They looks like tar kilns
Here in Scandinavia we used to make wood tar from pine trees
ore the bark from birch.
We used the root and parts of the stem who was damaged because they are loaded with resin in pine trees.
One of the byproducts from making wood tar is charcoal.
If they are tar pits I think you are lucky, maybe you should contact some local museum to determine if it is tar kilns.
Wikipedia link for tar kilns:http://en.wikipedia.org/wiki/Tar
