It is unclear how this passage tomb started life – there are two theories. The first is that it began as a henge monument with an internal stone circle (a circle henge) with a 32 metre diameter ditch and fourteen stones. Within this circle a pit was dug which was found to contain a burnt human ear bone, the pit being covered with a capstone and an ornately carved slab.
This appears to have been buried on its side but carvings on both faces and along the top suggest that it once stood upright – it bears some resemblance to carved stones found at similar sites in Ireland and also at Barclodiad-y-Gawres 11 miles (18km) due west. A replica of this slab now stands outside the rear of the tomb, close to where it was found.
Later the stones of the circle were broken and toppled and the tomb was built over the site. This consisted of a 26 metre wide outer double kerb of stones with a northeast 1 metre wide entrance that leads over 7 metres to an off-centre chamber made up of six uprights, one of which has a spiral carving. Within the chamber there was a freestanding cylindrical pillar and the whole interior was covered with two capstones.
The earthen covering that can be seen today was built after excavation and reconstruction of the site, it would originally have been larger.
A second theory suggests that there was never a henge structure at all, but that a smaller passage grave was built into the mound with an outer kerb of stones. At some later time both the passage and the mound was enlarged, the old stones covered and a new outer kerb built.
Among the finds finds from the site are two flint arrow heads, limpet and mussel shells, a stone bead as well as the remains of a small ox found beyond the mound.
Most people today would agree that the passage tomb was a later alteration to the stone circle of Bryn Celli Ddu. Some, however, have challenged this long-standing interpretation.
Steve Burrow, the Curator of Neolithic Archaeology at the National Museum of Wales has argued that the stone circle and the passage tomb co-existed at the same time, and that the former could have been erected as a ritual boundary outside of the latter.
And Dr. Seren Griffiths, of the University of Central Lancashire, verified the existence of humans at the site, “We know that Bryn Celli Ddu sits in a much more complicated landscape than previously thought.
In all, this Neolithic monument has served many purposes over the centuries, and remains standing as a testament to the understandings and observances of our ancient ancestors.
Burrow also tested an early 20th century idea that Bryn Celli Ddu was accurately aligned to coincide with the rising sun on the summer solstice , the longest day of the day. It took two tries before Burrow succeeded in recording the rays of light that penetrated down the passage way into the burial chamber on the dawn of the summer solstice.