock and other things like that, depending on the nature of the rock, it’s going to look different from the soil,” Betts shared. “What we can see is how people were using different kinds of soil to build up the mounds and how that looks different.”
As a whole, continued Betts, mounds are always more magnetic than the soil around them. It’s largely because the people who built them primarily—and it seems intentionally—used topsoil.
From the exterior, each of the mounds appeared to be uniform—largely unbroken and continuous, a seamless whole. However, the geophysical survey revealed that wasn’t the case. Varying levels of resistivity, and often a range of magnetic values, showed the linear mounds were discontinuous or segmented inside.
“When we see these differences, there’s something different inside of the mound in the kind of soil or how that soil was put there. They were built in segments or in parts or pieces. It’s different materials, it’s different methods, it’s different times,” Betts said. “That was something I thought was pretty curious. Why would that be? Once [archeologists] see patterns, we want to try to explain what those patterns are from, what they mean.”
Betts listed two suggestions, both of which he believes could be true. One is that the mounds weren’t all built at once.
“Maybe you build part of the mound and then you come back and you build the rest of the mound. You add to it. If you’re doing that, the way you build it one year may not be the same as the way you build the next,” he stated.
Or the builders constructed the mound at one time and specifically built one part of the mound one way and the other part a different way.
“That was part of the process,” Betts quipped.
He said previous archeological study revealed this was the case for other types of mounds too. There were different stages.
“We’re gonna do this first and at the appropriate time we’re going to come back and do the other parts of this,” he said. “One of the things we see as well is that mounds weren’t just built and they’re done. They were part of an ongoing ritual process. You have burials being added after the mounds were built. People still had a relationship with those mounds.”
Betts appreciates that these instruments allowed him to see details about the mounds without having to harm them.
“We don’t have to destroy, we don’t have to alter them, and that’s good,” he said.
Park Superintendent Susan Snow agreed. It’s through work like this, she reflected, that Effigy Mounds “can start to understand more about [the mounds] and more about the people that lived and worked in this area without having to use invasive techniques.”
