louisiana mounds
Mounds located on the Louisiana State University (LSU) campus have been identified as the oldest known man-made structures in North America.

The two mounds stand 20 feet tall and are coordinated in an alignment toward one of the brightest stars in the night sky. They are among more than 800 man-made structures built by indigenous people across Louisiana.

Many of these mounds have been destroyed over the centuries, while the LSU campus mounds have remained preserved, listed on the National Register for Historic Places.

Researchers from the University collected sediment core samples, revealing layers of ash from burned reed and cane plants, as well as remains of burned osteons that indicates that the mounds were likely used for ceremonial purposes.

Comment: Wikipedia: 'osteon (plural osteons or ostea) 1. (anatomy) Any of the central canals, and surrounding bony layers, found in compact bone.'

A radiocarbon analysis of the material suggests that the mounds were built over thousands of years, with construction of Mound B starting around 11,000 years ago.

Tree roots found in sediment layers shows that Mound B was abandoned around 8,200 years ago, a period when the northern hemisphere experienced a major climate event with temperatures suddenly dropping on average by about 35 degrees Fahrenheit, which lasted about 160 years.

Then, around 7,500 years ago, the indigenous people began to build a new mound, Mound A, just to the north of the first mound.

According to the new analyses, the indigenous people reconstructed the first mound during construction of Mound A, both being completed around 6,000 years ago.

"There's nothing known that is man-made and this old still in existence today in North America, except the mounds," said LSU Department of Geology & Geophysics Professor Emeritus Brooks Ellwood, who led this study, published in the American Journal of Science by Yale University.

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 coauthor Geoffrey Clayton, about 6,000 years ago, the red giant star Arcturus would rise about 8.5 degrees east of north in the night sky, which means it would have aligned along the crests of both LSU Campus Mounds. Arcturus is one of the brightest stars that can be seen from Earth.