New research reveals more information about the mounds on the Louisiana State University (LSU) Baton Rouge campus, including the discovery of charred mammalian bone fragments thousands of years old and a coordinated alignment of both mounds toward one of the brightest stars in the night sky. This new research offers more information about the oldest known man-made structures in North America.
The two large 20-foot-tall mounds on the LSU campus are one of more than 800 artificial hill-shaped mounds in Louisiana built by ancient Indians. While many mounds in the region have been destroyed, the LSU campus mounds have been preserved and are listed in the National Register of Historic Places.
Brooks Ellwood, a professor in the LSU Department of Geology and Geophysics, and his colleagues collected sediment cores from the two mounds found on campus along Dalrymple Drive to learn more about them. The cores revealed layers of burned reed and rush plant ash, as well as burned osteons.
Radiocarbon dating of the layers of material indicates that the mounds were built over thousands of years. These finds show that people began building the first mound about 11,000 years ago. Scientists believe that the sediments in the southern mound, which they have named “Mound B,” were taken from a location immediately behind LSU Hill Memorial Library because there is a large depression in the ground there.
The mound was built over several thousand years, layer by layer, until it reached about half its present height. Layers of ash and microscopic charred bone fragments may indicate that the mound was used for ceremonial purposes, which included burning reed and rush plants to build large fires that would have been too hot for cooking. Scientists do not know what kind of mammals were cremated or why. However, they found many microscopic charred bone fragments, known as osteons, the building blocks of large mammalian bones, in the ash beds of both LSU campus mounds.
Then, about 8,200 years ago, the southern B mound was abandoned. Tree roots found in the 8,200-year-old sediment layer indicate that the mound was unused for about 1,000 years. Also about 8,200 years ago, the Northern Hemisphere experienced a major climate event with a sudden drop in temperatures of 1.67 degrees Celsius, which lasted for about 160 years.
We don’t know why they abandoned the mounds around 8,200 years ago, but we do know that their environment changed suddenly and drastically, which may have affected many aspects of their daily life.Ellwood said.
So, about 7,500 years ago, the indigenous people began to build a new mound just to the north of the first one. This time, however, they took mud from the floodplain where the entrance to the LSU Tigers Stadium now stands, which at the time was an estuary. With this mud, they built the second mound, “Mound A,” layer by layer, to about half its present height. Mound A contains water-saturated mud, which liquefies when shaken. As a result, Mound A is unstable and degrading, so staying away from mounds is critical to preserving them.
Based on new analyzes of the sediment layers and their ages, it appears that the Indians cleared Mound B, abandoned in the first place, and began building it up to its present height before completing Mound A. Both mounds were completed about 6,000 years ago. and have a similar height.
The crests of both mounds are aligned along an azimuth that is about 8.5 degrees east of true north. According to LSU astronomer and study co-author Geoffrey Clayton, around 6,000 years ago, the red giant star Arcturus (Arthur) was rising about 8.5 degrees east of north in the night sky, meaning it would have lined up along the crests of both mounds on the LSU campus. Arcturus is one of the brightest stars that can be seen from Earth.
The people who built the mounds, some 6,000 years ago, coordinated the orientation of the structures to align them with Arcturus, which was visible in the night sky at that time.Ellwood said.
Louisiana State University | Brooks B. Ellwood, Sophie Warny, Rebecca A. Hackworth, Suzanne H. Ellwood, Jonathan H. Tomkin, Samuel J. Bentley, Dewitt H. Braud, and Geoffrey C. Clayton, The LSU campus mounds, with construction beginning at ∼11,000 BP, are the oldest known extant man-made structures in the Americas. American Journal of Science June 2022, 322 (6) 795-827; DOI: doi.org/10.2475/06.2022.02