I have prepared a map of the area with caves shown where you can vary the transparencies of each layer:
In the Abergavenny area, there are 8 caves over 1 km long with a further two close by. Of these 8 caves, Ogof Draenen is the second longest in the UK (with some of the largest passages and many of the best formations), Agen Allwedd is fourth longest, Ogof Daren Cilau is fifth (with the largest passage and many impressive formations), and Ogof Craig A Ffynnon is tenth. Ogof Craig A Ffynnon is considered to be one of the "best decorated caves in the UK". Eglwys Faen has so many entrances in one area that it should be a record of some kind but it's probably not.
In total, there is over 150 km of known cave within a 15 km radius of Abergavenny.
The only mountains in the Abergavenny area that can hold caves are to the South and the West. This is where the limestone reaches the surface. The main mountains in which caves can or have been found in the area are:
Caves tend to occur anywhere where there is limestone, and the Abergavenny area has an abundance of it. Also, it is at the South edge of the ice field from the last ice age.
The meltwater from the local glaciers is assumed to have flowed in the valley to the Southeast of Llanelly Hill. Following this, the route was thought to have changed to its current route, to the North of the Trefil Moor and Mynydd Llangynidr, in between Mynydd Llangattock and The Sugar Loaf, and around the edge of The Blorenge. This river is now called The Usk. When the meltwater reached the edge of Mynydd Llangattock, it took a multitude of large underground routes, creating Agen Allwedd, Daren Cilau and Craig A Ffynnon, with the possibility also of having created Ogof Draenen before it created the above caves. All these caves are thought to have resurged (where the water returns to the surface) at Pontnewynnydd, where Ogof Draenen now resurges. This is the area at the south of the map where the limestone outcrop stops.
As the river Usk cut down more, it cut below the level of the limestone and so took the overland route that it now takes. Later the river Clydach cut the Clydach Gorge, breaking into the drainage system of the Mynydd Llangattock caves causing them to resurge for the most part in the Clydach Gorge. Several local drainage caves developed in the River Clydach area including Llanelly Quarry Pot. Some more local drainage systems developed under the moors to the West including Ogof Carno, Ogof Cynnes, Ogof Ap Robert and Ogof Tarddiad Rhymney, the last two being just off the left (West) of the map.
Firstly, for those who don't know the difference, I will explain it. Caves are mainly horizontal, Potholes are mainly vertical, requiring ropes or ladders in order to negotiate them.
Potholes tend to develop in an area where the limestone is open to the surface. Also a steeply dipping limestone can cause potholes, and not caves to develop. In the Abergavenny area, the dip is a very gentle 3° to 8° and the limestone is covered by a cap-rock of Millstone Grit, which is almost impenetrable by water except where faults have occurred. This means that what tends to happen in this area is that water sinks near the bottom of the limestone and then runs along the bottom of it in the caves. If water sinks near the top of the limestone, it tends to fall quickly in a pothole style before running, again, along the bottom of the limestone, so once again creating caves and not potholes.
There are in fact a few potholes in the area, two of which are of significant depth. The first of these is Blaen Onneu Quarry Pot, to the east of Ogof Cynnes. The second is not much deeper, but more significant; Crescent Cave, to the West of Ogof Cynnes. These are almost certainly part of larger cave networks, the main cave parts of which have not yet been found.