The Old Red Sandstone in the Brecon Beacons (and some Quaternary topography)

There are probably more erudite accounts of this day, not least in the OUGS Severnside's next newsletter, the Sabrina Times. However, this is my own version of an interesting day out. Hopefully it is accurate as far as the geology goes (although by no means as comprehensive as Norman's newsletter report will be!) although with name changes going on it is quite hard to get your head round at times!

We met by Craig y Fro quarry, just down from the Storey Arms heading north where we were not viewed too favourably by the man in the burger van who thought we were taking up parking that his potential paying customers might use during the day!

John began by telling us the tale of how he had noticed pebbly conglomerates in the Beacons and knew there were pebbly beds in the Brownstones Formation in Pembrokeshire so he was keen to explore the area and work out links between them if possible.

Stop 1 - Craig y Fro quarry

Heading across the busy main road to Craig-y-Fro quarry

This quarry in sandstones and mudstones of the Senni Fm (below what was known as the Brownstones) provided research material for Dianne Edwards into the early plant fossils for which it is famous, and also for David Attenborough's Life on Earth series. These fossils are of small, branching, plants which have been carbonised. Unfortunately the site was over-collected in the 1960s and, to provide more material, the upper left hand corner had to be dynamited. It is now a Site of Special Scientific Interest (SSSI) and no collecting is allowed

The back wall of Craig-y-Fro quarry

 The back wall of the quarry shows a series of mudrocks/shales, and sandstones, and we were warned by John about the very slippery nature of the shales, particularly where they crop out in the stream bed! As an aside, a friend told me that their MRT used the waterfall for ice climbing practice!

Exposure on the north side of the quarry

The exposure includes a series of channels, filled at their bases with mudstone and calcrete clasts which have been eroded from the previous channel cycle. This channel series passes upwards into mudstones; the top of the sandstone has been shown to have plant fossils, and is particularly rich in large mica flakes, maybe 1 mm.

Cross stratified channel sandstone

Many of the mud clasts have eroded out, leaving holes. Calcrete clasts show white

A bed formerly rich in mud clasts (now eroded).

Military road north of Storey Arms

At the top end of the road, by the start of the Corn Du path, John gave us a mini-talk about the Lower Devonian Senni Beds/Brownstones "transition". A few points from it:

John Davies explaining the geology to us

• This boundary was, historically, described as transitional, even though it was placed on a conglomerate lithology, by reason of a gradual colour change from green up to maroon in the Senni Beds.
   
• The conglomerate contained exotic clasts
• It used to be said there were no fossils in the Brownstones but in fact this is "no body fossils" (plants and animals) since there are Beaconites burrows.
• The earlier Senni Beds have all sorts of Euripterids etc but are then topped by the conglomerate body. The Senni Beds are in a discrete block and may represent an area with a microclimate. The bed of coarse clastic material runs across from here to Ross on Wye and the Lower Brownstones there equate to the Senni Beds elsewhere (you can't tell them apart by colour)

At this point he broke off to describe the landscape in terms of its geology. Looking south we could see the Upper Devonian Plateau Beds at the top of the hill. Easy to see how they got their name!

The arm of the signpost is carefully aligned to point out the Plateau Beds on the ridge. 

Looking back north towards Craig-y-Fro we could see the gently southward dipping beds of sandstone and mudrock. These are broken by normal faults, downthrowing to the north. As I'm in the middle of working through a case study of the Book Cliffs in Utah I could see some similarities, despite the lack of exposure here! Another member of the group could as well but we aren't sure if we can carry the analogy very far since these are apparently interpreted as fluvial rather than marine/littoral deposits so it is hard to see how flooding surfaces, sea level rise and fall, sediment partitioning etc could apply here despite that superficial resemblance.

Beds of sandstone and shale above Craig-y-Fro

 He also described the hummocky terrain left in the valley bottom after the last glacial.

Looking north down the valley towards Libanus - the hummocky, stream incised valley fill is glacial material 

Location 2 - on the old Military Road north of the Storey Arms

We walked down the old road, past a quarry, with a good view of the exposures above Craig-y-Fro. Eventually we came to some small exposures of conglomerate in the path. This is the Lawrenny Cliff Fm which lies directly over the Senni Fm. The fabric shows matrix support and the clasts have been identified as from Mid-Wales, though some are very rounded and may be reworked.
Two identifiable conglomerate beds can be mapped from Pembrokeshire, through the Beacons, to the Ross on Wye area, in what used to be called the Brownstone Group.

Examining the conglomerate on the Old Military Road

The variety of clasts can be seen

From here we can see exposures up the hillside to the west which are the New Shipping Fm, a slightly younger part of the "Brownstones". More of that later.

Lunchtime

The afternoon stops were to be in the next valley west, so, since the Brecon Mountain Centre lies conveniently on the route, we went there to eat our lunch though, sadly, only the cafe/loos were open. However, the sun came out and we were treated to lovely views east to the high Beacons.

Brecon Beacons from the Mountain Centre on a beautiful October day!

Once again John described the connection between the landscape and the geology though unless you are on the spot it doesn't mean a lot! Except to say that between us and the Beacons the ridge is Senni Fm, topped by Lawrenny Cliff Fm

Location 3 - Above the Devil's Elbow

Amazingly we kept the convoy together through a number of left and right turns, negotiated the bends of the Devil's Elbow without damage to us or a campervan coming the other way, and arrived at yet another place with splendid views north.

A major feature here is the valley on either side where the Swansea Valley Disturbance cuts through from SW to NE whilst we are standing on crags of Lawrenny Cliff Fm.

The Carreg Cennan disturbance runs across below the distant skyline and the Swansea Valley disturbance from the valley top right across to the valley below us on the left.

Cross stratification and slumping in the Lawrenny Cliff Fm above the Devil's Elbow

And a close up - hard to see through the lichen but there is at least one well rounded quartz pebble!

 From here we continued over the watershed to the south then and stopped by the Afon Llia to look at exposures of the overlying New Shipping Fm which can clearly be seen dipping south here, in the same direction as the stream is flowing.

Last stop of the day by the Afon Llia, looking north, upstream

Bedding planes dip south, downstream, creating a series of water slides

Beds of the New Shipping Fm can be seen clearly in the far bank

And here, in the near bank.

So, having completed two transects under John's expert guidance, through the south-dipping beds of the north crop of the South Wales syncline, we headed home. Well not quite home as the chance of being near the whisky distillery at Penderyn seemed too good an opportunity to miss for a few of us!

According to their website, "‘Distilling in Wales was a lost art...’
But in the late 1990s in a pub in a small post-industrial Welsh valley town, a group of friends drank and chatted about establishing the first
whisky distillery in Wales in over a century. They dreamt of creating a whisky as pure and precious as Welsh gold, represented today by Penderyn’s ‘gold seam’.

The "gold" seam on Penderyn's logo

The journey home should have been straightforward along the Heads of the Valleys road and through Abergavenny to the A40 but roadworks between Merthyr and there forced us to take an interesting diversion, winding our way through little villages and round steep, tight corners. An unexpected addition to a good day out.