[1] A subsidiary top just over 1 kilometre (0.62 mi) to the north of the main summit is Carreg yr Ogof (meaning "rock of the cave"), whose name reflects its limestone geology.

Wrapped around its northern and western flanks is a narrow band of Carboniferous Limestone which gives rise to a karst landscape.

The Twrch Fechan Fault defines its eastern margin and brings Old Red Sandstone rocks into contact with the limestone on this flank.

A round barrow on Fan Foel was excavated in 2002-4 and showed a date of about 2000 BC with two separate burials in the cairn.

What makes them interesting is their great size of about 20 metre diameter, indicating their importance to the prehistoric local community.