Grew up on a farm in Scandinavia and have witness a few of these up close. The amount of debris they can accumulate is staggering - mostly nails, and corroded clumps of metal (probably from old farming equipment). There was nothing particular about the fields where their food came from, in fact it would be considered very clean, almost ecological. But hundreds of years of harvesting will leave a footprint. Also, in the earlier days when renovation was not set in system they used to bury their trash (what else could they do really).