Geologists’ Association - South Wales Group -- Cymdeithas y Daearegwyr - Grŵp De Cymru

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Geological Walks in Wales -

Cribarth to Craig-y-Nos

Cribarth lies north of the upper Tawe

(Swansea) valley. Its summit,

approximately 4km.

Location: Parking is available at

Craig-y-nos Country Park off the

A4067, 40 km NE of Swansea, grid

reference SN 8400 1554, Post Code

SA9 1GL.

Maps: Ordnance Survey 1:50,000

Landranger Sheet 160 (Brecon

Beacons) and 1:25,000 Outdoor

Leisure Sheet 12 (Brecon Beacons West and Central); British Geological Survey 1:50,000 Sheet 231 (Merthyr Tydfil)

is grey, with white, crescent-shaped fossil shells; Marros Group (formerly called the Millstone Grit) has round, creamy

coloured pebbles cemented together.

The stones on the path are limestone, made of pieces of fossils cemented together. The fossils include brachiopods,

corals and crinoids that lived in a warm sea that covered southern Britain in the early Carboniferous period of time,

some 350 million years ago - long before today's

hills and valleys formed.

The British Isles were then south of the equator:

continental drift has since moved them

northwards.

Look closely at the limestone - a magnifying glass is

Geologists’ Association - South Wales Group -- Cymdeithas y Daearegwyr - Grŵp De Cymru

WWW.SWGA.ORG.UK

Take the path marked "Geological Trail". It is marked by white posts. After 400m, stop at a crest with crags of

limestone, at a point near a fence where there is an open view.

Locality 2. Across the valley is a limestone quarry at Penwyllt. Higher ground to its right is made of Marros Group,

which overlies the Carboniferous Limestone Supergroup - it formed later, and is younger.

The layers - beds- of rock are typical of sedimentary rocks, that

built up on the sea-bed over millions of years. The beds were

once horizontal and covered a vast area. They are now tilted

(dip) to the south, and erosion has carved out the landscape.

is older than Carboniferous Limestone supergroup. To the

north-west the Marros Group has scarp-and-dip features like at

Penwyllt. In between is the Carboniferous Limestone

Supergroup. The buildings left of the road are Dan-yr-ogof, with its caves and imitation dinosaurs. Closer to you, on

the extreme left, is a steep-sided valley with a large saucer-shaped depression, called "The Crater", near its head.

There are smaller depressions to its right.

Bearing Left, continue upwards; and up to the stile.

Locality 3. There are many fossils in the limestone blocks of the

wall - do not collect from these! There are also blocks of red rock:

these are erratics of Old Red Sandstone, carried here from the

north by ice during the last phase of glaciation, about 22,000 years

Geologists’ Association - South Wales Group -- Cymdeithas y Daearegwyr - Grŵp De Cymru

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Locality 4. These pale crags have rounded, fluted surfaces. They are natural limestone pavements, formed by solution

of the limestone when it was covered by soil. Notice that the bedding here is nearly horizontal.

Locality 5. This path follows the line of a tramroad, built in the 19th century when there were over 30 quarries on

Cribarth. Limestone was taken to the Swansea Canal at Abercrâf for agricultural use and later for the iron industry.

Fossils include sponges - large oval shapes with a delicate, cell-like structure. Bedding is tilted steeply to your left

(south. There are blocks of brown sandstone that were discarded during quarrying. Sandstone beds within the

limestone were deposited by rivers during the Carboniferous period when sea-level fell, forming islands like the

Bahamas today.

Return to the Path and head SW; after 1 km, stop by a stone wall.

Locality 7. The tall crag ahead, near the corner in the wall, has

The Cribarth Anticline continues the fold on Craig y Rhiwarth (Locality 2) and is part of a narrow zone of bent and

broken rocks called the Swansea Valley Disturbance.

If you knock together pieces of

limestone here, they make a

sulphurous smell, caused by the

decay of organic material.

Related processes result in the

formation of crude oil. Pale,

oolitic limestones form the

eroded core of the anticline:

nodules in the Chalk of southern

England.

Geologists’ Association - South Wales Group -- Cymdeithas y Daearegwyr - Grŵp De Cymru

WWW.SWGA.ORG.UK

In some places chert grew as tiny crystals: weathering dissolved the limestone, leaving a porous rock called

rottenstone, that was quarried as a polishing agent (see Locality 8). You can also find white, glassy crystals of calcite,

Turn left at the corner in the wall. Ahead to the right it is an easy ascent to the triangulation pillar at the summit

Locality 8. The Swansea Valley is in line with the ridge of Cribarth, because erosion has picked out weaker rocks along

the Swansea Valley Disturbance. The valley has steep sides and a broad floor, typical of erosion by glaciers. During

the last phase of glaciation, 22,000 years ago, a glacier flowed from the Brecon Beacons along the Swansea Valley.

Marros Group, including pieces of conglomerate - cemented

North from here you can see Pwll-yr-Wydden (see Locality 3).

Dark, lumpy ground to its right is where rottenstone was

quarried in the 1820's. The ditches between Localities 8 and 9

may be trial pits for rottenstone.

The Marros Group here is tilted steeply to the north-west.

Across the valley below you can see bedding surfaces sloping

towards you. This valley follows another fold - a downfold or

syncline - running alongside the Cribarth Anticline. Limestone

areas are well-drained, with short grass and thin soils,