Known. The largest dirt mound at Poverty Point was built quickly.
Poverty Point is in northern Louisiana. People raised the earthworks between about 1700 and 1100 BC. There are five dirt mounds, a flat plaza, and six nested ridges. The largest mound is 72 feet tall, 710 feet long, and 660 feet wide. Dated layers show that the earthworks were built rapidly.
Why the mounds were raised is still not certain. One researcher thinks the site was probably a religious place that people travelled to.
https://www.ancient-origins.net/news-history-archaeology/poverty-point-0015779