There are several cairns scattered across Gelligaer Common. One of the best preserved can be found at SO098033. This cairn is about 5 metres across with a large central cist and a capstone about 1 metre square – supported on edge of cist. It was painfully cold in the bitter wind, with snow a couple of inches thick lying all around. Needless to say I didn't hang around too long.
There is also a cairn at SO098031 (which I failed to spot) and further cairns a little North West of the parking area at SO105031 (which I was too cold to look for!)