Not all of Anglesey's burial chambers were covered by a mound of earth and stone, a new investigation reveals. Though over 40 of these monuments have been recorded on Anglesey, only 16 are still extant. It is believed they were built during the Late Stone Age, or Neolithic, spanning a period from about 4200 BC to about 2200 BC.

Often referred to in Welsh as a cromlech, a burial chamber is a group of vertically raised stones, on the tops of which a horizontal rock, called a capstone, was placed. The individual stones of the cromlechi, and the rocks forming standing stones and stone circles found all over the British Isles, are often referred to as megaliths in archaeology.

These megaliths were put together by human creativity, and the whole stone structure was often covered by a mound. A fine example of this feature, and the only one on Anglesey, can be found at Bryn Celli Ddu (pictured) near Llanddaniel Fab. At this site, the interpretation of burial chambers as a metaphor for the Earth goddess's womb becomes all too clear, and is well worth a visit.

Archaeologists investigating burial chambers in the 20th century discovered that all sites excavated (bear in mind that only eight sites have been excavated with academic intentions) had traces of a mound, apart for the site at Lligwy (Moelfre).

The remains of the cairn originally covering the burial chamber at Trefignath (Holyhead) can still be seen, for instance. A cairn is essentially a mound formed entirely of rocks, and is often found on the summit of mountains or along old trails in the uplands; cairns are a typical feature of later prehistoric and medieval field-clearance.

Studies of the cromlech at Lligwy (not to be confused with Din Lligwy), on the other hand, not only revealed no signs of a mound, but suggested that it was designed to provide its builders and later generations of visitors with a spring and autumn equinox alignment. This feature can still be appreciated today, on March 21st and September 21st, and this function would be rendered impossible had the megaliths been hidden by an earth and rocky cloak.

Research discovered that the giant capstone was not raised in place by human effort, forming a chamber enclosed by vertical stones. Rather, a cavity was dug beneath a large slab of bedrock (the capstone) which was then supported in three places by megaliths and further enclosed by other upright stones.

During the equinoxes, the last rays of the sun as it descends below the horizon cast a shadow stretching across the chamber floor to the monument entrance. Inside the subterranean space the light is gentle, an eerie illumination manifested by sunlight reflecting on the pale limestone surfaces of the megaliths.

Beyond, the shaded passage gives the spirits of the entombed dead access to unknown spheres, while also giving access to the living as they search for the mystic human experience.

• To find Bryn Celli Ddu (OS map 114: SH 506701) take the A4080 (Brynsiencyn) road past Plas Newydd, turn off for Llanddaniel Fab and follow the signs.
• Lligwy Burial Chamber is off the A5025, 1km/0.6mls north of Llanallgo.