Meeting at Craig Derlwyn	Geraint tells us where we are	This deserves a closer look	Permafrost remnants in cross-section

The next two stops were to examine the Millstone Grit. The first (720 161) was most ungritty. It was grey shales (with phosphatic iron nodules) and a fossiliferous (Gastrioceras subcrenatum) marine band. But the next (726 181) was in the Basal Grit with a white quartzite and quartz conglomerates. The site was a quarry providing material for refractory bricks used in the South Wales metal industries. A characteristic of this locality were many large plant debris fossils.

... and this is what you will see.	Plant debris in the Millstone Grit	In the Carb Limestone quarries. It's either a coral or a sponge -	or a mark on the rock

The Carboniferous Limestone at Black Mountain summit (735 188) was characterised by a multitude of sea floor fossils, exposed in abandoned quarries. Walking westward along the limestone, a fault can be demonstrated. One finds that one is no longer on limestone, there is quartz sand under ones feet. One is back into the Basal Grit of the Millstone Grit. On this day, this easily observed fault was being demonstrated not only to us, but also to several coachloads of Birmingham University geology students. They looked far more interested than I recall my fellow students being in similar circumstances, several decades ago!

Near the fault were several “shake holes” - the Welsh for sink-hole or doline. One in particular had been cleaned out by some aspirant caver. We did not feel inspired to check out his cave. Lunch was taken at the top of the Old Red Sandstone - the Grey Grit. (726 192) This is a quartz pebble conglomerate.
 

A shake hole	another view	Cwar Glas Quarries	Cwar Glas Quarries

We could see over the low ground underlain by the bulk of the ORS which is soft sediment of various types. There are few convenient places to examine this so we ignored it and went straight to the Upper Silurian at Pont ar Llechau (727 248) to look at the Ludlow Series. These are steeply dipping sandstones and shales, packed with shallow water fossils, especially in the “rottenstone” bands. As ever, no trilobites were found.

The last stop of the day was at Carreg Cennen Castle (668 191). The castle, and its steep crag, are built of Carboniferous Limestone, but it is surrounded by Old Red Sandstone. Obviously faulting is involved to bring the two rock types into contact - and it must be large scale faulting. This is the Carreg Cennen Disturbance, which trends in a Caledonian direction, from Milford Haven up to Church Stretton, and controls the south-eastern side of the Palaeozoic Welsh Basin. The older faulting was reactivated in Variscan times. Spectacular faulting leading to a spectacular site.

This excursion was a very pleasant and educative experience held in spectacular scenery. We learned a lot not only about the geology but also the human and physical geography of the Black Mountain. What at first appears untouched by man has been quarried for iron, silica and limestone. The glacial history was touched on and could form another excursion. And, of course, Geraint Owen explained it all clearly and with infinite patience. One could not expect a better excursion.

Here is Geraint Owens Handout - a model of its kind!

Geology of the North Crop of the South Wales Coalfield
Sunday 6th April 2003

The Black Mountain is the western massif of the Brecon Beacons, which form the highest ground in South Wales. The area lies on the North Crop of the Coalfield, providing exposures through a complete succession from the Coal Measures through the Millstone Grit, Carboniferous Limestone, Upper and Lower Old Red Sandstone, to Silurian and Ordovician rocks of the Tywi Valley.

SAFETY: Please take care to ensure your own safety and that of others around you. In particular, beware of slippery or loose rocks, scree and slopes; never climb rock faces; keep away from steep drops; never stand under overhangs or on them; wear a safety helmet in quarries or near cliffs, be aware of traffic when near roads; do not hammer rocks more than is necessary, and always wear goggles when hammering rocks; be aware of other people around you and consider their safety as well as your own.

We will stop at as many of the following localities as time allows.

1.Craig Derlwyn - east (SN 725 157). The "Farewell Rock" is a thick sandstone unit at the base of the Westphalian Coal Measures. Its sedimentary features are consistent with deposition on an alluvial plain with a source area to the north. The _view from here takes in the strike valley along the outcrop of the Lower Coal Measures, with working opencast coal sites, and the Pennant escarpment and plateau beyond.
2.Craig Derlwyn - north (SN 720 161). This site exposes grey shales with ironstone nodules, and a thin marine band that yields goniatites and bivalves (Dunbarella) from the Gastrioceras subcrenatum marine band, marking the base of the Westphalian. The shales belong to the Shale Group of the Millstone Grit.
3.Pen Rhiw-wen (SN 726 181). The Basal Grit comprises white quartzite and pebbly quartz conglomerate with cross-cutting channels. Impressions of plant fossils are common. The Basal Grit outcrop is scarred with small quarries that worked the rock for the manufacture of refractory silica bricks.
4. Black Mountain summit, south (SN 735 188). Disused quarries in the Carboniferous Limestone display bioclastic and oolitic limestones dipping gently south. Fossils include corals (solitary and compound), brachiopods, crinoids and gastropods. Karstic landscape features include shake holes, or dolines. A major fault - the Cwmllynfell Fault - lies west of the road, and large shake holes are developed close to the fault.
5. Black Mountain summit, north (SN 726 192). This parking area provides a spectacular view over the Lower Palaeozoic outcrops of the Tywi valley and the hills of central Wales. In the middle distance the Silurian ridge of Trichrug is prominent, deeply incised by the River Sawdde. Red soils can often be seen in the intervening ground, indicative of the Old Red Sandstone outcrop. The Black Mountain summits to the east are capped by hard units in the Upper Old Red Sandstone known as the Plateau Beds. Just past the hairpin bend is the 'Petrified Waterfall', a tufa-coated limestone crag.
6.Cwar Glas Quarries, Pont ar Llechau (SN 727 248). Safety helmets should be worn. The quarries expose steeply-dipping sandstones and shales of the Silurian Ludlow Series. There are wave-rippled surfaces, undulating parallel laminae (hummocky cross-stratification), trace fossils, and abundant body fossils, including brachiopods, bivalves, gastropods, crinoids and trilobites. Near the bridge the rocks in the river gorge become red, indicating the transition from marine Silurian conditions to continental Old Red Sandstone conditions. This regressive episode occurred here well before the main Caledonian (Acadian) deformation in the mid-Devonian.
7.Carreg Cennen Castle (SN 668 191). Carreg Cennen Castle is perched on a high crag of Carboniferous Limestone surrounded by Old Red Sandstone, and bounded by faults. It lies along the line of the Carreg Cennen Disturbance, one of several narrow zones of intense deformation in South Wales with a broadly Caledonoid trend. The Carreg Cennen Disturbance can be traced laterally into the Church Stretton Lineament, forming part of the Welsh Borderland Fault System, that controlled the south-east margin of the deep-water Welsh Basin during the early Palaeozoic. The present structure may be due to Variscan reactivation of the fault line. There will be time to visit the castle if people want to.