I have done posts before on the incomplete / destroyed ring cairn near Gernos Fach -- most recently following a visit by Chris and myself in 2016:
I have had another look at it, and am still rather mystified as to why only a part of the embankment is present. The southern part of the embankment is largely intact, and is easy to spot. The northern segment is missing, and there are signs that it might have been deliberately destroyed -- a few subtle digs or scrapes on the ground surface........ Also, it's possible that there might have been other stones on the embankment which have been taken away. Only a couple of standing or leaning stones survive at what might have been the entrance, but there are signs of other recumbent stones embedded in the embankment.
Was this an embanked stone circle? Maybe the archaeologists will look at it when they descend on Wain Mawn in September.
Now we have another ring cairn, which I had not noticed on previous visits. It's right next to the hill summit on Cnwc yr Hydd, about 30 metres from the summit cairn. It's much smaller than the Gernos Fach feature -- only about 10m in diameter. Again, the southern part of the bank is still present, and is about 50 cm high. The northern part of the bank is missing, and here the evidence of destruction is much easier to see -- there are hollows and cuttings littered with bedrock debris. One cutting, in the NE flank of the cairn, is about 4m in diameter. Could there have have been a hut here? It does look similar to the hut circles on Foel Drygarn and Carn Ingli..........There are no signs of standing stones associated with this feature.