Odd bits of Mound-Builder-related art

I am currently working on a story that involves archeological hoaxes and the Mound Builder myth. One research book is Mound Builders of Ancient America, by SF's own Robert Silverberg. He has written several pleasing historical works during his career, and this one is complete, well-researched, and well-written.

One thing that strikes me is the cover illustration. which is identified as "An American Battle Mound" from a book called Traditions of De-coo-dah, by William Pidgeon (1858), an imagina...