The most interesting thing about these mound-builder sites is that they generally (not always) turn out to be part of a layout that resembles cities in the pyramid-building civs of Central America and Mexico.
Some time in the early to mid 50s my father bought a barn near Swanton, Nebraska for its lumber. We spent a number of winter Saturdays disassembling it and hauling the lumber home. One day he took a different route and pointed out a series of 5-10 ft. high mounds in what was then a cow pasture. He said they were Indian mounds. There might have been a dozen or so. Years later, I looked for them both with a fly over with my brother in his small plane and later driving around. Never did find them again. My guess is that they bulldozed them down & farmed the ground.