Large tombs or sepulchers were used by the greater ancestors before the native Americans today. With the relics and art that is buried with the great number of large skeletons, it is easy to see that the previous generations obtained a higher degree of culture and society.
The mounds seem to have been occasionally taken over by the natives
The skeletal remains are easily distinguishable from the previous residents by their great size and their great stature.
As the natives took over the mounds of the previous occupants many of the skeletons and their legacy may have been discarded by the latter culture.
~Chris L Lesley
History of Logan County, Illinois, 1886
Fritz Zimmerman: The Nephilim Chronicles, Angels in the Ohio Valley