Summer weekend North Norfolk 2017

This year the club decided to take a look at the Quaternary geology of north Norfolk, which would be a complete change from our local geology. The leader was Tim Holt-Wilson who is well known for his work on the geodiversity of Norfolk. His knowledge and previous work in the area led to a highly interesting and informative weekend.

The solid geology of Norfolk is mainly covered by Quaternary deposits, although in some areas the underlying rock does appear at the surface as at Hunstanton where the cliffs are composed of Cretaceous age rocks. The oldest rocks are to be found in the west and these dip gently eastwards towards the north sea basin. The oldest rocks are Lower Cretaceous sandstones and clays, which form an eroded escarpment between Heacham and Castle Rising, and underlie much sandy heathland locally. Rock types vary from sands and impermeable clays to tough, iron-cemented sandstones. Overlying these rocks are rocks of the Upper Cretaceous chalk which contain numerous flint nodules.

As you move eastwards the chalk becomes overlain by Pliocene-Pleistocene marine sands and gravels, these being the Norwich Crag and the Wroxham Crag Formations resting unconformably on the chalk. Other “pre-glacial” deposits are the freshwater and estuarine deposits known as the Cromer Forest Bed.
The overlying superficial deposits are very complex and it is these that we had really come to see. Ice-sheets and seas transgressed and regressed many times over this landscape which left a complex sequence of marine and terrestrial sediments. Between about 2.6 million and 900,000 years ago, Southern England was joined to France, at the Straits of Dover, which now links the North Sea to the English Channel. Numerous small-scale oscillations from warmer to cooler episodes occurred resulting in frequent but relatively small falls and rises in sea level, which would have affected the Norfolk coastline.

Around 900,000 years ago, a marked climatic shift occurred, resulting in a climate dominated by extremes both globally and locally. During this time over 20 major climatic shifts from ‘warm stages’ (or interglacials) to ‘cold stages’, (or glacials), have been recognised globally, with each lasting several tens of thousands of years. During the maximum extent of glaciation, ice covered the whole of the British Isles north of Bristol and London. The thickness of ice over Norfolk is estimated to have been between 2 and 3km. At least three major glaciations have been recognised: the earliest of these is called the Happisburgh Glaciation and started around 640,000 years ago. The second, and largest glaciation is the Anglian Glaciation and occurred around 450,000 years ago. The third glaciation, which didn’t extend as far into Norfolk, is known as the Devensian glaciation which was at its coldest around 25,000 years ago. Each of the glaciations dramatically modified the geography of Norfolk and deposited vast quantities of sediment.

Sandringham warren

The first location on the Saturday morning was Sandringham Warren and Dersingham bog (TF 6694 2839) where Tim showed us landscape features related to Lower Cretaceous geology. It is here that sands of the Leziate Beds ( Sandringham Sands Formation, Upper Jurassic to Lower Cretaceous) are capped by harder, iron-rich layers of basal Dersingham Beds (Dersingham Formation). Leziate Beds consist of loose, quartz rich sands and iron-cemented sandstones and were laid down under marine conditions.The Dersingham Beds the basal of which are iron-cemented sandstones, overlain by sands, silts and clays. The Dersingham Beds produce the plateau feature seen at Sandringham Warren. Tim explained that a series of bluffs separated by dry valleys is thought to represent a line of a former cliff and the dry valleys a product of a process known as “spring sapping” where groundwater outflow undermines slopes and therefore cuts back into the slope forming a valley. Dersingham bog which is a SSSI, is developed on layers of poorly draining sandy “head” washed down from the Leziate Beds and is now an acid mire. It has many rare plants like bog ashphodel, round-leaved sundew and cranberry.

Snettisham Common Pit

We then drove to Snettisham Common Pit (TF 6710 3362) which is a county wildlife site and turned out to be a superb sandstone quarry from which high silica sandstone had been quarried in the 19th century for use as foundry sand. The quarry has vertical exposures up to 5m in height of Leziate Beds topped by strata of the Dersingham Beds. The sands showed good examples of cross-bedding and the walls of the quarry were peppered with the holes from solitary bees. One important feature in the quarry is faulting which may be associated with glacial loading or periglacial cambering.

Lunch at Ringstead Common

A short stop at Heacham chalk pit where the Lower Chalk (Grey Chalk, Upper Cretaceous, Plenus Zone)is exposed. It was here that Mike Hodgson found a very large ammonite, and although worn, was impressive. The next stop was Ringstead Common nature reserve, for a well earned lunch before we looked at the geology. Middle Chalk ( white chalk, Upper Cretaceous, Labiatus Zone) is exposed here. Ringstead is an example of an outwash valley. During the decline and melting of the North Sea ice-sheet valleys in central and western Norfolk were deeply excavated by the action of torrential melt-water streams issuing from the margin of the retreating ice. Ringstead is the result of a former drainage overflow channel that was cut by the melting ice water. Lots of examples of chalk flora were also present.

Cliffs at Hunstanton

We returned to Hunstanton ( our base) and after an ice-cream on the sea front spent the remainder of the day observing the cliffs at Hunstanton (TF 673415). The cliffs extend from Hunstanton to St. Edmunds Point, Old Hunstanton, about 1.5km away, and the whole is a SSSI of Cretaceous deposits. The colour contrast of the rocks is impressive with, at the base, the rusty brown of the Carstone Formation to the red of the Hunstanton Chalk Formation ( Red Chalk) to the white and grey of the Ferriby Chalk Formation. As the cliffs dip to the east, at St.Edmund's Point the Red Chalk is at beach level and the Carstone is no longer visible.

The Carstone is formed of coarse sand particles interspersed with rolled pebbles indicative of deposition in a high energy, shallow sea, near shore environment, with strong currents. The lower Carstone is Aptian in age (112-116ma) whilst the upper Carstone is Middle Albion (105ma). When it is fresh the Carstone is grey-green in colour but darkens to brown with weathering. On the foreshore the Carstone forms a strange pattern of ‘tuffets’, where the sea has preferentially eroded joints in the rock.

Above the Carstone is the Red Chalk (101ma) and it is thought there was a gradual transition from one to the other. The more pebbly Carstone being laid down in a higher energy and shallower environment to the finer Carstone. The Red Chalk being laid down very slowly in deeper water but close enough to land to receive iron-rich clay. When the sea-level rose sufficiently the land was completely flooded and only white chalk was deposited. The Carstone contains fossils including ammonite fragments, bivalves and traces of burrowing organisms, whilst the fossils of the Red Chalk include belemnites, brachiopods, echinoids and corals. The lowermost bed of the White Chalk also contains numerous fossils including, bivalves, brachiopods, belemnites, echinoids. Between the top of the Red Chalk and the base of the Ferriby chalk is the Paradoxica Bed. This was originally named from what was thought to be a sponge but in fact turned out to be sediment-filled, anastomosing Thalassinoides burrow systems.

Paramoudra or Pot Stone
On the Sunday our group went east to spend the day to try and gain a basic understanding of the Quaternary geomorpholgy. Our first stop was west Runton. ( TG 185432) where the Cromer Forest bed is the Type site for the Cromerian Interglacial Period. The area is a SSSI because of it's rich fossil beds and one of the most important Quaternary sites in the UK.It is famous for the elephant or steppe mammoth, Mammuthus trogontherii, which was discovered in 1990 and is one of the oldest and best preserved fossil elephants ever found in the UK.

Commencing on the beach we are standing on a chalk wave-cut platform and there are large flint circles and other flint structures which resemble garden pots. These are the Paramoudra or “Pot stones” which are trace fossils of an unknown creature and referred to as Bathicnus paramoudrea. As we work up the succession the Wroxham Crag lies unconformably on the chalk. This is a shelly deposit and is pre-Pastonian to Cromerian in age. The shells are marine bivalves that are cold water species and herald the onset of the glaciation.
Overlying the Wroxham Crag are the Cromer Forest Beds which according to the BGS Lexicon can be described as follows: “The formation encompasses the fluvial, lacustrine and organic deposits of the postulated "Ancaster River", which is believed to have flowed from the Pennines to the North Sea along a line corresponding approximately to the present north coast of Norfolk. Four members are recognised in north Norfolk coastal sections. The Sheringham Member (lowest member) comprises freshwater organic mud, clay and sand. The Runton Member includes laminated freshwater silty clay. The West Runton Member comprises layers of alluvial clay and organic freshwater mud. The Bacton Member (highest Member) comprises clay and organic mud.”

The Cromer Forest bed has yielded the richest Pleistocene vertebrate fauna in UK and include, fish, amphibians, reptiles, birds and mammals. The upper part of the Cromer Forest Bed the layers become gravelly and marine bivalves are present indicating a return to marine conditions.This, along with cross bedding suggest esturine conditions. Above the Cromer Forest Bed Formation are glacial deposits belonging to the Anglian Stage. They consist of a complex sequence of fluviatile sediments and contorted drift which demonstrate a climate cooling from the interglacial to glacial conditions. They include river sands, gravels, glacial till and outwash sands. There is remarkable folding in the glacial sediments the consequence of glacio-tectonic deformation.

Glacio-tectonised strata at Weybourne Hope

From West Runton we travelled to Weybourne Hope. The overall first impression was of high cliffs of contorted soft sediment very unlike any glacial till in mid-Wales. The lower cliffs are composed of the Weybourne Chalk Member (Norwich Chalk, Upper Cretaceous, Campanian), which are glacio-tectonised where sub-horizontal shear planes have occurred due to glacial ice movement. This is chalk breccia in which fractured flints and fragmented shells occur. Above the chalk is the Wroxham Crag Member (Early Middle Pleistocene) as seen at West Runton but is disturbed and distorted due to the effects of glaciation. Above the Wroxham Crags Formation are Members of the Sheringham Cliffs Formation (Middle Pleistocene), the Runton Till Member a matrix-supported diamicton and the chalky Weybourne Town Member which is a highly consolidated matrix-supported diamicton. Above these there is a sharp erosional boundary and then the Briton's Lane Sand and Gravel Member of the Briton's Lane Formation (Late Pleistocene).

We then visited Blakeney Esker ( TG 031422), apparently the best in England. An esker is a long winding ridge of sand and gravel which has been deposited within a tunnel made by a subglacial stream cut into the base of an ice sheet. They are not normally preserved very well as the loose sand and gravel are washed away by melting ice sheet. The esker is 3.5km long and runs from Blakeney to Wiverton, with a base width of 100m. This is also a wildlife site and the differences in the geology are reflected in the plants at the top and bottom of the esker. Those at the top are acid heath type plants where as those at the base are growing on clay.

Our final stop was to St. Mary's church Brancaster to look at the building stones in it's walls. This is something the club is well used to undertaking. The church building is known for the re-use of Roman stone in the walls. There are the remains of two Norman windows in the south side of the west end of the chancel and these have been blocked in by stone taken from the Roman Fort of Branodunum. It was also used as a facing stone and isolated stones were found dotted around the walls. The stone is a very distinctive quartzite and comes from quarries around Castle Rising.
Building Stones Brancaster Church

2016 Annual Weekend Field Trip – Lake District

Keswick at Night

The Club’s Annual Summer Weekend, 8-11 July 2016, was spent in the Lake District, based in the delightful lakeside town of Keswick, among the northern lakes, and admirably led by Cumberland Geological Society stalwart, John Rodgers. The Lake District, which is also the Cumbrian Mountains, is some 30 miles square of Ordovician and Lower Silurian rocks surrounded by younger Carboniferous rock. A band of Ordovician Skiddaw Slate lies across the north and a band of Silurian rock across the south of the district. Between them is a band of igneous rock, mostly lavas and ash flows, the Borrowdale Volcanic Group, hosted by Ordovician rock. This formed due to arc volcanism, caused by a subduction front as the Iapetus ocean closed only 40 km to the north, across the Solway Firth. An underlying batholith of granite caused the original uplift of older rock. During the Ice Age this erosion resistant dome was sculpted by glaciers to form the picturesque lakes and lakeland scenery.

On the Friday evening our leader gave an introductory talk on the lakeland geology. A month spent there would not do justice to the district, so our two days had to be very selective. On the Saturday we explored an area north-east of Keswick in the Skiddaw Group, around Mungrisdale and Mosedale Bridge, moving on in the afternoon to the nearby valley between the dramatic Bowscale and Carrock Fells. These localities allowed us to examine structures in the Skiddaw slate, and look at the effect of metamorphism on the slates, particularly rocks spotted with cordierite, and containing acicular andalusite.

Mosedale wall showing hornfels

In the valleys of the Caldew and Grainsgill Beck we could see both granite outcrops and glacial action. The weather was mostly kind but, this was our twelth annual weekend excursion, and we can’t expect perfect weather all the time. It literally blew a gale when we walked up to the Tungsten mine. Saturday evening ended with a discussion back at the B&B;before the group dispersed to various Keswick restaurants.

Group at Friar's Crag Derwent water

Sunday began at Friar’s Crag looking south down Derwent Water and discussing the working of the glacier which formed this, generally flowing north towards us, round the dolerite intrusion on which we stood, continuing north of us to excavate also Bassenthwaite Lake. This area is on the northern part of the Borrowdale Volcanic Group, so again we were able to combine consideration of the local volcanics - this area was a caldera – with an appreciation of the extraordinary erosive power of the glaciers. We drove down the west bank of Derwent Water to see roche moutonneés, lava flows and columnar cooling among many other features.

Walking the Rosthwaite moraines

In the afternoon we walked the Rosthwaite moraines just south of Derwent water, in the glacial valley now occupied by the north-flowing river Derwent and its tributaries from Seathwaite and Stonethwaite to the south, endeavouring to understand the glacial movements which left this debris. Another splendid weekend ended on the Monday morning, for most us with a look at Keswick and the Threlkeld Museum just east of the town.

Arnside & Sandside, north of Morecambe Bay, 2015

Led by Westmorland Geol Soc (Mike Balderstone, Colin Patrick, Mike Dewey)
Stayed: Willowfield, 53 The Promenade, Arnside, Cumbria LA5 0AD
Kent Estuary Arnside

Twenty members enjoyed the 2015 Summer Weekend at Arnside, north east Morecambe Bay, on and around the beautiful estuary of the River Kent (the yellow sand came up on the tides, not down the river). Several members of the Westmorland Geological Society acted as leaders. We were on the southern side of the estuary; the river forms the south-east boundary of the Lake District National Park, the hills of which could be seen in the distance.
The Lower Carboniferous limestones are underlain here by older Silurian rocks outcropping around Kendal to the north, and were once overlain by younger Carboniferous Millstone Grit now outcropping around Lancaster to the south. These Dinantian rocks were deposited mostly as carbonate muds, with both oolitic and shelly content. We examined three of the six Dinantian sequences laid down during the period 360 to 325 Ma, in descending order: Urswick, Park & Dalton Limestones. Each was formed by a cycle (a mesothem) of sea level fall (regression), exposure of the limestone, erosion, and then sea level rise (transgression) to a maximum. Global sea level rose in the the early Carboniferous; the periodic regressions were caused by subsidence during crustal extension. The widespread carbonate platform – extending to eastern Poland at the time – forms some good building stone, the Urswick Limestone in particular is a well bedded grainstone (grainy and without lime mud). Around Arnside there is almost no brick, even in modern buildings.

Group discussing iron mining at one of the adits

Saturday began at Arnside Knott (SD 4499 7737) with landscape interpretation followed by a walk down the hill to the coastal section, on the way observing the hematite adits. Iron was deposited in faults from hot mineral-rich fluids replacing limestone, possibly during the Cretaceous; and exploited by local iron works. We saw a dolomitised quarry in the Red Hill Limestone Fm, lying below the Dalton Lst in the same mesothem. The coastal traverse extended from the Dalton Lst, which makes a tolerable building stone, up-sequence through the fractured and rubbly-looking Park Lst, deposited in shallow water, a poor building stone containing more lime mud; and up again into the Urswick Lst. Each formation is c150 metres thick. The very open folding is clearly visible along the coast, when viewed from well out on the sands. Faulting and fault breccia are noticeable. Red hematite shows in faults. Lunch (SD 4375 7667) was on a high bank over the sandy beach. In the afternoon on the Urswick Lst wave-cut platforms [447 762] we saw abundant large syphonophillia solitary corals, and syphonodendron colonial corals, the latter apparently all upside-down as a result of breaking off during storms and resting top (ie wide side) upwards.

Siphonophilia and Siphondendron corals

Sunday began in the rain with a walk west along the waterfront from the hotel to discuss an unusual 50m section of highly disturbed Dalton Lst for which no satisfactory explanation yet exists. It may have something to do with the shale partings, up to 2 m thick, which lubricated the movement of the more competent limestone during the Variscan Orogeny. These shales were eroded from Silurian rocks in the Lake District. Then we drove to Sandside quarry to see the Park Lst at the entrance, with its pronounced mudstone layer, and from there to Throughs Lane, Storth. The area is much affected by the Silverdale Disturbance, a 0.5 km wide band of rock through which an earlier, underlying Caledonian fault was reactivated, passing up through the Carboniferous rocks, as a reverse fault in the Variscan Orogeny. This created a monocline (i.e. a step rather than an antiform). On the step the sequence is vertically bedded, and the Woodbine Shale, here up to 10 metres thick, in the Lower UL, has been removed by erosion, creating a deep cut, which became a road called Throughs Lane. The monocline soon disappears, probably because the land surface is tilted across the vertical plane of the step.

Beds of Dalton limestone with shale partings

Next stop was the Ship Inn, Sandside, where an excellent lunch was laid out for us in a marquee. The afternoon session involved a walk half a mile south back along the coast road to St.John's Cross. The ridge behind is formed of Dalton Limestone and the gateposts are Urswick Limestone. A walk back up the busy road brought us to Bromersha Bay. Here we had a briefing on the structure of the district in a low lying field beside the coast road. These ‘shale hollows’ are formed where a dipping shale bed has been eroded out of the limestone. Then we made our way 500m along the former railway cutting, now a footpath, back to the Ship Inn, first in Dalton Beds and then in the Park Lst, all displayed steeply along both sides of the path. The strike of the steeply dipping beds curves sharply because of Variscan strike-slip faulting and buckling, and the lubricating effect of the evident numerous shale partings. Slickensides are pronounced and have confusingly different alignments; brittle fractures are evident, suggesting the cracking was shallow in the crust, only a few kms. At the road bridge over the footpath the pathside rock suddenly becomes more rubbly, suggesting a change from Dalton to Park Limestone, accompanied by a fault. Arriving back at the Ship Inn we said goodbye to our excellent leaders. Sunday evening (supperless after a huge lunch) was spent at Leighton Moss NSPB bird sanctuary, to have drummed into our resistant heads by Michele & Richard the names and behaviours of various wading birds.

Gait Barrows NNR limestone pavement

On Monday morning, as usual we led ourselves, first to Gait Barrows NNR (around SD 481 774) and some extensive limestone pavements in the Urswick Lst; quite the best we have ever seen. En- echelon quartz-veined cracking is widespread. This area, including the Yorkshire Dales has the best limestone pavements in Britain. When a horizontally bedded limestone is close enough to the surface for glacial action exactly to scrape away superficial deposit, the acid rain collects and incises the surface, particularly along the joints, leaving clints (upstands) and strongly aligned grykes (fissures). Finally we drove to Trowbarrow quarry, close to Leighton Moss. Here the vertically bedded Urswick Lst (probably on the step of the monocline) has been removed. On the west side a final few metres of UL remains, now used as a climbing wall, with the underlying fractured and rather useless Park Lst emerging behind it. On the east side a few metres of UL remains with the overlying, and also rather broken, Gleaston Fm visible behind it, the only time we glimpsed the Gleaston.

Annual Summer Weekend 2014

Annual Summer Weekend 2014
Minehead, Somerset, 30 May – 2 June
North Devon-Somerset Coast

The 2014 Summer Weekend was spent near Minehead, exploring coastal sections from Kilve in west Somerset to Lynton, just over the border in Devon. Seventeen people enjoyed a long weekend brilliantly led by Chris Darmon, of Down to Earth magazine, and Geosupplies, well known as a field trip leader, and as chairman of the Youth Hostel Association, and president of Herefordshire and Worcestershire Earth Heritage Trust. We began on Friday evening with a geological briefing.

The district lies at the juncture of two different geological regions. To the west, in Devon, is rock formed from thick marine sediments deposited during the Devonian period, uplifted and folded on an east-west alignment by the late Carboniferous Variscan Orogeny. By the end of the Carboniferous period the region was mountainous and eroding. To the east, in Somerset, is younger rock, formed from Triassic and Jurassic sediments deposited by a rising sea encroaching westwards onto the eroding Devonian landscape (Carboniferous and Permian are missing in an unconformity). The lacustrine environment of the Triassic was followed by shallow marine limestones of the Jurassic. These younger rocks display a north-south alignment, overlying the east-west alignment of the older rocks.

Saturday was spent on the younger rocks to the east, around Kilve and Watchet, beginning with a single 1916 oil retort, which only briefly distilled oil from early Jurassic shales (ST 1451 4464) of the Blue Anchor Fm. Volume commercial oil extraction requires the organic content to have been matured (cooked) in situ at exactly the correct temperature: too high and it volatilises and escapes; too low and it fails to migrate to reservoirs. Bedded black shales were beautifully exposed on the beach. They smell oily when struck, and fizz furiously with acid. Sharp contacts with interbedded limestones testify to ancient sudden environmental change. Pronounced jointing shows in limestone beds but not in the incompetent shales. Loose blocks of limestone on the beach are imprinted with Psiloceras ammonites to 25 cm diameter.

Altrnating limestone and shales jointing present in the lomestones

Burrows on the base of limestone beds suggest oxic bottom water. Were the shale beds deposited in deeper anoxic waters? There looked like burrows on the shale beds; if so, then sea level change did not account for the different depositional environments. Walking west (1411 4430) we speculated about reasons for these sharp alternations in lithology. One possibility is pulses of stratification anoxia during the shale intervals, caused by sudden global warming due to release of methane sequestered on the sea floor as methane hydrate. Frequent clasts of petrified wood in the limestone confirmed shallow marine conditions locally during deposition. Bedding lines on the foreshore provided geometric evidence of faulting in the limestones, and fault drag could be discerned in some cases.

Red and tea-green marls

After lunch in Watchet we drove to Blue Anchor Bay (010 450). Triassic limey muds on the foreshore – the red & tea green marls - result from oxidising and reducing environments. Formed in salty playa lakes, these marls contain evaporites, here with extensive and spectacular sheets of pink alabaster (fine-grained gypsum, hydrous calcium sulphate), a secondary product of the halite crystals which would have precipitated first. The low cliffs of red mudstone, are part of the Mercia Mudstone Grp.

Fault placing pale grey limestone beds next to red Mercia Mudstone

At the east end of the bay a fault places pale grey limestone beds beside this red Mercia Mudstone, with fault drag showing. Is this the Triassic/Jurassic boundary? Perhaps not, because a little further east round the corner is more gypsum, suggesting that we are not quite finished with the Triassic. The day’s geology finished in the café at the west end of the promenade.

Sheets of pink alabaster

Sunday was spent further west on the Devonian rocks at Lynton, close to the fatal 1953 Lynemouth floods (car park 7108 4974). The ‘Valley of Rocks’ is a hanging valley, now mostly dry, curiously running along the coast of the Bristol Channel but a km or so inland. Originally the northwards East Lyn river failed to reach the coast and turned west to discharge a mile or two further along the coast, perhaps at Lynton, or perhaps further along at Lee Bay. Coastal erosion and glacial meltwater finally connected the East Lyn directly to the sea and left the Valley of Rocks high and dry, 145m above the East Lyn. Evidently once glacial, it ends at Lee Bay, where the base of the cliff shows an early raised beach of sand and pebbles, topped by river deposits of sand and partially rounded stone, and then by glacial head with large angular stones and a preferred orientation. This was a major fluvial and glacial discharge before the upstream end of the valley broke through to the sea.

View from North walk

During the day we walked up the natural rock castles, noting jointing; and the cleavage in less sandy beds (7101 4991). An excellent cliff path (North Walk) along the coast was built in 1817 for the ‘genteel’. Looking north, the headland is comprised of Devonian sandstone – the Hangman Fm, extensively used as a building stone in Lynemouth. We also walked the west section of the path, noting the effect of sea level change on cliff shape, and shelly marine fossils pathside, high above the present sea. After lunch (7053 4979) we examined more tors, including Castle Rock. These tors are sedimentary, not igneous, horizontally bedded and with widely spaced joints, making them stable and erosion resistant, so they tend not to tumble, but stand high above the surrounding surface, which was carried away by solifluction during the permafrost.

Monday morning was spent in seaside Minehead, the name possibly a corruption of Mynydd, alluding to the hill on which the old town stands. Many buildings use the local stone: Devonian ORS, Permo-Triassic NRS, and Jurassic limestone. Half a dozen old lime kilns are situated near the harbour. Several times, both day and night, we looked across the Bristol Channel from Minehead and guessed at the towns we saw: Barry and Penarth slightly to the east, perhaps Porthcawl slightly west. But we were cautioned to remember that there may have been substantial Variscan strike slip along the Bristol Channel, so that the land opposite was slid into place from elsewhere.

Annual Summer Weekend - South Pembrokeshire 7-10th of June 2013

Orielton Field Studies Centre. Leader Sid Howells

This year’s Summer Weekend was spent at Orielton Field Studies Centre, just south-west of Pembroke, where twenty members enjoyed good food and comfortable accommodation. Arrangements were excellent, even extending to a minibus for transport. Seven years ago we spent the weekend on the geology of north Pembrokeshire; so in 2013 it was time to see the Old Red Sandstone and Carboniferous rocks of the spectacular coastal sections of south Pembrokeshire.

The weekend began for some with a Friday afternoon visit to Pembroke Castle, built on and of Carboniferous Limestone exposed in the Pembroke Syncline. Beneath the castle is the Wogan, a vast natural cavern in the limestone, giving the castle occupants a basement access direct to the banks of the river Pembroke. The weekend proper began with supper on Friday followed by an introductory talk by our leader Sid Howells. South Pembrokeshire was folded by the late Carboniferous Variscan Orogeny when Africa collided with southern Europe, and the district was folded on a WNW axis. The southern coast is a Carboniferous limestone, exposed in the Bullslaughter Bay Syncline. This passes north into the Orielton Anticline, exposing older Devonian Old Red Sandstone, and even older Silurian and Ordovician in a few places. North of this lies the Pembroke Syncline, which again exposes the younger Carboniferous Limestone.

Saturday started on the sands of Freshwater East Bay [SS016978], an anticline within the wider Orielton Anticline. The cliffs show steeply dipping beds near the core of the anticline. We studied deposition patterns in the watery sand of an ebb tide and then walked across the bay, down-sequence from Younger ORS on one side, through Silurian mudstone and sandstone in the middle, and back up-sequence into the ORS. East along the coast for several miles are high ORS cliffs with a series of embayments where conjugate fault sets nearly normal to the fold axis are points of weakness and hence erosion.

At the far north of the bay is a pronounced conglomerate, the base of the Lower ORS, not to be confused with the extensive Ridgeway Conglomerate which marks the top of the Lower ORS. These sediments were deposited in semi-arid, sometimes waterlogged, Devonian braided rivers and coastal plains. We saw the evidence: red sandstones deposited in oxidising conditions, lie beside greenish sandstones deposited in a waterlogged environment; pronounced dessication cracks preserved in the mudstone; fine sandstones intercalated with thin lenses of grits due to surges of coarser material in riverine conditions; calcretes (carbonate nodules) formed as limestone precipitated from evaporated flood soils on exposed sand banks. Quartz and pink (potassium) feldspar grains were probably sourced from igneous rock to the north-west.

Conglomerate

A few miles south-west the Carboniferous Limestone appears in the Bullslaughter Bay Syncline and we had lunch there at the picturesque Stackpole Quay [SR993958].
Axial core of syncline

At Quay Cove we saw the axial core of a tightly folded plunging asymmetric syncline and Sid Howells excavated the foreshore beach stone to uncover a huge Lithostrotion (colonial rugose coral); it has to be re-exposed at every visit! There are also brachiopods (productids), solitary corals (Caninia), large crinoid ossicles and very large worm casts (Palaeophycus?).

Lithostrotion

Next came Stack Rocks [924945] in the Upper Carboniferous, on the MOD range. Caves are often eroded in softer horizons on either side of promontories, and frequently they meet in the middle to form natural arches, and then collapse to leave a tall stack just offshore.
Green Arch

These form splendid safe havens for colonies of nesting seabirds. West of the stack lies the Flimston Fault, running south-east across the Bristol Channel. We could see, looking along the headland, where a lateral displacement of some 500 metres had shifted the rocks to produce a visibly different fold structure on the other side.

Devil's Couldron

We moved on to the Devil’s Cauldron, Flimston Bay and, finally, a brief visit to Freshwater West [885994], where the Flimston Fault runs along the wave-cut platform. Here, at Great Furznip, the Skrinkle Sandstones of the Upper Devonian lie unconformably above the Ridgeway Conglomerate of the Lower Devonian, and below the Lower Limestone Shales which mark the start of the Carboniferous. Back at Orielton we finished the day with supper and a second talk from Sid Howells.

Most of Sunday was spent walking the lovely beach from Saundersfoot Harbour [138047], across the barnacled boulder bed of Monkstone Point, to Trevayne [147029], in Carboniferous sandstones of the Middle and Lower Coal Measures. Once again, looking west into the cliffs cross-sections of numerous short range folds, faults and low angle thrust structures are presented. Near the harbour in the cliff faces there are ironstone nodules, formed by chemical replacement as iron in the pore waters precipitated as siderite (iron carbonate). The folding varies from tight upright folds to recumbent folds with thin anthracitic coal seams, which perhaps lubricated the folding.

Ladies Cave Anticline

Not far south of the harbour is the Ladies Cave Anticline [139040], the most frequently illustrated geological feature of the district. Alternating thick sandstones and shales have allowed a sharp asymmetrical chevron fold to remain protruding well above the foreshore sand. The outer shape is protected by thick sandstone beds. The inner part has been eroded and hollowed out by hard quartzite storm beach boulders. A similar anticline appears fifty metres north along the beach, but now fully collapsed and only just emerging from the sand. Anticlinal rocks emerging from the sand become more widely separated towards the sea, showing the structure plunges north. The less steep side of the asymmetrical anticline dips roughly south, showing the Variscan Orogeny which caused it occurred to the south. After crossing Monkstone point, Tenby comes in sight and we made the long, steep climb up the cliff steps to the minibus waiting at the top.

Next stop Lydstep Point [088976], on the Carboniferous Limestone. As so often on the South Pembrokeshire coast, west-east faults have led to progressive land displacement westward, and conjugate fault sets very roughly aligned north have caused embayments – and beautiful sandy havens. Caldey Island lies some 3km west and was probably once a fault-shifted peninsula, now separated from the mainland by erosion. On Lydstep Point we saw the Gash Breccia, a wide horizon in the vertically dipping limestone. It appears in many places and is thought to be unique to this district. Was it caused by limestone cave collapse, or brecciation during the Variscan Orogeny, some sort of fault crush perhaps, or something else? Much discussion has not reached agreement, or even consensus. Some of the blocks are bus sized, some of rock crushed much finer.

Lydstep Point

The final stop was 1 km west at Skrinkle Haven [083974]. From our vantage point, the nearest half of the cliff of this small bay is Carboniferous Lower Limestone Shale; the other half is the Skrinkle Sandstone which forms the Upper Devonian of the district. The Middle Devonian has been lost in an unconformity. The near side of the bay (north) is eroded on a fault within the shale, leaving a large, sheer wall-like cliff. At the far end of the bay the promontory is caused by the Ridgeway Conglomerate, marking the top of the Lower ORS. Unfortunately the far cliff face was in strong shadow, obscuring the boundary, but the nature of the fallen rock below each unit was clearly different. Sunday ended with supper and a quiet evening in the common room.

Colin at Manorbier

Monday morning we visited the shore of Manorbier Bay [062974], in sight of the castle, to look at a cross-section of vertically dipping ORS, some 500 metres south of the Ridgway Conglomerate. Each of six beautifully exposed deposition cycles begins with a coarse, mud rip-up lag in red silty sandstone, and then fines upwards through channel fills of grey sandstone with parallel lamination, and some rippled lamination in the upper part. Each shallowing cycle is then topped by purple silty mudstone, with calcrete nodules formed from the soils which began to be established on the sand bars of a braided river, before avulsion (sudden change in direction) inundated the new vegetation and began another cycle of deposition. Most members left after the morning, with a few stalwarts staying on to search for fossils further up the succession. All agreed, one of our very best annual weekends.<