We were talking with the local Water engineer one day, as his Mother used to live in our Cottage and he was interested to see the Pear tree which she had planted as a child. He said "I levelled this garden for the previous owners."
We said afterwards that if that was his idea of level then we would not want him to make a lawn for us. Later we realised that he meant that he had come in with a bulldozer and spread the remains of the other three houses out across the land.
When we we making one bed we found an area like the ones mentioned, full of broken glass etc.. I dug down along side it, and realised that as I got deeper and deeper the glass in it got older and older. In the end when I had gone down over 4 feet I gave up and just removed the top yard or so and covered the rest with top soil.
Fun though is it not waiting to see what else one turns up?