Andy Farrant said:it is quite likely that Twll Ddu is a natural shaft which had been blocked, possibly during construction of the tramway?. The top of this has been excavated, so there is a debate to be had as to what extent is the entrance a natural feature.
When i first saw the entrance before all the damage (presumably by the anti-entrance fraternity), It was though what looked like boulder clay to within 2 to 3 cm of the surface, then topsoil and grass. I failed to see any chippings or clinker. My photos don't show any either.
So it seems natural all the way to the surface. I wonder if the pile of clay and debris at the base of the shaft hides any bones etc, if it was ever an open shaft. Andy maybe able to postulate so some scenarios. It should have an excavation done on it before its sealed. What body would be responsible for doing that I wonder ?