Craig Ruperra is situated on the southeastern rim of the South Wales Coalfield which stretches north to Merthyr Tydfil and west to Llanelli and Kidwelly. The coalfield rocks, which are about 300 million years old, are bent into a downfold, with rock layers on the northern rim sloping gently southwards and those on the south rim sloping more steeply northwards. The southern rim is marked by several parallel ridges of hard sedimentary rocks like sandstone and limestone, separated by parallel valleys of softer rocks like mudstone.

Craig Ruperra is formed of a series of hard reddish sandstones called the Brownstones Formation. You can see these rocks sloping northwards at about 25º from the horizontal at the side of the track which leads up to the summit of Craig Ruperra from the west. The Brownstones lie underneath most of the reserve.

The Brownstones Formation forms a prominent ridge around the South Wales Coalfield. It continues west from here as the forested ridge of Coed Coesau-whips to Craig Llysfaen, Craig Llanishen and Coed y Wenallt. It is why Caerphilly Road rises steeply from Thornhill to the Travellers’ Rest. Northeast from Craig Ruperra, the Brownstones continue on the other side of the Rhymney River as the wooded slope of Graig Wyllt above Plas Machen.

These rocks then run north past Cwmbran to form the slopes of the Blorenge overlooking Abergavenny, as well as the slopes of the Black Mountains. They even form the Brecon Beacons. The rocks of Pen y Fan are formed of the same rocks as Craig Ruperra, although the Brownstones are much thicker in the Brecon area than they are here.

The Brownstones are part of a sequence of mainly reddish-coloured rocks deposited by rivers flowing southwards from high mountains across a wide coastal plain about 400 million years ago during the period of time known as the Devonian. Collectively these rocks are called the Old Red Sandstone.

Craig Ruperra falls away steeply on its southeastern side because beneath the Brownstones is a sequence of mainly soft rocks like mudstones, which, along with the Brownstones, belong to the lower part of the Old Red Sandstone. The St Maughans Formation, and the Raglan Mudstone Formation which lies beneath it, form all of the lower ground from here southwards towards the River Severn. Occasional harder sandstone beds within the mudstones form low ridges such as at Cefn Mably. Most of this lower ground is also covered with rock debris, called boulder clay or till, dumped by ice during the last glaciation, about 20,000 years ago.

Near the top of the St Maughans Formation is a thin bed of grey limestone known as the Ruperra Limestone. The outcrop of this rock runs close to, parallel with, and just within the southeastern boundary of the reserve and also just to the west of Ruperra Castle. The limestone was formed by limy nodules growing in the soil in the hot semi-arid climate of South Wales about 400 million years ago. The limestone was quarried locally for agricultural lime.

The northern slopes of Craig Ruperra are also underlain by the Brownstones Formation, but towards Draethen, the steep slope which descends down to the valley of Nant y Draethen is formed of rocks which lie on top of the Brownstones. These are again mainly sandstones, but this time more yellow or green in colour, and with some bands of coarser pebbly rocks. They are known as the Quartz Conglomerate Group and they belong to the upper part of the Old Red Sandstone. The same rocks occur on the Blorenge and cap the summit of the Sugar Loaf above Abergavenny.

Looking west from the summit of Craig Ruperra, you can see the steep, narrow valley of Nant y Draethen. This valley has been eroded along the outcrop of some softer rocks which lie above the Old Red Sandstone, although the stream itself runs on rocks close to the top of the Quartz Conglomerate Group. The geology changes here from sandstone to limestone. The limestone sequence belongs to the next, younger, period of geological time, the Carboniferous. The deposits begin with thin limestones (the Castell Coch Limestone Formation) and soft mudstones (the Cwmyniscoy Mudstone Formation) and these have been readily eroded by the stream to form the valley. This is an exceptionally good example of a valley eroded along the outcrop of a particular rock unit, a feature known as a strike valley.

The limestones mark a change in the environment of South Wales. The rivers and coastal plain of the Devonian Period were flooded by the sea and South Wales became a shallow tropical sea.

The northern side of the valley rises just as steeply as the southern side as the softer rocks give way to a sequence of hard limestones known as the Pembroke Limestone Group. These limestones form another prominent ridge around the edge of the South Wales Coalfield. Farther west, at Taff’s Well, they form the ridge on which Castell Coch is built as well as the Lesser Garth on the western side of the Taff where there is a large quarry. The limestone is quarried all along its outcrop and from Craig Ruperra you can see quarries at Cwm Leyshon on the edge of the ridge to the west and to the northeast at the long, narrow Machen Quarry.

To the left (north) of Machen Quarry, the dip in the skyline is formed of the coal-bearing rocks and then rises to coal-tip covered Mynydd Machen which is made of hard sandstones. These sandstones also form the distant hills you can see to the west, beyond the limestone ridge. The coal and sandstone marks another change in the environment of South Wales, as the shallow seas silted up and became humid equatorial swamps.

Next time you’re struggling up the path to the motte at the summit of the ridge, you’ll know you can blame the geology!

Geological map of the Ruperra area.

Reproduced with kind permission of the British Geological Survey

Geology

(With thanks to Tom Sharpe).

At 182m (591 feet) above sea level, the summit of Craig Ruperra provides a wonderful view of the surrounding country, south towards the River Severn and north into the hills and valleys around Machen. It is a great spot from which to see how different kinds of rock affect the shape of the land. The rocks here are all sedimentary rocks, rock layers deposited under water, either under the sea, or laid down by rivers.