There are more than 800 pyramidal mounds in North America. New research has shown that some of these mounds are among the oldest manmade structures on the continent. Furthermore, each mound's crest aligns with an azimuth that is about 8.5 degrees east of true north.
Researchers have discovered charred mammal bone fragments and a coordinated alignment of both mounds in the direction of one of the brightest stars in the night sky as they learn more about the LSU Campus Mounds.
As a result of this new information, we have more insight into the oldest manmade structures in the Americas.
Over 800 man-made, pyramidal-like mounds have been found in Louisiana, two of which are on LSU’s campus. Although many mounds have been destroyed in the region, the LSU Campus Mounds have been preserved and listed on the National Register of Historic Places.
This study was published in the American Journal of Science by Yale University.
To gain a better understanding of the two mounds on LSU’s campus along Dalrymple Drive, Ellwood and colleagues collected sediment cores. Fire-burned reeds and cane plants, as well as mammal bones, were found in the cores.
The layers of material indicate the mounds were built over thousands of years according to radiocarbon dating. There is evidence that the first mounds were built about 11,000 years ago.
There is a large depression in the ground behind LSU’s Hill Memorial Library, so the scientists think sediment for the southern mound, named “Mound B,” came from there. About half of the mound’s current height was built up over a few thousand years.
Ashes and microbes charred onto microscopic bone fragments suggest the mound was used for ceremonial fires that burned reeds and canes too hot to cook on.
There is no information about the type of mammals cremated or why the animals were cremated. In the ash beds at both LSU Campus Mounds, they discovered microscopic, charred bones called osteons, which make up large mammal bones.
Eventually, the southern Mound B was abandoned around 8,200 years ago. A sediment layer found in the mound that dates to 8,200 years ago contains tree roots that indicate the site was not used for about 1,000 years.
There was also a major climate event around 8,200 years ago in the northern hemisphere, when temperatures dropped by an average of 35 degrees Fahrenheit over a period of 160 years.
Ellwood said that the environment in which they lived changed suddenly and dramatically around 8,200 years ago, affecting many aspects of their daily lives.
The indigenous people then began building a new mound just north of the first mound around 7,500 years ago. In this instance, mud was taken from a floodplain where the entrance to LSU’s Tiger Stadium is now located, which at that time was an estuary.
Layer by layer, they constructed Mound A, the second mound, to about half its current height. It contains water-saturated mud that liquefies upon agitation. To preserve the mounds, it is critical to stay off Mound A due to its instability and degradation.
In light of new studies on sediment layers and their ages, it appears indigenous people cleared the abandoned first-built Mound B before completing Mound A, then began building it up to its present height.
A similar height was achieved by both mounds around 6,000 years ago when both are believed to have been completed.
Each mound’s crest aligns with an azimuth that is about 8.5 degrees east of true north.
Approximately 6,000 years ago, the red giant star Arcturus would have been rising about 8.5 degrees east of north in the night sky, aligning with both LSU Campus Mounds’ crests, according to LSU astronomer Geoffrey Clayton. One of the brightest stars visible from Earth is Arcturus.
Ellwood said the mound builders aligned the mounds’ orientation with Arcturus, which was visible in the night sky at the time.
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