The low spot I'm filling is directly outside of my kitchen, where I installed a drywell a few years ago. Since my house is so old, when I excavated I found lots of broken china, glass bottles, unidentifiable small metal items, etc. I dug up lots of large rocks, I assume they were left over from the foundation and used to fill in and raise the grade around the house. About 6" below the surface was a layer of an ash like substance which I now think is buried coal ash.
The best find were a few pieces of the top of a long gone kitchen coal stove, I found one of the round plates and the removable piece that separates the round plates.