The historic sites (blue) appear to be confined to an eroded terrace while the prehistoric sites (red) occupy both eroded terraces and floodplain. The contour maps suggest that we would encounter some truncated and eroded historic period features.
Those areas in which few artifacts were found are poorly-drained and low-lying. It is possible that they represent ancient stream channels. If there was a road passing through the field in the 18th and 19th centuries, it isn't obvious now.