Indian mounds

The various cultures collectively termed Mound Builders were inhabitants of North America who, during a 5,000-year period, constructed various styles of earthen mounds for religious and ceremonial, burial, and elite residential purposes. These included the Pre-Columbian cultures of the Archaic period; Woodland period (Adena and Hopewell cultures); and Mississippian period; dating from roughly 3500 BCE (the construction of Watson Brake) to the 16th century CE, and living in regions of the Great Lakes, the Ohio River Valley, and the Mississippi River valley and its tributary waters.

Since the 19th century, the prevailing scholarly consensus has been that the mounds were constructed by indigenous peoples of the Americas. Sixteenth-century Spanish explorers met natives living in a number of later Mississippian cities, described their cultures, and left artifacts. Research and study of these cultures and peoples has been based mostly on archaeology and anthropology.

During the 16th through 19th centuries, Europeans and Americans generally thought that a people other than one related to the historic Native Americans had built the mounds.

The namesake cultural trait of the Mound Builders was the building of mounds and other earthworks. These burial and ceremonial structures were typically flat-topped pyramids or platform mounds, flat-topped or rounded cones, elongated ridges, and sometimes a variety of other forms. They were generally built as part of complex villages. The early earthworks built in Louisiana c. 3500 BCE are the only ones known to have been built by a hunter-gatherer culture.

...
Wikipedia