The Ancient Mounds Where the Ash Vanished
Bright bands inside two ancient Louisiana earthworks were read as ash from intense fires. A synchrotron has found soil instead — and erased a supposed centuries-long pause in construction.
The Location
Two earthen mounds stand on the campus of Louisiana State University in Baton Rouge. Their true age is still disputed, but a prominent 2022 study proposed that construction of Mound B began more than 11,000 years ago, potentially placing the pair among the oldest surviving human-built structures in the Americas.
A new American Antiquity study, highlighted by Louisiana State University, did not retest that headline age. It aimed X-rays at a different part of the story: how the mounds were assembled and what their pale internal bands contain.
What's There
Earlier core analysis divided construction into two phases separated by a pause of roughly 500 to 1,000 years. Light-coloured layers were interpreted as ash left by intense fires, possibly connected with ceremonies or cremation.
The new team examined core samples with two synchrotron techniques:
- X-ray fluorescence mapped the elements across the proposed construction boundary.
- X-ray absorption spectroscopy tested chemical differences between pale and dark material.
- The non-destructive beams preserved the archaeological samples during analysis.
The History Rewritten
If construction had stopped for centuries, the researchers expected soil weathering and environmental change to leave a chemical break. They found no elemental shift across the supposed gap.
The pale bands also failed the ash test. Their chemical signature did not differ significantly from the darker material around them, leading the team to identify them as soil rather than burned remains.
The layer that looked like a ritual fire may never have burned at all.
The Mystery
The findings remove two dramatic pieces of the earlier reconstruction, but they do not settle the biggest question. The new work did not date the mounds, so the claim of an 11,000-year beginning remains neither confirmed nor disproved by these X-ray results.
A separate team is now using optically stimulated luminescence to estimate when soil beneath the first construction layer last saw sunlight. That method could provide an independent clock for the start of building.
For now, the mounds have become stranger in a quieter way. High-energy light stripped away a story of fire and interruption, leaving continuous earth — and an age still waiting to be measured.