A confused set of fences and gates at the edge of access land appears to bear no relationship to what the map is showing me and I emerge into an area of limestone outcrops and broken pavement, in which somewhere - so the map tells me - are two cairns, the first stop of the day. Rather overshadowing everything is the panoramic view to the northeast, sweeping across the reservoir to the central Beacons peaks.

I wander around amongst the limestone for a while, not really looking in the right place and finding nothing cairn-ish. Eventually I come across the northern cairn, a turfed-over mound with limestone blocks protruding here and there. The centre of the cairn has been scooped inevitably, but not recently if the covering turf is any indication. Treasure seekers rather than walkers have disturbed this one, it seems. The view of Pen y Fan is obscured by a small stand of trees, but would otherwise be the perfect backdrop. I fail to find the other cairn and eventually decide that bigger and better sites await.

To the north of here the map shows an enormous expanse of quarry, so my route cuts directly east up the slopes of Cefn yr Ystrad. This proves to be much harder going, the grass masking lumps and bumps of limestone and hollows that could turn an ankle with ease. I'm relieved to reach the ridge and even more relieved to see the day's main objective, the enormous bronze age cairn of Carn y Bugail ("Cairn of the Shepherd"). It's still some way off, and the intervening terrain is not the easiest to cross. What looks like a smooth grassy plateau actually takes a tiring 10 minutes of route-picking and step-watching even in this dry weather.

But the effort is entirely justified. The OS map shows two named cairns here, but our friends at Coflein are not content with that and have added another two. The named cairns are the real beauties, despite the efforts of many visitors to hollow out their interiors. Carn y Bugail has been moulded into a rather peculiar shape, two piles of stones heaped up on top of the mound giving it an oddly horned shape, like a toad or lizard. Despite this, it's a huge cairn, 3m high, as big as any I've visited and boasting terrific views to the central Beacons and across to the Black Mountains to the northeast. The view north is blocked by the equally massive Garn Felen. ("The Yellow Cairn") and the prominent mound that Garn Felen III sits atop, forming the end of the summit ridge. Beyond that Waun Rydd fades into the deepening haze as midday approaches. To the immediate northeast of Carn y Bugail are a collection of enormous (presumably natural) limestone blocks that form the outer extent of the cairn.

To the east of Garn Felen is a small pyramidal modern cairn, with a wooden cross set into its top. This monument to the crew of a Wellington bomber, marked in Gladman's fieldnotes, is indeed poignant. Even more so when you see that small fragments of twisted and melted aluminium surround the base of the cairn, the remains of the plane itself. Cause to stop a while. Despite the sadness of such a sight, there are worse places to be remembered. And remembered the fallen airmen obviously still are.

Garn Felen cairn is a match for Carn y Bugail in size. The top has been similarly scooped, but without the pointy rebuild. It remains a seriously impressive monument though, the plentiful limestone scattered all over the mountain's top being an easy source for such a monster. From here the obvious focal point is actually Garn Felen III and the Waun Rydd summit beyond, with a deep valley in between. So it's to Garn Felen III that I head next.

The obvious cairn here is a small, pointy, modern thing, but it sits on a great rounded mound of limestone blocks that forms the northern end of the long summit ridge. Coflein has recognised this for another bronze age cairn, although the OS don't mark it. Beyond, the ground falls steeply away, to a lower shelf where Garn Felen enclosure is visible. The landscape below the cairn is a weird, pock-marked sea of natural sink holes and possibly some human intrusion, like a turf-skinned holey cheese. The bigger scarring of the modern quarry is just visible over the ridge beyond.

I head back across to the SW to the summit trig point. I think this marks the highest point of the mountain, but the substantial nature of the main cairns means that they may rise above it. The trig has been well placed for the better sight-lines over to the west though. From here the three big cairns are laid out in profile, and what an impressive trio they make. Interestingly there is a flattened, circular patch of limestone blocks surrounding the trig. Could this be the remnants of yet another cairn? It certainly seems possible, although the Uplands Survey recorded the trig pillar but didn't comment on this in doing so.

Looking westwards, the ground drops away into a little cwm. On the slope opposite are the remains of Garn Felen II, a shattered cairn in a slightly odd situation. All that remains is a turfed over doughnut, with a scatter of exposed limestone blocks on the downslope side, the whole thing perched halfway down the slope. Compared to the other three cairns it is slight and has no impressive views either to or from it (although the prominent bump of Garn Felen III is in clear view). But it does make for a nice sheltered spot to sit and contemplate the minds of the people who came to this exposed, rugged mountain top millennia ago. They left behind monuments that survive so well and I'm sure they would be pleased to know that the places still exerts such a pull on this visitor.

I head across to the enclosure. Oblong in shape, the stonework of the walls still stands to a few courses high. Much more limestone lies around and about, so building material was certainly not an issue. What is rather less clear is why the structure has been built around a shake hole. I assume (geologists, please help) that the hole was already there when the walls were put up around it. It's not very big, so its mysterious portal-to-the-underworld qualities are fairly limited. Odd.