The winds of time blow sand, dust, dirt at a rate of a few feet every hunderd or so years; a prime example of this can be found in Isreal when visiting the Crusader Castles. You can see how during unoccupied periods between Crusades that the sand built up and when the next group came into the fortifications, they literally built up from that level a newer and higher level, which was done a total of three times.
Thank you Jumper! In Yorkshire a medieval Viking site has been covered by more than eight feet of soil in circa 1000 years.