A team of scientists has determined that a large mound on the campus of Louisiana State University is the oldest known human-made structure in the Americas, according to a paper published in the American Journal of Science.
The scientists behind the recent work radiocarbon dated two mounds—both roughly 20 feet tall—on LSU campus. One of the mounds (“Mound B”) began around 11,000 years ago, when people started taking soil from the surrounding areas and piling it on one site. If the scientists are correct, that age would make Mound B the oldest known human-made structure in North or South America.
“There’s nothing known that is man-made and this old still in existence today in North America, except the mounds,” said Brooks Ellwood, a geologist at LSU and the study’s lead author, in a university release. The team was mostly made up of LSU geoscientists, as well as one astronomer.
Though the structures aren’t the oldest worldwide—Turkey’s famous Göbekli Tepe has them beat at an estimated 12,000 years old—they’re much older than other known structures in North or South America. LSU’s mounds had been studied previously and are just two of some 800 mound structures constructed by Indigenous Americans in Louisiana. The mounds have been excavated before; a dig in 1985 yielded some human-worked objects, and research in 2012 revealed some stone artifacts. But the recent study is the best dating of the structures yet.
Besides the mounds’ superlative age, the researchers also found thousands of ancient charred mammal bone fragments and the ashy remains of reed and cane plants. They believe the mounds were aligned towards Arcturus, a bright star in the night sky.
Indigenous people in the Americas built earthen mounds for a variety of reasons. Some mounds may have had civic importance, while others were ritual sites for worship. Perhaps the most famous mounds in North America are the 80-odd that make up Cahokia, an Indigenous settlement in Illinois that was larger than London in 1250 CE.
Some mounds contain handcrafted objects of ritual significance or, in the case of the LSU mounds, charred remains. In other cases, the mounds themselves were the symbolic objects, and some were even made in the shapes of animals. These effigy mounds are primarily found in Iowa, Minnesota, and Wisconsin, and depict animals like bears, lynx, deer, turtles, and panthers.
Ancient mounds have been damaged almost wholesale in modern times. In Missouri, dirt from mounds was used in the construction of railroads. In Illinois and Minnesota, mounds were cleared for farmland. In Oklahoma, mounds filled with valuable objects like carved conch shells, pearl beads, and copper breast plates were plundered and destroyed with explosives. And in 2009, dirt from a 1,500-year-old mound in Alabama was reportedly used to fill a supermarket chain’s foundation; another was “flattened like a pancake” in 2010.
The LSU campus once had several other mounds, but those, too, were destroyed. Mounds A and B are what remain.
The ash layers within the mounds provide a record of their construction. Around 8,200 years ago—several thousand years into its construction—Mound B was abandoned, the researchers said. Tree roots found in the mound indicate it wasn’t used by humans for a millennium. Then, Mound A’s construction began, and shortly after, construction on Mound B resumed. The mounds were completed about 6,000 years ago—a time when woolly mammoths still walked Earth.