1

Dear Members

Despite the disappointing weather so far this summer, our programme has run

successfully. Only a few people dropped out early from the Rudry trip due to the

heavy showers! We enjoyed fair weather in Snowdonia where Elaine Burt from the

BGS explained the challenges of mapping an area where the adjacent maps are so

old that they needed some degree of re-interpretation. By the time the rain closed

in on Monday we had retired to the Cafe to enjoy ice-creams and lunches.

The sun shone on us in Ogmore and drew out the crowds. There was a good turn

out and most of us enjoyed scrambling around the rocks to find the fossils. (If only I

hadn’t sprained my ankle the week before....) Although cool, the weather for

Duncan Hawley’s trip to Llangattock was dry. Hopefully, John (our Editor) will

receive write-ups of all these events for you to peruse at your leisure.

Our next event will be on 29th July with Tony Ramsay to Cribarth, North of Swansea.

This will involve a short but steep climb to reach the ridge but it is a very interesting

area, all within the boundaries of Fforest Fawr Geopark. As we have a longer foreign

trip in September to Gaspe, the next day trip after this will be our Introductory Day

on 18th November, in its new Autumn slot. Stephen Howe, from the National

Museum will lead this day at Penarth. This is an excellent location for students new

to Geology. Of course it is not just for beginners, all are welcome.

I hope to see some of you at the Symposium in Northampton 17th – 19th August.

This event is always full of interesting lectures , field trips and social contact. You

should come along. This year it celebrates 40 years of OUGS. Many of the

speakers are ex-Presidents and the lectures cover the past and the future of Earth

Science. For those of you who have been members for a long time, the organisers

are looking for photographs for a display of past events. Can you help? If so, please

contact Don Cameron.

Our Day of Lectures is scheduled for Sunday 8th December in the National Museum

in Cardiff. Details of speakers will appear in due course. In line with last year, in the

afternoon, we hope that some members will be willing to give a short talk / slide

show of geology of somewhere they have been - 10 minutes maximum. All

presentations must be in Powerpoint. If you are willing to participate, please get in

touch with me.

Best wishes

Janet

Branch Organiser’s Report

SABRINA TIMES OPEN UNIVERSITY GEOLOGICAL SOCIETY

SEVERNSIDE BRANCH

JULY 2012

D8

Rudry, South

Glamorgan

Ogmore and

Dunraven Bay

Events

Branch Committee

and Editorial

Contacts

In this issue:

2

10

12

13

14

Sabrina - Goddess of the

River Severn

Wood carving at The Old

Station, Tintern,

Monmouthshire

2

Rudry, South Glamorgan – 22 April 2012

Rudry, South Glamorgan – 22 April 2012

Severnside had a field trip to the Mynydd Rudry and the surrounding area on the 22nd April 2012.

The following pages are copied from the handout prepared by the leader, Tom Sharp together with a few photographs

taken during the day...............

The area around Rudry southeast of Caerphilly displays a section through the south-eastern margin of the South Wales

Coalfield from the early Devonian Lower Old Red Sandstone to the late Carboniferous Pennant Sandstone Formation.

Although exposure is generally poor, it is sufficient to demonstrate the clear relationship between geology and landscape

here. In addition, high ground both to the north and south of Rudry €can, on a clear day, offer views extending over 40

kilometres in each direction, from the Brecon Beacons in the north to Exmoor in the south.

Our route will be dependent on the weather, but the aim is to walk a transect down the sequence from the Pennant

Rhondda Member to the Devonian Brownstones Formation then head up the sequence to the Black Rock Limestone Group

and the overlying Maenllwyd lnn for lunch. Our return route will depend on time and weather, but we will probably return

along the limestone ridge. lf the weather is particularly bad, we have the option of a direct return along the road.

Introduction to the geology of the South Wales Coalfield

The South Wales Coalfield is a plateau about 500-600 m above sea level, dissected by many approximately NW- SE flowing

rivers which occupy deep valleys. Almost all of the industrial development of the area has taken place along the valley

floors and sides, leaving the barren moorland of the ridges between the valleys to sheep.

The rocks of the Coalfield were formed in the Carboniferous Period, between about 350 and 299 million years ago. Older

rocks, mostly red and grey sandstones and mudstones of the Devonian Old Red Sandstone lie to the north, east and south

of the Coalfield.

The Carboniferous rocks of the South Wales Coalfield provided the raw materials for the industrial revolution, and much

early industry was located along the fringes of the coalfield. Here, iron ore, limestone, wood (for charcoal) and, later, coal,

were easily accessible.

Pembroke Limestone Group

The Pembroke Limestone Group is a sequence of grey limestones deposited in a warm, shallow sea during the early part of

the Carboniferous Period, about 360 million years ago. At this time South Wales lay in the tropics just south of the equator.

Much of the calcium carbonate (lime) of the rock was derived from the shells of the huge numbers of animals which lived

in the shallow waters. The limestones have been quarried for centuries for use as agricultural lime, as a flux in iron-

making, in construction, and as roadstone and railway ballast, Quarrying continues in a number of places along the

outcrop, for example, at Penderyn and Vaynor in the north, at Machen in the east and at Taffs Well in the south.

In some parts of its outcrop, the limestone has been altered from its original composition of calcium carbonate. Around the

eastern part of the rim, the limestone contains magnesium carbonate, dolomite, and in the south, in a discontinuous band

between Rudry and Llanhany, some of the limestone has been replaced by iron ores, mostly hematite, the red iron oxide,

Fe2O3.

Along the eastern and southern edge of the Coalfield, the limestone outcrop forms a prominent ridge, especially between

Taffs Well and Machen. The thickness of the limestone beds thins towards the north and towards the east, suggesting that

the original shorelines of the Carboniferous sea lay in those directions.

Marros Group

During the early part of the Carboniferous Period, the shallow sea which had covered South Wales began to silt up, as

gravel banks were deposited by rivers flowing from hills over Mid Wales. These gravels are now the conglomerates of the

lower part of the Marros Group, the Twrch Sandstone Formation, and are made up almost entirely of well-rounded pebbles

of white vein quartz, with occasional pebbles of jasper. Plants grew along these gravel banks and deltas, and are

sometimes preserved in the conglomerates. Above these beds is a sequence of shales and sandstones, the Bishopston

Mudstone Formation, which includes both marine and non-marine deposits, Along the southern margin of the coalfield, the

Marros Group rocks are much more muddy, with much thinner beds of conglomerates than occur along the northern

margin.

3

Rudry, South Glamorgan – 22 April 2012 (contd.)

South Wales Coal Measures Group and Pennant Sandstone Formation

The rocks of the Upper Carboniferous are divided into two main units, the South Wales Coal Measures Group,

comprising the Lower and Middle Coal Measures Formations, and the overlying Pennant Sandstone Formation. The

rocks are mostly sandstones, mudstones and coals. They were deposited along a coastal plain as river channel,

floodplain and delta deposits. For much of early and middle Coal Measures times, the rivers flowed from the north,

but later, as the mountain building of the Hercynian Orogeny raised land areas to the south of Wales, rivers began to

flow from south to north, carrying sands into the South Wales area, the Forest of Dean, and the Bristol area. These

sediments are the Pennant Sandstone which is found only in these southern coalfields. The hard Pennant Sandstone

forms prominent escarpments, such as Craig y Llyn near Hirwaun, the Garth at Taffs Well, and Caerphilly and Rudry

mountains.

In the late Carboniferous, Wales lay at the equator, and the hot, humid climate encouraged the growth of dense

vegetation on the coastal plains and deltas. Repeated episodes of submergence of the forest vegetation, either by

river floods or by incursions of the sea, killed the forests and buried the vegetation beneath a layer of sediment. In

time, the deltas were re-established and the forests re-grew.

Coal

Coal is the compressed remains of plants. As peat and decayed plants accumulated on the floor of the Carboniferous

forests, and the area was repeatedly flooded, the plant material was squeezed and altered. In the South Wales

Coalfield, we have coals which vary in their quality from the bituminous or house coals of the southern and eastern

parts of the coalfield, the steam coals of the central part, to the high quality anthracite of the west and north-west.

The distribution of the different qualities (or rank) of coals in the South Wales Coalfield has not been fully explained.

Bituminous coal is about 80-90% carbon, burns with a smoky flame, and leaves quantities of ash. It is soft, and

friable, often with a blocky structure and containing bright and dull layers. Bituminous coal occurs mainly to the east

of the Taff and along the southern margin of the Coalfield. It was mined for domestic use, for locomotives, and for

gas-making. About 30% of the South Wales coals are bituminous.

The structure of the Coalfield

The Coalfield is an elongate, saucer-shaped basin 90 km long by 30 km wide, stretching from Pontypool in the east

to Kidwelly in the west and from Taffs Well to Merthyr Tydfil. The edge of the coalfield is defined by the outcrop of the

Carboniferous Limestone which forms a discontinuous rim from Kidwelly, through the southern part of the Black

Mountain and Brecon Beacons to the Blorenge near Abergavenny, then south through Pontypool to Risca and then

west through Taffs Well to Porthcawl. The beds of rock slope towards the centre of the basin, but the rim is lopsided,

with a dip of about 10° to the south along the northern rim, and about 50° to the north along the southern rim. The

detailed structure is more complicated, and there are several other east - west folds in the rocks within the coalfield.

The main ones, from south to north are the Llantwit-Caerphilly Syncline, the Pontypridd- Maesteg Anticline, and the

Gelligaer Syncline.

The youngest rocks lie in the centres of the synclines in the areas of Caerphilly and Gelligaer. The Pontypridd-

Maesteg Anticline brings the rich coal beds of the Lower and Middle Coal Measures closer to the surface in the area

of the lower Rhondda valleys.

The NNW-SSE direction of most of the Valleys is partly the result of the rivers following lines of weakness or zones of

broken rocks (faults) within the rocks. The Taff Valley, for example, is eroded along the line of a series of stepped

NNW-SSE faults. Where the River Taff breaks through the limestone rim along the southern edge of the coalfield, it

follows the Tongwynlais Fault.

4

Rudry, South Glamorgan – 22 April 2012 (contd.)

Fieldtrip around Rudry

The geology of the area around Rudry is straightforward, with strata dipping at between about 25° and 45° towards the

northwest and with outcrops trending northeast-southwest. They are cut by only one significant fault the Caerphilly Fault,

which lies to the west of Mynydd Rudry. The oldest rocks, red mudstones of the Lower Old Red Sandstone St.Maughans

Formation occur to the southeast of Ruperra and the youngest, the Hughes Member of the Pennant Sandstone Formation,

on the northern slopes of Mynydd Rudry.

Geological section across the Rudry area

Geological sketch map of the area around Rudry

5

Rudry, South Glamorgan – 22 April 2012 (contd.)

6

Rudry, South Glamorgan – 22 April 2012 (contd.)

Rudry Common and Mynydd Rudry

The car park at Rudry Common sits on low ground underlain by soft mudstones and coals of the Lower and Middle Coal

Measures sequence, between two higher ridges to the northwest and southeast. Although much hidden by vegetation,

particularly bracken, in the summer, it is still possible to recognise the sites of early coal- workings called bell pits on this

lower ground.

Bell pits were shallow vertical shafts sunk to exploit ironstone or coal seams close to the surface. Upon reaching the coal

or iron ore, the shaft was widened to extract as much material as possible without collapse. They varied in size depending

on the nature of the rock, but on average they were 5 to 7 metres deep, about 2 metres in diameter at the top, widening to

about 4 metres at the bottom, giving them a bell-like shape. Once the ore or coal was extracted, the pit was abandoned

and filled with material excavated from a new adjacent pit. Areas of bell pit working are pockmarked by small crater-like

depressions which can often be seen only in certain light conditions.

Coal mining in this area is recorded as far back as 1316 and continued intermittently for the next 600 years.

The ground rises towards Mynydd Rudry as the softer Middle Coal Measures give way to the coarser and more resistant

Pennant Sandstone Formation. The lowest unit of the Pennant is the Llynfi Member which soon passes up into the

Rhondda Member. As the slope steepens, we cross the outcrop of several coal seams in the Rhondda

Member, the No.1 and No.2 Rhondda Seams, although they are no longer exposed. The summit of Mynydd Rudry lies on

the outcrop of sandstones of the Rhondda Member which exhibit features typical of the Pennant Sandstone. The rock is an

impure sandstone, with cross-bedding which in places shows its derivation from the south. The north-westerly dip of the

strata is apparent in these outcrops. Pennant Sandstones of the Brithdir and Hughes Members form the northern slopes of

the hill.

On a clear day, the view from this hill is superb. To the north lies the Caerphilly Basin, not only a topographic hollow but a

structural depression too, lying in the centre of the Llantwit-Caerphilly Syncline, one of the major structures of the Coalfield.

The syncline contains some of the youngest rocks in the Coalfield, the Grovesend Formation, with the Mynyddislwyn Seam

near its base. On the northern side of the syncline, on Mynydd y Grug above the tips of Bedwas Colliery, Hughes and

Brithdir Members dip southwards at about 14° -21°.

On the basin floor, the town of Caerphilly and its castle are built on ridges and mounds of drift deposits, mostly sands and

gravels, from the last (Devensian) glaciation, with waterlogged peat-filled hollows between them.

To the west of Rudry Common, Van Road turns northwest through Coed Parc-y-Van, following a hollow eroded along the

line of the Caerphilly Fault. This is one of many NNW-SSE faults which cross the South Wales Coalfield and largely control

the orientation of the valleys. The Caerphilly Fault extends from the eastern side of Caerphilly, where it is joined by the

Energlyn Fault, to the eastern end of Craig Llysfaen where it dies out in the Lower Old Red Sandstone Brownstone

Formation. Its downthrow is about 14-16 metres towards the southwest.

Thin beds and nodules of clay ironstone occur in the shales of the Lower and Middle Coal Measures. These are formed of

the iron carbonate siderite, and have an iron content of between 20 and 38%. They were worked in bell pits, open pits and

levels and were the main source of iron ore for the iron industry in South Wales until the mid 19 th century. These

ironstones were known as 'mine', and were worked by miners. Coal was worked by colliers.

Between 1828 and 1834 an ironworks operated at Ty'n-y-coedcae on the Rhymney River about 2 km northeast of Mynydd

Rudry. The ore came from workings on Mynydd y Bwlch about 1 km southeast of the works, but it was expensive to extract

and the works closed after just six years. An earlier ironworks, the Caerphilly Furnace at Penyrheol, operated from about

1680, using iron ore from bell pits on Rudry Common and Wernddu, limestone from Cefn Onn and charcoal from Machen

Forest. The Caerphilly Furnace continued to work through the 18th century and closed around 1819.

Although exposure in this area is poor, the sequence is known from underground coal workings and from the Rudry

Borehole which was sunk in 1954-55 at Garth Place about 1 km to the northeast of Mynydd Rudry. The borehole reached

a depth of 491.5 metres, passing from the Pennant Sandstone Formation Rhondda Member, through the Middle and

Lower Coal Measures and the Marros Group and into the top 3 m of the Pembroke Limestone Group.

Looking east from Mynydd Rudry, the large quarry in the Pembroke Limestone Group at Machen can be seen.

The limestone ridge continues south-westwards to form the ridge of Cefn Onn to the south of the car park.

7

Rudry, South Glamorgan – 22 April 2012 (contd.)

Cefn Onn

From the car park, a minor road rises to the south onto the ridge of Cefn Onn. The outcrop of the Marros Group is passed,

but there is no exposure. About 47 m of the Bishopston Mudstone Formation were encountered in the Rudry Borehole,

with the Twrch Sandstone Formation below it with a thickness of only 3.8 m. Once into the beech woods, beds of dark grey

limestone dipping north-westwards at about 26° crop out. The Pembroke Limestone Group here on the south-eastern

margin of the coalfield is just over 300 m thick. The limestone was formerly assigned to just one stratigraphic unit, the

Rudry Dolomite, but the sequence may extend from the Barry Harbour Limestone Formation, the lowest unit of the Black

Rock Limestone Subgroup to the High Tor Limestone Formation and Hunts Bay Oolite Subgroup, although these

constituent formations have yet to be formally recognised in this area.

A short diversion to the route described in the handout was made along the ridge of Cefn Onn to the disused quarry at ST174852 where the Pembroke limestone could be seen dipping at about 45°

South of Cefn Onn Farm, the road descends into the valley of Nant y Cwm. This is a fine example of a strike valley, eroded

along the strike of the outcrop of a series of softer rocks, the Avon Group, which occurs at the base of the Carboniferous

sequence. The Avon Group here comprises an upper shale unit, the Cwmyniscoy Mudstone Formation, and a lower

limestone unit, the Castell Coch Limestone Formation. To the west, near Tongwynlais is an additional limestone, the

Tongwynlais Formation below the Castell Coch Formation, but it is not present here.

At the hairpin bend just north of the valley, the rocks are cut by the Caerphilly Fault. On the south-western side of the fault

here, the dip of the limestone has been recorded as 70°. The most obvious effect of the fault on the landscape is

demonstrated by the low wooded ridge in the strike valley. This ridge is formed of the Castell Coch Limestone Formation

the outcrop of which has been shifted a short distance northwards on the eastern side of the fault relative to the outcrop

on the south-western side of the fault. This demonstrates that the Caerphilly Fault downthrows to the southwest.

Once across the stream, the Castell Coch Limestone is exposed in a roadside outcrop, but as the road rises past Pant-glas

Farm, the roadside outcrops are of green, yellow and brown micaceous sandstones and red mudstone of the Devonian

Upper Old Red Sandstone. These dip towards the NW at between 12° and32°.

Craig Llysfaen

The road reaches the ridge of Craig Llysfaen where a track heads north eastwards along the ridge. Sandstones and

conglomerate boulders form the base of the track. From the trig point at 264 m above sea level, on a clear day there is a

fine view south across the coastal plain on which Newport and Cardiff are built, to the Bristol Channel, the islands of Flat

Holm and Steep Holm, the Somerset coast and the hills of Exmoor. To the north, across the Coalfield, the Brecon Beacons

may be visible. The Upper Old Red Sandstone and the underlying Brownstones here dip northwards; the same rocks dip

southwards on the summits of the Beacons. The steep south scarp of Craig Llysfaen is the equivalent of the north face of

the Beacons, where the harder sandstones of the Brownstones Formation give way to softer, underlying mudstones of the

St Maughans Formation. The Brownstones Formation here is about 150 m thick, compared with 330m in the Brecon

district. The Senni Formation. 300 m of sandstones between the Brownstones and the St Maughans Formation in the

Brecon area, is not distinguishable from the Brownstones here.

A path descends through Coed Coesau-whips to the strike valley of Nant y Cwm. In the stream bed, grey. Green and red

sandstones and red and green mudstones of the Upper Old Red Sandstone are exposed, but a short distance up the

northern side of the valley, the Castell Coch Limestone is exposed. The limestone is also exposed in the stream section a

few hundred metres downstream.

At Craig y Llan, a path rises from the Avon Group onto the Barry Harbour Limestone Formation which is exposed along the

path and in a small quarry above a ruined limekiln. The path leads to the Maenllwyd Inn. The pub is probably situated on

the Gully Oolite Formation as the hollow to the north in which Dan-y-Graig Farm is located is considered to be have been

eroded along the outcrop of the Caswell Bay Mudstone Formation.

Rudry lron Mine (ST 2040 8643)

About 300 metres south of the Maenllwyd in the area of Cwm Farm is the site of the Rudry lron Mine. Here the Barry

Harbour Limestone Formation is locally replaced by hematite. An incline was driven down the base of the limestone to the

east of the road. About 2,000 tons of ore yielding 50% iron were obtained between about 1868 and 1876 when the mine

closed due to drainage problems which were probably related to groundwater flow at the contact of the Barry Harbour

Limestone with the underlying impermeable Cwmyniscoy Mudstone Formation. There is still a spring or well on the western

side of the road here situated at this stratigraphic contact. Hematitized limestone with calcite veining is visible at the

rather overgrown mine entrance just north of the road junction. There was also a shaft in the field immediately to the west

of the road. This has reopened through collapse several times in recent years, and on each occasion has been filled in.

8

Rudry, South Glamorgan – 22 April 2012 (contd.)

Hematite replacing limestone is common in the outcrop from here westwards through Taffs Well to Llantrisant.

The hematite is probably derived by groundwater leaching from clay ironstone nodules and bands within the Lower and

Middle Coal Measures.

The Castell Coch Limestone Formation seems to be absent for a short distance here.

Nant y Cwm lies in the floor of the valley near the base of the Avon Group, flowing northeast along the strike of the Upper

Old Red Sandstone. To the south, the road rises through grey sandstones of the Upper Old Red Sandstone, visible dipping

northwards at 28°, in a small roadside quarry, to the Brownstones Formation near the top of the hill. A footpath leads

gently back down through woods to Nant y Cwm upstream from Cwm Farm. This joins other tracks which can be followed

back to Cefn-on Farm and Rudry Common.

Cwm Leyshon Quarry (ST 2116 8686)

The Black Rock Limestone Subgroup is exposed in the disused Cwm Leyshon Quarry east of the Maenllwyd.

Much of the limestone sequence in this area has been altered to dolomite (calcium magnesium carbonate). Such

dolomitization is common in the limestone around the Coalfield. Its origin is uncertain but it may have occurred soon after

deposition by the percolation of groundwater through the sediment.

Coed Cefn Pwll Du

The forested hill north and east of Cwm Leyshon Quarry was the site of extensive lead mining from Roman times until

1872. In 1782 it was recorded that several Roman coins had been found in a disused mine here, and further coins were

found next to an ancient hearth and potsherds during underground excavations in 1965. It has been suggested that lead

from these mines may have been used for pipes at the Roman fort at Caerleon. Working continued intermittently through

the centuries. The tips and workings are now much overgrown, sometimes with substantial yew trees which are evidence

of the antiquity of some of the workings.

The limestone is cut by two sets of mineralised fractures, a main set trending east-west and a subordinate set NNW-SSE.

These trends can be seen in the orientation of the lines of pits and opencut workings in the forest. They appear as zones of

shattered limestone cemented by barite with galena, along with calcite and quartz. Sphalerite is rare, but secondary

minerals formed by decomposition of sphalerite such as hemimorphite and smithsonite are more common.

The mines have been known by various names, such as Clive Mine, Clive West and Clive United. Most of the mine sites are

fenced and not accessible.

Other mine sites lie on the ridge southwest of the Maenllwyd in Coed Cefn-on.

Further reading

Bevins, R.E. & Mason, J.,2000. Welsh metallophyte and metallogenic evaluation project. Results of a minesite survey of Glamorgan and Gwent compiled by the National Museums & Galleries of Wales. Countryside Council for Wales Science

Report No 386.

Evans, D.E., 1971. A traverse across the eastern part of the South Wales Coalfield via the Rhymney Valley. In Bassett, D.A.

& Bassett, M.G. (eds), Geological excursions in South Wales and the Forest of Dean. Cardiff, Geologists' Association South

Wales Group, pp.56-73.

Squinell H.C. and Downing, R.A., 1969. Geology of the South Wales Coalfield, Part 1, the country around Newport (Mon.). Explanation of One-inch Geological Sheet 249, New Series. Third edition. London: HMSO, xiii +

333pp.

Strahan, A., 1899. The geology of the South Wales Coalfield. Part 1. The country around Newport. Memoirs of the

Geological Survey of England and Wales, 249. London: HMSO, vi + 97pp.

9

Rudry, South Glamorgan – 22 April 2012 (contd.)

Rudry Iron Mine Spring near Rudry Iron Mine

Quartz Conglomerate — Marros Group Quarry at ST174852

Tom Sharp and Severnside at Rudry Common Bell pit — Rudry Common

Caerphilly from Mynydd Rudry View South from Mynydd Rudry towards ridge

of Cefn Onn

10

Ogmore and Dunraven Bay , Vale of Glamorgan – 26 May 2012

Leader Geraint Owen

It was a sunny day with a welcome cooling breeze when we gathered at the

car park in Ogmore with the objective of exploring the Carboniferous

limestone and its relation to the overlying Permian/Triassic beds, both in

time and depositional environment.

Geraint led us to the flat top of an anticline, formed during the Variscan

Orogeny, where there was a vast array of fossils displayed in the well-

bedded Carboniferous limestone. He pointed out the Productid brachiopod

Delepinea, about 20cm in size, which had grown large in a calm seabed

environment. We located one specimen in life position with the two cup-

shaped shells visible but most had been inverted by storm events. The lack

of fragmentation was

repeated for other species –

colonial coral, Lithostrotion,

and the solitary coral,

Caninia. The preservation of

trace fossils such as Zoophycos, from a worm which created fan-shaped

ridges during its feeding, indicate a low-energy environment in a warm sea

far from the influence of land derived sediments.

A few steps to the south we found a

change in lithology, a conglomerate

consisting of Carboniferous limestone of

mixed sizes and shapes, metre-sized

boulders to large pebbles, from well-

rounded to angular. Discussion led us to

the conclusion that we were above an

unconformity in a wadi deposit produced

in desert conditions from intermittent

torrential rain.

Closer observation of the unconformity

contact showed a infilled karstic

landscape within the limestone, with

potholes and cave systems filled during

the Permian or Triassic. Dating is

problematic through lack of fossils. The

infilling showed evidence for fluid-

deposited minerals, pyrites, calcites,

galena and barite which may have post-dated the initial deposition. Further

south Geraint pointed out the spoil heaps where an attempt had been

made to mine the lead.

As we walked south we encountered a change in lithology above the

unconformity. Sutton Stone, a limestone deposited in the Early Jurassic in a

near-shore environment as indicated by the fragmented fossils and lack of

structures. These contribute to the quality of the rock making a freestone,

quarried locally for use in construction.

Severnside at Ogmore

Productid Brachiopod Delepinea

Lithostrotion

Zoophycos trace fossils

Unconformity — Triassic wadi deposit above

Carboniferous limestone Remains of Lead Mine at Ogmore

11

Ogmore and Dunraven Bay , Vale of Glamorgan – 26 May 2012

Averil Leaver

After a lunch break we reconvened on Southerndown beach and walked over to the Trwyn-y-witch headland. From a

distance we observed the structures in the headland, finding the unconformity between the Sutton Stone and the overlying

Lias beds with some stepping to the west and complex folding in the Sutton Stone. After examining some fallen blocks of

the limestone to identify original chert layers and redeposited chert bands in the Lias we moved to the corner of the bay to

examine a fault system. To the north of the fault the Sutton Stone is absent, movement at the fault having shifted it at

least 30m below the present surface. This corner has seen recent erosion and landslip and some of the structures I

observed 8 years ago are now no longer there.

Surfaces of the Blue Lias revealed fossils of Gryphaea (the Devil’s toenail), Pinna (an upstanding bivalve) and ammonites.

These complete fossils indicated that conditions were calm and muddy, a deep sea environment.

The changes in the rocks reflect the changing landscape over the

millennia. In the Carboniferous there was land to the north and a

large sea filling the Bristol Channel area. During the Permian and

Triassic the sea retreated and deserts covered the region, with a

playa lake at some times during the period. When the sea level rose

again in the Early Jurassic eroded hills became islands which were

then submerged in deep sea.

Dunraven Bay, Southerndown — looking

South

Dunraven Bay — Fault and folding Dunraven Bay — Severnside with Geraint Owen

For comparison these pictures were taken by

Averil and the editor in 2004 and 2002 and show

the folding which is now covered by fallen rock

2004 2002

12

Severnside Branch Events

National Events

40th Anniversary Symposium

17th to 19th August 2012

This event will be held at the University of Northampton and the theme will be Earth Sciences at the OU and OUGS:

Past, present & Future.

The organisers would like to hear from founder members and wonder if anyone has photos from the early years

that they would be willing to lend to form part of an exhibition?

DATE EVENT LEADER DETAILS

2012

July 29thCribarth

Swansea ValleyTony Ramsey

Quaternary,Carboniferous, Tectonics and Natural History

Meet at Craig-y-Nos centre car park at 10.30 AM

September 13th

to 23rdGaspe, Canada Prof. Brian Williams

Waiting list only

November 18th Penarth, Glamorgan Stephen Howe

Introductory Field Trip - Looking at the sedimentary beds along

the foreshore at Penarth.

New students - come and see what the OUGS do !

More details later

December 8thMuseum of Wales

Cardiff

Day of Lectures

Meet at 10am Tea and coffee will be served.

Lecture 1 - tba.

Lecture 2 Mark Brandon,OU Milton Keynes. the making of

Frozen Planet.

Lunch at the Pen & Wig then presentations by members

Invitation from Fforest Fawr Geopark to a day of talks & presentations

The Evolution of the Natural & Human Landscapes of Fforest Fawr Geopark

All welcome to the whole day or a part. Free. No booking needed.

Wednesday October 17th 2012, 10.00am to 4.30pm Garwnant Forestry Commission Centre SO 003131

(signed off the A470 5 km north of Merthyr Tydfil, parking charge)

13

THERE IS A NOMINAL FEE OF £2 ON DAY TRIPS (UNLESS OTHERWISE STATED) TO COVER LEADERS EXPENSES.

ALWAYS WEAR APPROPRIATE GEAR...WARM CLOTHING AND HIKING BOOTS. BEAR IN MIND THAT MANY SITES, ESPECIALLY QUARRIES,

REQUIRE THE WEARING OF HARD HATS. ACTIVE QUARRIES REQUIRE HARD HATS AND YELLOW JACKETS.

IMPORTANT

Each person attending a field meeting does so on the understanding that he/she attends at his/her own risk.

The OUGS has Public Liability Insurance cover for field and indoor meetings but Personal Accident and Personal Liability cover remain

the responsibility and personal choice of the participant.

There may be an element of appropriate cover included in house or travel insurance. Although OUGS activities are not particularly

dangerous members are advised to check whether exclusions apply to activities in which they propose to participate in case they wish to

arrange further cover. An annual travel insurance may be the best solution for any member who regularly attends field events. This is

again a matter of personal choice.

All members participating in overseas events will be required to have travel insurance for the duration of the event. Participants should

be covered for Medical, Repatriation and Personal Liability expenses. The Personal Accident element remains the personal choice of the

member.

Disclaimer

None of the information in any of the advertisements for field trips or other events in this newsletter constitutes a brochure under the

Package and Travel Regulations (1992)

Transport

If you are able to offer a lift to any event or if you have local knowledge regarding bus or train services etc. that might help those

without their own transport to attend an event please let Janet know.

Updates

Why not try the following link to a service for keeping an eye on changes in the events list.

http://www.changedetection.com

Event Notes

Note: EVENT DETAILS CAN CHANGE

Any last-minute changes of times or arrangements are on the OUGS website.... www.ougs.org

Or.......Contact Jan Ashton-Jones Tel: 01432 870827 Email: janaj1009@gmail.com

If you are not receiving email reminders from Janet regarding forthcoming events or if you get a new email address please let her know

so that she can keep in touch.

OUGS Severnside Branch Committee

Janet Hiscott - Branch Organiser 01633 781557 (evenings) severnside@ougs.org

Averil Leaver - Treasurer 01446 418592 averilandjohn@ntlworld.com

Jan Ashton-Jones - Events 01432 870827 janaj1009@gmail.com

Jan Boddy 01793 762575 janet.boddy@cheerful.com

Kath Addison-Scott 01179 538085 kathaddisonscott@yahoo.co.uk

Anthony Bukowski 02920 300080 abukowski@hotmail.com

Philip Clark 01885 483697 chipgc@btinternet.com

Elizabeth Edmundson 01792 863119 igneeous@talktalk.net

John de Caux - Newsletter 01633 875955 johndecaux@sky.com

Editorial

To receive the colour version of this newsletter by email in .pdf format just send a request to johndecaux@sky.com

Why not share your geological experiences with other members? All contributions for the newsletter will be very welcome.

We can read or scan most data formats or documents. For photos please send the images as separate files, preferably in

jpeg (.jpg) or bitmap (.bmp) format.

Contributors should note that reports, photographs and other contributions may also be reproduced in the OUGS

newsletter.

Library

The Branch Book and Map Library list is online

The library is in the care of Branch Organiser Janet Hiscott Request a catalogue from....... ssilibrary@ougs.org

OUGS Sales Is your fieldwork kit lacking a few items? If it is check out OUGS sales!

They can supply all you require............Hand lenses, Lanyards, Notebooks, Reference Cards and much more!

Click on ‘sales’ on the OUGS website www.ougs.org

14

It’s worthwhile taking a look at what other branches are doing. Many of their events could be convenient to attend.

OUGS Oxford Branch Organiser: Sally Munnings 01635 821290 oxford@ougs.org

OUGS Southwest Branch Organiser: Chris Popham 07971 930431 south.west@ougs.org

OUGS Wessex Branch Organiser: Sheila Alderman 01935 825379 (eve) wessex@orgs.org

OUGS West Midlands Branch Organiser: Linda Tonkin 01902 846074 west.midlands@ougs.org

Many of our members belong to other Earth Science related organizations which also do interesting events.

Welsh Stone Forum http://www.museumwales.ac.uk/en/364/.

Geologists Association Secretary: Sarah Stafford 020 7287 0280 geol.assoc@btinternet.com

Affiliated Regions:

South Wales www.swga.org.uk

West of England www.wega.org.uk

Bath Geology Society www.bathgeolsoc.org.uk

Avon RIGS Group The Avon RIGS Group are re-launching their newsletter, Outcrop, as an on-line blog.

The relevant websites are:

www.avonrigs.org.uk http://avonrigsoutcrop.blogspot.com The Russell Society www.russellsoc.org.uk

OUGS Neighbours

Other Organisations

Dinosaur footprints at Barry

Answer to picture puzzle in April issue

15

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