Summer Weekend Field Trip to Suffolk 2021

As covid gradually recedes, the club has restarted its annual weekend away field trips. This year we were in Suffolk being guided by Tim Holt-Wilson. We stayed around Saxmundham, and journeyed north on Saturday and south on Sunday.
Tim gave us an introduction to Suffolk geology on the Friday evening. The oldest beds we saw were the London Clay formation – Eocene marine mudstone at 52Ma. We also saw the Coralline Crag Formation (early Pliocene sandy, fossiliferous marine limestone around 4Ma) and the Corton Formation (glacial till and outwash deposits around 0.5Ma).

Bawdsey East Lane beach. The grey at the base of the cliff is the London Clay formation. The higher brown layer is the Coralline Crag formation. Extensive erosion is evident. The pipe dangling down is a field drainage pipe left exposed by rapid erosion.
There is 48Ma missing between these two beds; but the unconformity is not evident in the cliff face.

The “rock” forming the wave-cut platform of the beach is actually London Clay – very slippery and treacherous at times.

Many of the pebbles on the beach are actually rounded pieces of London Clay (scale is an A6 field notebook).

The Coralline Crag formation in-situ with pieces of shell protruding from the eroding cliff face (dangling drainage pipe is the scale)

A piece of eroded coralline crag material on the beach showing numerous fossil fragments. Nearly all of the fossils are fragmented, so identification is difficult (scale is the camera lens cap)

Cross bedding in the Corton Formation at Pakefield Cliffs

Suffolk was at the edge of the ice sheet in recent glaciations. During glacial times, there were three large rivers draining from higher ground further west across present-day East Anglia into the North Sea: one in the north arising from the south Pennines, a central one arising in the West Midlands and a southern one arising in the London area. These conveyed stones from these areas which are now found on East Anglian beaches.

A wide variety of pebbles at Pakefield beach – these reflecting the draining from a wide area of the west Midlands, possibly as far as Wales.

London flints on the beach at Benacre Broad.

Coastal erosion is occurring rapidly at many locations, although sand does move up and down the coast, and some areas show increased deposition. Dunwich was a major population centre in the Middle Ages, but erosion resulted in loss of >90% of the town, so only a small village remains today. We saw a fascinating model of Dunwich in the 12th century in the village museum with a yellow line half a kilometre inland representing the current coastline.

The Martello Tower at Bawdsey East Lane beach. These towers were built around the coast during the Napoleonic Wars to protect Britain. Now we have to protect them by installing rock armour to stop the sea eroding them away!

A tree recently lost to coastal erosion at Shingle Street

A sand bar at Shingle Street showing the deposition that occurs side-by-side with the coastal erosion.

One other feature of trade in the Middle Ages was that boats would come from many countries in Europe with local stones as ballast, then leave the stone as they loaded up their cargoes. Because there is virtually no building stone in Suffolk, these ballast stones would be incorporated in the walls of the churches and houses built at this time.

Flints incorporated into the wall of Orford Church. Look particularly at the second and third row up from the bottom

The second row up is formed of Swedish flints brought here as ship’s ballast. Note the “milky way” background

English flints in the third row up. Note the smooth glassy texture.

The Church of St Andrew at Covehithe. Originally a very large church, in 1672 the parishioners were given permission to knock most of it down and rebuild a much smaller church within it. The old church incorporates interesting building stones, many of which were re-used in the walls of the small church.

The old Franciscan monastery of Greyfriars at Dunwich. Tim Holt-Wilson (sitting far left) is going to show details of the building stones to members

Field trip to Waun Marteg July 25th 2021

The field trip on 24th July took us to the Waun Marteg forest, near Bwylch y- Sarnau. led by Tony Thorp

The four quarries visited took us through a sequence from the late Llandovery ( Telychian stage) to early Wenlock ( Sheinwoodian stage) The exposures form part of a syncline and the route took us from west to east from oldest to youngest rocks.

We parked at the first quarry (location one SO 009 770) which exposes the junction between the Dolgau Mudstones and the early Wenlock Nant-ysgollen Mudstones.
The Dolgau Mudstones (Llandovery, Telechyian stage) are grey green, turbidite mudstones with burrowed hemipelagites. An example of the burrow-mottling was found in the scree and can be seen below. During the late Telychian tectonic events had become quiet. This allowed the re-establishment of oxic slope-apron sedimentation within the Welsh Basin which is characterised by the Dolgau mudstones.

Burrow Mottling

The Nant-ysgollen formation is the earliest strata in the Wenlock series and overlies the Dolgau mudstones. It marks the return of oxygen-poor bottom conditions and deeper water. It consists of thinly interbedded turbidite mudstones and laminated hemipelagites. The formation is in the Cyrtograptus centrifugus biozone and the base of the Wenlock series and the Sheinwoodian stage is equated with the base of the centrifugus biozone (although Mullins and Aldridge (2004) suggest that the actual level is at the top of that biozone or at the base of the C. murchinsonii graptolite biozone). The formation showed well developed cleavage which had a high angle dip and pencil cleavage was apparent.

First Quarry,SO 009770, with the junction between the Dolgau Mudstones and the Nant-ysgollen Mudstones

At the second location ( SO 013 771) the Nant-ysgollen mudstones were the predominant lithology. Bedding of shallow dip was apparent as were concretions and well developed pencil cleavage. Whereas at the third location ( SO 016 773) although the hemipelagites and pencil cleavage of the Nant-ysgollen mudstones were still present, towards the top of the exposure a fine sandstone probably of the Penstrowed Grits Fm was apparent. Within the scree pale calcareous mudstones were numerous.

Second location,SO 013771, shallow dip in the Nant-ysgollen mudstones

Horizontal bedding in the Penstrowed Grits Fm.

Location four (SO 012 776) was a large quarry within the base of the Penstrowed Grits Fm. with good example of sandstone mudstone turbidites. The Penstrowed Grits is an extensive sandstone lobe facies that was supplied from the south west end of the basin. The medium- to thick-bedded, high-matrix turbiditic sandstones are interbedded with thin siltstone turbidites and laminated hemipelagites. The muddy turbidity currents were probably supplied from the adjacent platform and slope rather than from much further south. The bedding was almost parallel at the SW corner of the quarry but the sandstone beds at the NE end of the quarry became jointed and the mudstone had developed pencil cleavage.

Pencil Cleavage

At the end of the visit a few of the members decided to carry on to a fifth location close to Bwlch-y-Sarnau (SO 035 750). A NW/SE fault crosses the quarry. The junction between the Nant-ysgollen Mudstones and the Penstrowed Grits Fm was exposed. The bedding in the Penstrowed grits was at a very steep angle and calcareous mudstones were again present. Good examples of current aligned graptolites were found.

Quarry Bwylch-y-Sarnau with member along the line of the fault

Current aligned graptolites found at the quarry

NOTES

Hemipelagite is formed by the slow accumulation of biogenic and fine terriginous material onto the sea floor. There are two types of hemipelagite laminated and burrowed. laminated hemipelagites occur under anoxic bottom conditions are dark grey and have a bedding parallel lamination, whereas burrowed hemipelagite occur under oxic bottom conditions is pale grey/green with darker burrow mottles and lamination is prevented by burrowing and scavenging benthos that constantly rework the sediment.

Pencil Cleavage
“Pencil cleavage is a low-temperature tectonic fabric formed due to shortening of clay-rich sediments like mudstones”
When mudstone or clay-rich sediments are deposited they will tend to break along bedding planes as the platy clay minerals give a natural bedding parallel fissility. If at a later time the rock is subject to another deformation then a second cleavage may develop with a different orientation to the first. This is due to pressure solution and reorientation of clay minerals. At some point this secondary cleavage will be just as pronounced as the first cleavage and the shale will fracture along both planes and form pencil like structures. Pencil cleavage can also develop where two tectonic cleavages develop in the same rock.

Mullins, G. L. & Aldridge, R. J. 2004. Chitinozoan biostratigraphy of the basal Wenlock Series (Silurian) Global Stratotype Section and Point. Palaeontology 47, 745–73

Field trip to the Ring Hole Dolfor on 16th June 2021 - led by Tony Thorp

Field trip to the Ring Hole Dolfor on 16th June 2021 - led by Tony Thorp

The Ring Hole is a deep semicircular declivity within a huge thickness of glacial till. It is an active landslip area formed by the constant removal of till from the base of the slope by the river Teme. As the till is removed the gradient of the slope is maintained above a critical angle of stability. (BGS)

The Ring Hole

At the meeting place, above the Ring Hole, Tony explained that we were on a bleak plateau at some 450 m surrounded by a number of rounded hills, of about 500 m height. This is held to be part of the Mid Wales “peneplain”, a proposed pre-glacial erosion surface, established during Mesozoic times and since then uplifted, dissected and eroded by ice and water. The generally rounded nature of the topography results from the uniform sedimentary nature of the Silurian bedrock.

Scree at base of landslip

We descended into the Ring by taking the path round to the west, from which the whole structure could be seen. It is some 150m wide and over 50m deep. The near side shows a near vertical face in the till with scree slopes below. At the base of the landslip we stopped to examine the scree which was composed of sandstone and siltstone similar to the local rocks. We then continued our descent to the bottom of the gorge and the brook ( infant river Teme) crossing the brook to examine the rocks exposed in the bottom of the gorge

Undisturbed beds in the Bailey Hill formation

The rocks exposed are from the upper Bailey Hill Formation ( Ludlow Epoch) and comprise thinly bedded sandstones and argillaceous siltstones. As you walk along the exposure it becomes obvious that there are different areas where firstly the rocks are undisturbed with very shallow dips to rocks that are disturbed showing clear soft sediment deformation with complex folds.

Recumbant fold - disturbed beds

The disturbed beds here and in other exposures nearby are indicative of the original palaeoslope onto which they were deposited. This is different from the various sole structures (flute casts, ripples, etc.) which indicate the depositing current direction. They enabled Bailey and other geologists to postulate the "Montgomery Trough", a north east to south west declivity going through the region into which the slumping was directed. The depositing currents, in contrast, were generally from the south west.

Concentric fold - disturbed beds

Field trip to Woolhope Dome led by Dr. Geoff Steel on 22/9/2019

Report of a field trip to the Woolhope Dome led by Dr Geoff Steele

Sunday, 22nd of September, we took part in one of the best field
trips of the season. We had a bit of drizzle at one location, but otherwise the weather was ideal for walking, cloudy with occasional sunshine.

The Woolhope Dome is a classic Silurian location, being where one of
the earliest geological field trips took place and after which the
Woolhope Naturalists Field Club (est 1851) was named.
It is roughly elliptical and about six miles long, exposing rugged
Silurian rocks surrounded by the lower relief of the “Old Red
Sandstone”.

During Wenlock and Ludlow times alternating beds of limestone and
shale were laid down and were subsequently folded into a dome-like
anticline. Erosion acted unevenly, leaving the harder limestones as
high escarpments, rather like ramparts, separated by roughly circular
valleys corresponding to the shales. The older rocks are exposed in
the centre, with younger strata surrounding them

The meeting point was the large car park at Founhope Recreation
Ground, where initially Geoff explained we would see a classic eroded
anticline similar to e.g. the Zagros Mountains in the Middle East, but
on our doorstep and covered in vegetation. In some ways it is similar
to the Hafren Forest area where Ordovician rocks form an inlier within
the Silurian.

The geology is contemporaneous with the Silurian nearer home where it
is more basinal. The Dome comprises a “shelf” facies and Geoff had
scaled equivalent sections to compare shelf with basin, the total
thickness of the shelf deposits being about one third of that of the
basinal ones. This reflects the much greater subsidence in the basin,
occasioned by movement on the NE to SW Caledonian fault system along
the shelf edge.

For convenience, we left one car and took the other to our start point
on the geologically lowest stratum at the highest point in the centre
of the dome in Haugh Woods.

The trip was carried out as a short, but stiff, three mile walk back
to the village, going up the sequence. (We made one concession to
convenience, by calling by car at what should have been the last
location. This was in the Pridoli Downton Castle Formation, comprising
reddish thin bedded sands and silts, somewhat reminiscent of the same
formation in Downton Gorge. Had it been present, the bone bed would
have been somewhere in the scree at the bottom.)

Pridoli Downton Castle Formation

Wild Service Tree

In Haugh Wood, after a diversion to look at rare specimens of the Wild
Service Tree (which is a Sorbus, like the rowan and whitebeam, whose
berries were used to “bitter” beer before hops were used and whose
leaves showed an interesting evolutionary stage within Sorbuses
between the simple palmate of the whitebeam and the ash-like ones of
the rowan.) we made a passing acquaintance with the Haugh Wood
Formation in the forest path. This was a greeny-grey poorly
fossiliferous thin bedded limestone which lay just below the Woolhope
Limestone. This we soon found well exposed both in a track and in a
nicely cleaned up quarry which is an SSSI, so no hammers.
where we had lunch. The bentonites are at the bottom of the exposure shown.

Woolhope Limestone

We did find three bentonite beds exposed in medium bedded limestones
with some more massive limestone beds. There were plentiful crinoid
ossicles, some bryozoans and some rhynchonellid brachiopods. Picking
suitable rocky seats, we enjoyed our sandwich lunch in the sunshine
before walking down the wooded dip slope into the surrounding valley,
where the more easily eroded Wenlock Shales were exposed in a stream
bed.

Wenlock shales in stream bed

Crossing two level grassy fields, we ascended the wooded scarp
slope of Wenlock Limestone. This was exposed in some small disused
quarries, stuffed with fossils. After fossiling, once again, we walked
down the dip onto the Lower Ludlow Shale where the shale comprised
level pasture before climbing up the wooded scarp of Aymestry
Limestone. At the top, we could examine the rock where it was not too
well exposed alongside the track, before walking down the dip slope
and back into the village of Founhope. There we come off the dome onto
the level ORS Raglan Mudstone as we returned to our start point.

For a short walk, we had seen an extraordinary geological section
through an eroded anticline which is a location of both geological and
historical significance.

Section through Woolhope Dome from information board

I have previously visited the dome in the distant past, with other
societies, but only as a multiple location car tour. I realise now
that was the wrong way to approach it, giving a poor representation of
its structure. It was far more instructive to take it, as now, as a
short, but sometimes steep, walk. Very rewarding, as the walk up the
scarp can be tough, but one can relax on the gentler descent down the
dip!

Field trip to the South Wales Coalfield, Sunday June 9th 2019

Members enjoyed a trip round the Pontneddfechan area led by Dr Gareth George. Gareth also led a field trip on the Mumbles Coast for the Severnside group of the OUGS on the Saturday, so all our members attending went to both meetings, and we were joined by some Severnside members for our Sunday field trip. The weather was rainy in the morning, but it brightened up later on.

We visited four separate sites during the day: Penwyllt Quarries, the River Tawe at Ynyswen, Henrhyd Falls and Craig y Dinas.

The Penwyllt quarry area includes limestone quarries, quartz-arenite quarries and a large area of old industrial sites including brick-making and an old rail track-bed.

The limestone quarry has thick basal beds which represent the top of the Oxwich Head limestone with thinner overlying beds which are the bottom of the Oystermouth series.

Limestone quarry with the Oxwich/Oystermouth junction

In some areas there is a paleosol between the Oxwich & Oystermouth; but it was inaccessible in this particular quarry.

Left on the quarry floor were some blocks with a good demonstration of hummocks and swales: a particular sedimentary feature which usually indicates formation during stormy weather.

The curved upper surface of the block is the hummock

hummocky pattern of the laminae on the side of the block

We then went to a quarry in the Twrch sandstone where there is an excellent example of an old channel formation: a localised lens-shaped sandstone structure within regular beds.

old river channel in the Twrch sandstone

We then went to the River Tawe at Ynyswen where the river forms a deeply incised meander with multiple sandstone beds exposed in the vertical valley side. Within this exposure is a distinctive bed called the Twelve-foot sandstone. The river is actually running over the top of the Twrch sandstone which we had previously seen in the quarry at Penwyllt.

the twelve-foot sandstone high in the vertical valley side

the top surface of the Twrch sandstone forming the river bed

The next stop was Henrhyd Falls – the highest waterfall in South Wales, with a drop >90ft. The water falls over a resistant sandstone layer known as Farewell Rock. The name came from the 19th century coal miners who recognised that if they dug down to this particular rock layer, they could say farewell to any chance of finding coal beneath it!

Henrhyd Falls, where brave souls can walk behind the falls!

The final stop was at Craig y Dinas. From there, it was a short walk along the Sychryd valley to see Bwa Maen – a spectacular folded block of limestone within a fault zone. Bwa Maen is “bow of rock” in English. The South Wales coalfield has several extensive fault systems. The old miners did not know about faults; they just knew that there were lines where you could not follow the coal seams any more, and they called these “disturbances”. The Vale of Neath Disturbance is one such major fault system, with the main splay of the fault (the Dinas Fault) going along the Sychryd Valley.

Bwa Maen

the apex of the fold