The administrative centre of Caerphilly is Hengoed. Other principal towns are Bargoed, Newbridge, and  Rhymney.

The chief rivers of Caerphilly are the Rhymney and Sirhowy.

The main rivers of Monmouthshire are the Wye and Usk, which are famous for salmon and trout fishing.

The lowlands of Monmouthshire have rich mixed farming, with arable crops, including wheat, being important. The coast of Monmouthshire is exposed to high spring tides which rush up the Severn in a `bore´ from the Bristol Channel, rising at Chepstow sometimes to 18 m / 60 ft. The southern part, east and west of the Usk, comprises the Caldecot and Wentloog levels, which are protected from the sea by sea walls. North of the Caldecot level, between the Usk and the Wye, the surface is undulating. The north of the county is more mountainous. About 7 km / 4.3 miles from Abergavenny is the peaked mountain called Pen-y-Fal or Sugar Loaf (596 m/1,955 ft), over 8.1 sq km/3.1 sq mi of which have been presented to the National Trust. Between Abergavenny and Usk is the wooded hill-fort of Coed-y-Bonedd, one of several Monmouthshire camps. Skirrid Fawr (486 m/1,595 ft), known locally as the Holy Mountain, has views of the Black Mountains, the Usk valley, and the Sugar Loaf.