Didn't a Saxon settlement grow up in the remains of Wroxeter after it was a Roman town? I'm guessing, that cos' most saxon buildings were wood, mud and thatch, and depending how long it was occupied like this, I'm guessing the accumilation of delapidated buildings, parts of the site stones beind removed/ knocked down and being used elsewhere, parts being farmed, cattle/sheep/people etc. would have meant quite a lot of organic matter built up over time, and when it finally fell into tital disrepair, trees/shubs/nature takes over. It's only a matter of time till stuff gets 'submerged', esp under brambles. Also, don't underestimate 'worm action', those fuckers in sufficient numbers can move/disrupt tonnes of earth in no time. Chedworth villa was buried too, and I'm sure some of those Orcadian tombs have been found by accident when buried, so It's not just thr Roman stuff that 'disappears'.

Also, most organic (and sometime non organic) household refuse was dumped on fields as fertalizer during most periods of farming prior to 20th century, so ground levels on good farmland can raise substantially over time when it is mixed in to soil during ploughing.