The bones are safe until they uncovered, and that could happen thousands of years later during a pluvial. The boundary between the arid and wet zones in the Midwest runs pretty much along the 100 degree meridian these days. We know from other studies that it was sometimes 10 degrees further West and at other 10 degrees further East, and there were extensive salients here and there all across the continent.
This could happen repeatedly and you'd end up with bones from 250,000 years ago ending up with bones from 100,000 years back, and 10,000 years back, and 2 years back. These hills tend to form, on average, in the same spots in every interstadial ~ rivers, though, change differently even though the drainage basins that give rise to them might not change appreciably.
***It piles up in hillocks of Wind Blow Loess and any bones lying about would be covered up.****
The bones I mentioned were found above ground and not fossilized.