Each stone is roughly three metres tall and one metre wide at the base, but only about twenty centimetres thick - tall, broad slabs of stone, like the page of an enormous book stood on edge. They are situated about three metres apart on flat grassland behind the farmhouse of Plas Meilw, two kilometres southwest of Holyhead and a similar distance south of Holyhead Mountain. They are aligned along their long axes in a northeast-to-southwest direction. From their location you can look out toward the coast in one direction and back toward Holyhead Mountain in the other. They are a Scheduled Ancient Monument under Welsh heritage law. Their estimated age - between 3,500 and 4,000 years - places them squarely in the Bronze Age, contemporary with the height of monument-building across western Britain.

Almost everything you might say about why the stones are there is either speculation or has been disproved. The most common theory in older literature is that they are the surviving members of a larger stone circle, with the other stones either removed or buried over time. No archaeological evidence has ever been found to support this. Another older story holds that there was once a stone cist - a small box-shaped burial chamber made of stone slabs - between the two standing stones. Again, no evidence has been found to support this. There is a possibility the stones marked the entrance to such a burial chamber, or were paired ritual markers, or served as boundary stones, or held some astronomical alignment - but in the absence of finds, each remains a hypothesis. What is definite is that they were placed deliberately, in this position, by people who intended them to stand.

About a hundred metres from the standing stones is the Plas Meilw hut circle - a separate Iron Age dwelling site that is much smaller and less well-preserved than the famous Holyhead Mountain Hut Circles a couple of kilometres to the north. The proximity is suggestive. People lived close to the standing stones for at least two thousand years after the stones were planted - which means that for most of the Iron Age and probably for centuries beyond, anyone living at Plas Meilw saw these slabs every day, attached significance to them or did not, told stories about them or did not, and passed those stories down. Whatever the stones meant to the people who erected them, they meant something different to every later generation that lived within sight of them. By the time anyone wrote anything down about Anglesey, the original meaning had been gone for centuries.

Walk to the stones across the open grassland above Porth Dafarch and they appear in profile against the sky, tall and thin and somehow improbably balanced on the soft ground. The wind on this side of Holy Island is constant and damp - the same wind that has been weathering these slabs for millennia. Approach closer and you can see lichen, weathering, the slight lean that one of them has acquired. The setting matters: the stones were placed where they would be visible from a distance, against the horizon, framed by the low hills on either side. Whoever erected them wanted them seen. Walk past with your back to them, look toward Holyhead Mountain, and you understand at least the geometry of their placement - they were planted on a line between the sea and the mountain, at a spot where both are equally part of the view.