Last year I had a large oak dropped - it was two trunks coming up from a large common base. The tree guys left too much of the stump sticking up so I paid a guy to come cut it flush since my saw is way too small for a job like that. He hit a rock inside the stump and had to stop. I was able to identify the perimeter of the rock and cut around it, which still left a substantial mini-stump sticking up. Yesterday I put a burn barrel over it and burnt most of the mini-stump down and learned the rock was a football size chunk of concrete. I believe my neighborhood was built on an old farm about 35 years ago. Why would a tree have concrete poured into it? Clearly it was done when the tree was much younger but I don't see signs there was a cavity or anything. Are there certain reasons why people may sometimes do this?