Discovered in 1965, a series of mass graves at the prehistoric city of Cahokia [near St. Louis, Mo.] reveal "one of the most extravagant acts of violence ever documented in ancient America: A thousand-year-old pit found under a tall earthen mound, lined from corner to corner with skeletons — 53 in all — neatly arranged two bodies deep, each layer separated by woven fiber mats." Source: Western Digs (2013).
Note: "The victims all appeared to be women, mostly in their late teens or early 20s. Evidence suggested they were strangled, or perhaps cut at the throat, at the edge of their shared mass grave, and then interred, meters away from an ornate burial of two men thought to be clan elders, political leaders, spiritual guides, or all three."
More pits discovered on the other side of the burial mound contained "some 270 human remains, each betraying signs of various degrees of violence — from having their jaws broken to being buried alive." So much for the image of the peaceful Native Americans.
Related: Cahokia Mounds State Historical Site