Charcoal Remnants

In the installation of our new geothermal heating system last week, we
uncovered some ancient coal about 12 feet down in the gray clay.
It was intermittent and was surrounded by a coat of brownish clay.
The small one has a bark center. This was an Oak Savannah for
centuries. Perhaps it was before the last glacier too.
How it go down so deep is a mystery to me unless it was in the glacial
till.