Thanks to Hugh and Geo, we have homed in on a rather interesting circular embanked feature not far from Carn Goedog. According to Dyfed Archaeological trust, its origins are mysterious, because there does not seem to be a ditch and bank arrangement (either outside in or inside out) and because the central enclosure is sunken and soggy. When I read the site description I immediately thought "Pingo?" -- and this possibility is worth considering.
Here's the description of the Nant Cledlyn SSSI in Ceredigion: "Nant Cledlyn Pingos SSSI is of national importance because it provides some of the best- preserved pingos in Wales. Each individual pingo comprises an elevated rampart that surrounds an internal basin that in some cases contains a significant thickness of peat. It is thought that the pingos were formed during very cold climate (permafrost) conditions that prevailed approximately 10,000 years ago, and that sediment deposition commenced at approximately the same time."
There are quite a few fossil or relict pingos in Wales, and they were properly described for the first time by Edward and Sybil Watson from Aberystwyth Univ. Geography Department -- a somewhat eccentric and wonderful couple who were inveterate field geomorphologists. Here is their map from a 1974 paper in Geografiska Annaler:
The Ceredigion examples are highly degraded, and sometimes all we can see today are slightly curved sections of embankments or ramparts. Here is another example from Thompson Common in Norfolk:
These are very subtle features in the landscape, dating for the most part from the Younger Dryas around 10,000 years ago, and it's very easy to confuse them with ring cairns or other man-made features. But geomorphologists have now found them all over the British Isles, and they are indicative of very severe permafrost conditions where peculiar hydrological conditions apply. When I was in East Greenland in 1962 some of my colleagues were studying a group of pingos in Pingodal which were in various stages of disrepair. They are ephemeral, and may last for a few decades or centuries. They have a core of ice -- an ice lens -- which tends to grow each year as more water arrives by subsurface flow (beneath the permafrost layer) and then freezes onto the lens. They can grow to 30m in height, and in Siberia and Alaska they are very spectacular features of the tundra landscape, as shown in the photos below:
They are also called "hydrolaccoliths" and "pseudo-volcanoes" -- the latter name is quite descriptive, because when the ice lens expands to the point where the surface sediment is too thin to provide effective insulation, it all starts to melt. The surface subsides, bits of the lens are exposed, and a "crater" begins to develop. This fills with meltwater, and that accelerated the melting of the lens until it is entirely wasted away, leaving a circular depression with an irregular rampart around its edge.
We should use the term "pingo" for something which is active, as in the Arctic examples above, and the term "fossil pingo" or "relict pingo" for the sort of thing we see in Wales.
There are two types of pingo -- open system and closed system -- but we won't go into the technicalities here. The most spectacular examples tend to be located on wide open plains or gravelly river valley floors, but they are also found in undulating or sloping terrain.
So could the Preseli example be a pingo? Quite possibly -- there are some small relict pingos on Trerhos Common, to the west of Wolfscastle, and others on the south side of Preseli. More to the point, the SSSI citation for Mynydd Preseli mentions relict pingos on Brynberian Moor, Gors Fawr and Waun Isaf -- and this is the type of territory we are talking about here.
Watch this space....... I suspect that the more one looks for ancient pingos, the more one will find.