Stumps from an old forest, or "sea grove". There is often peat mixed in. Peat is the tar-like substance of decayed vegetation, including trees. Most of the peat is in the Seacrest area. The Seagrove stumps may be from a different type of trees or ecosystem, or where trees were not as thick, or maybe younger, or perhaps there is a lot of peat a bit deeper. I don't know just speculating.
Photos from Seacrest in this thread, where you can also see stumps ...
It's Peat!