In 1723 the antiquary Henry Rowlands - a parson and amateur archaeologist born on Anglesey - visited Bryn Gwyn and recorded the site in his book Mona Antiqua Restaurata. He described a 'ruinous circle of eight stones', some of them already toppled, but enough surviving for him to read the original shape. By 1797, when the second account appears, six of those stones were gone. The treasure hunt had taken them. Anglesey in the late eighteenth century was a poor place; the idea that the ancient Britons might have buried gold beneath their monuments was widespread, and any large stone with no obvious agricultural use was at risk. The Bryn Gwyn stones were too big to move whole, but the digging beneath them had collapsed the pits, and farmers simply broke up the stumps to clear the field.

The two survivors lived on largely because someone built a cottage against the wide slab in the eighteenth century, using it as the building's end wall. Notches cut along its top show where the roof timbers were fitted into the stone. The cottage occupied a patch of ground inside the original circle, sharing its space with whatever ritual geometry the Neolithic builders had laid out four thousand years before. By the time the cottage was demolished, the stone had become both Neolithic monument and recycled domestic architecture - the kind of layered survival you find only in places where nobody got round to clearing the site.

Modern archaeology has caught up with the antiquary. In 2008, an excavation found three pits where standing stones had once stood, two of them still containing the broken stumps - consistent with Rowlands's record of a partly ruinous circle. Two years later, further excavations in 2010 identified three more removed stone pits, bringing the total to seven of the eight expected. A further pit inside the circle held what archaeologists call a 'blade-style' stone, a thinner upright with an alignment toward both the summer solstice sunrise and the winter solstice sunset. The same dual alignment shows up at other Neolithic monuments along the Atlantic seaboard - including Bryn Celli Ddu, ten kilometres to the north-east. Whoever was building on Anglesey five thousand years ago was watching the sky carefully.

The Bryn Gwyn stones do not stand alone. About 280 metres to the north-east is Castell Bryn Gwyn, a substantial earthwork that has been variously interpreted as a Neolithic henge, a Bronze Age enclosure, and an Iron Age fort - it was probably all three at different times. Further to the north-east, at a place called Tre'r Dryw Bach, eighteenth-century visitors recorded another large stone circle. That one has been cleared away entirely; there is nothing on the ground today. What remained at Bryn Gwyn, what survived at Bryn Celli Ddu, what got swept off the land at Tre'r Dryw - the difference between visible prehistory and invisible prehistory on Anglesey is often a single landowner's decision in a single century. The two stones now leaning into a modern hedgerow are the lucky ones.