In which midwestern U.S. state is this historical site located, an ancient Columbian Native American city?

My great grandparents had a farm in that area. My mother tells me of the time she went there and they had skulls on each fence post and bushel baskets full of items. It saddens me that they didn’t realize the treasures they had at that time. The only thing we have is one arrowhead.

Pretty cool place. lve been there