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NEWSLETTER, WINTER 2010

PEMBROKESHIRE, APRIL 18-23, 2009

Staying in St Davids at the comfortable Old Cross Hotel in the city centre meant that we

didn’t have far to drive to our first site, Caerfai Bay (SM 759243), on the morning of Sunday

19th

April. In this part of the St Davids Peninsula about half of the Cambrian succession is

exposed, with beds dipping to the south on the southern limb of an anticline. From the cliff

top at Caerfai we looked over the 60-metre (200 ft) peneplain, which dominates the western

part of Pembrokeshire. It is thought to be Pliocene or early Pleistocene in age, but only head

and glacial deposits of the Last (Devensian) glaciation occur on it, so dating is difficult. The

green St Non’s Sandstone forms the landward margin of the bay, and seawards this is

followed by red shale and the red Caerbwdy Sandstone. Ripple marks indicating shallow

water during deposition of the Caerbwdy Sandstone are emphasized by grey, quartz-rich

sand. Within the bay three closely packed N-S faults have produced a fault breccia, and the

junction between the St Non’s and Caerbwdy Sandstones is also faulted.

We then moved on to Caerbwdy Bay (SM 769249). Descending to the beach, we admired

spring flowers of gorse, thrift, scurvy grass, violets and bird’s foot trefoil. Stone for St

Davids Cathedral was quarried on the western side of the bay from the Caerbwdy Sandstone.

On the beach we found a boulder of halleflinta, a fine silicified tuff, and on leaving the bay

we found an outcrop of the same rock near where we had parked. The rock here dips to the

north, on the northern limb of the anticline. The halleflinta is about 650 million years old and

is part of the Pebidian group of late Precambrian rocks.

After lunch in St Davids, we went on to Whitesands Bay (SM 734272), where the Cambrian

rocks are overlain unconformably by Lower Ordovician (Arenig) and intruded by the Carn

Llidi gabbro. The Middle Cambrian Solva and Menevian rocks seen towards the southern

margin of the bay are heavily cleaved, with the result that the bedding is difficult to

recognize, but various bedding structures at an acute angle to the cleavage suggest a N-S

strike. Peter demonstrated cleavage refraction resulting from textural variation in the

sediment and the greater spacing of cleavage planes in coarser beds. Middle Cambrian

dolerite forms an E-W dyke exposed in the middle part of the bay. On the northern side of

the bay we examined the Lingula Flags, but found no specimens of the brachiopod Lingula.

The Arenig rocks are exposed in the next bay to the north, from where we obtained a good

view to the west of St Davids Head, which is composed of Ordovician quartz gabbro. Carn

Llidi, also composed of quartz gabbro, is a large hill overlooking the northern side of

Whitesands Bay.

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On Monday 20th

April, we visited Abereiddi Bay (SM 797314), which exposes Lower and

Middle Ordovician graptolitic shales, and is famous as the type locality of the graptolite

Didymograptus murchisoni. Here we were in a syncline, with the cleavage parallel to the

bedding, commensurate with good preservation of fossils. Together with the dark colour of

the pyritic shales, this bode well for graptolite-hunting, but most of us were unlucky – except

Mervyn who found a nice specimen near the old quarry called the Blue Lagoon (SM

795315).

We next visited Mathry ((SM 890323), which is the site of an impressive kettle hole formed

in glacial deposits of the Devensian Stage (Last Glaciation). John explained how the kettle

hole formed by collapse of the deposits on the site of a buried block of glacial ice, which

slowly melted in situ. Because it is underlain by impermeable till, the hole is permanently

water-filled. The ice probably came from the Welsh Mountains, though the north coast of

Pembrokeshire was also invaded by Irish Sea ice from the Lake District and Scotland. Lunch

was then taken beside a misty Fishguard Harbour, though some walked into the town to visit

pubs and cafes. On the south side of the harbour we examined outcrops of the Fishguard

Volcanic Series, which consist of basic lavas and tuffs intruded by dolerite. Black shales

interbedded with the lavas help to date the volcanics as Ordovician, but again we found no

graptolites. The volcanics probably originated from an island arc within the Iapetus Ocean.

We then drove to Cwm Gwaun (SN 006349), which is one of a series of deeply incised

valleys north of the Preseli Hills. Previously interpreted as cut by overflow from glacial

lakes impounded between the Irish Sea ice and the hills, the valleys were later shown to have

humped (up-and-down) long profiles, and therefore probably originated by subglacial

meltwater under pressure, which can locally flow uphill. Strumble Head (SM 895412) was

wreathed in fog when we arrived. Near the car park, we visited a little quarry, where the

green mineral epidote could be seen on joint surfaces in basic pillow lavas of the Fishguard

Volcanic series. Below the nearby lookout building, the pillow lavas were better expressed.

On Tuesday April 21st, we drove to St Martin’s Haven (SM 762089) to examine the coarse

porphyritic and finer basalts of the Skomer Volcanic Series on the beach. These are Upper

Llandovery (Lower Silurian) in age. The associated sediments are mainly coarse sandstones

and quartzites deposited near the shore of the closing Iapetus Ocean, in which the Ordovician

shales seen on Monday had been deposited in somewhat deeper water, when the ocean was

wider. At Marloes Sands on the southern side of the Marloes Peninsula, we saw younger

Silurian (uppermost Llandovery to Wenlock) sediments, including limestones crowded with

crinoid ossicles, corals, brachiopods, bryozoans and trace fossils (burrows). The near vertical

dip of the beds here results from the Armorican earth movements. Tim led us to a very

fossiliferous block of downfaulted Wenlock deposits.

After lunch at the pub in Dale, we drove to St Ann’s Head (SM 805030), where we had

wonderful views of the Old Red Sandstone, which forms the south-eastern edge of Marloes

Bay. In Cobblers Hole, we admired a large concentric fold with an E-W axis. Tim reported

that a shale bed nearby contains Devonian spores. At Musselwick (SM 851128), we stopped

to view a cliff section, which exposed Precambrian rocks thrust over Coal Measures during

the Amorican orogeny.

On Wednesday April 22nd

, we drove along the coast of St Brides Bay to Little Haven (SM

857128), where the cliffs expose sandstones, shales and coal seams of the Coal Measures,

again folded in the Armorican orogeny (Fig. 1). The folds are well displayed in the cliffs.

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Fig. 1. Coal seams at Little Haven

Fig. 2. Gash breccia in Carboniferous Limestone at Trevallen

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Other folds seen between Broad Haven (SM 862140) and Little Haven constitute typical thin-

skin tectonics, with folds riding over others piggy-back style. After lunch in Pembroke, we

drove to Freshwater East (SS 016977), where Silurian rocks are exposed beneath the Old Red

Sandstone in a major E-W Armorican anticline. The southern margin of the bay is formed of

Ludlow beds containing Rhynchonellid brachiopods and gastropods. These are overlain by

grey and red sandstones and conglomerates of the Old Red Sandstone.

At Manorbier (SS 064977), we walked to the beach past the imposing castle. As before, in

the shore exposures we could pick out the near-vertical bedding in the Old Red Sandstone by

green marl beds, whose colour resulted from reduction of iron in the presence of organic

matter in local ponds and lakes. The bedding was also picked out crudely by pink calcium

carbonate nodules, which were formed by rapid evaporation in the arid desert environment of

the Old Red Sandstone.

Thursday April 23rd

was devoted to the study of gash breccias in south Pembrokeshire in

the company of Peter Walsh and Sid Howells. These enigmatic deposits consist of large

jumbled angular blocks of Carboniferous Limestone filling huge vertical pipe-like

depressions in the in situ Carboniferous Limestone of the Bullslaughter Bay syncline. We

met the leaders at the National Trust’s Broadhaven car park (SR 976938) and walked to the

first site, close to the Trevallen army firing range (Fig. 2). Here Peter outlined the numerous

theories put forward in the past to explain the breccias. He favoured an origin as Cretaceous

hydrothermal diatremes, that is formed when steam under pressure forced its way to the

surface from considerable depth, thus creating a cylindrical pipe of shattered rock. There are

11 other examples all within a relatively small area, but it isn’t clear why they do not occur in

the Carboniferous Limestone elsewhere. The Trevallen gash has a total volume of at least

200,000 cubic metres, though the depth is unknown. The gashes clearly predate the 60 m

peneplain of Pembrokeshire, but this doesn’t help much with their precise dating. They often

contain red sediment, probably Triassic, and are often cemented with secondary carbonate.

After lunch, we travelled to Lydstep Haven, where the Carboniferous Limestone has a dip of

110o, i.e. has been overturned beyond the vertical by Variscan earth movements. Lydstep

Point (SS 095976) to the south exposes another gash breccia. From the cliff top here, we

looked across using binoculars to Valley Field Top and St Margarets Island (SS 119973),

where there are two more gash breccias. The last of these is 200 m across, 50 m wide and at

least 30 m deep, though the northern and western margins are concealed beneath the sea; the

total volume is at least 250,000 cubic metres. We then went on to Lydstep Cleft, to examine

a vertical karstic depression in limestone filled with reddened material, which raised yet more

apparently unanswerable questions!

On Friday April 24th

, the group split up, individuals choosing their own sites to visit,

including a boat tour of Ramsay Island to look at seabirds, seals and the cliff exposures of

Ordovician sediments, quartz porphyry and gabbro. Whatever was done, everyone seemed

very happy at the end of the day with their respective choices. It was a fitting end to a lovely

week, blessed with good weather, interesting geology and a welcoming hotel. Our thanks to

Peter Banham, Peter Walsh, Sid Howells and John Catt for organizing and leading the week.

It was a pity illness prevented another leader, Barrie Rickards from Cambridge, from joining

us.

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BERKHAMSTED, MAY 9, 2009

By John Catt

For this excursion we were joined by members from several other societies, including some

who had travelled that morning from Suffolk, Bedfordshire and the East Midlands. Initially

we met to park cars on the bridge where Swing Gate Lane crosses the new A41 on the

southern side of Berkhamsted (SP 997065). From this point we walked down the lane to

Bottom Farm on the floor of the Hertfordshire Bourne valley, where we were met by John

Marsland, the present owner of Bottom Farm.

The Hertfordshire Bourne was made famous by John (later Sir John) Evans, who was the first

in 1878 to describe its intermittent flow. Later, whenever the bourne flowed, it was visited

by members of the Hertfordshire Natural History Society, often under the leadership of John

Hopkinson. An early myth was that the bourne flowed every seven years, but the records by

Evans, Hopkinson and others indicated more frequent flow at less regular intervals.

Whenever rainfall in the previous calendar year exceeded 32 inches, flow occurred for a

period between January and June. However, various parts of the valley were occupied by the

surface stream in different years or even the same year. Very often flow starts at a series of

springs just above White Hill (SP 991052), and sometimes a small lake accumulates here

beside the road, but at other times it has started a short distance downstream below Mounts

Hill. Sometimes the flow terminates in a swallow hole just upstream from Bottom Farm, but

more often it flows through the garden of Bottom Farm and across the meadow to the east

(Fig. 3), then terminating either in a large gravel pit (TL 005061), which acts as another

swallow hole or, when this overflows, the stream can extend through a culvert under the new

A41 to the appropriately named hamlet of Bourne End, where the bourne joins the River

Bulbourne.

On our visit, the first swallow hole near Bottom Farm was almost full of stagnant water, but

flow into it had ceased. John Marsland showed photographs of the bourne flowing through

his garden in early 2001 and 2007, and also made others available on his website:

http://picasaweb.google.com/john.marsland. We then walked through the very pleasant

flower meadow below the farm to the gravel pit, which still contained about 0.5 m depth of

water. The dry course of the bourne could be traced across the meadow into the pit, but not

in the arable field beyond the pit. The water levels in the swallow hole and gravel pit

therefore indicated the temporary level of the water table beneath the valley floor, and flow

over the surface occurs only when the water table rises above the level of the channel in any

stretch of the valley.

After returning to the cars, we dispersed for lunch at various pubs in the Berkhamsted area,

and then re-assembled in Castle Hill, Berkhamsted (SP 993085) to examine several large

blocks of Hertfordshire Puddingstone on the grass verge outside the house (No. 13) of

Professor Eric Brown. The largest block is partly sarsenstone (silcreted sandstone usually

devoid of flint pebbles), though the sarsen part contained a line of dispersed pebbles. The

importance of this specimen, demonstrating the close relationship between puddingstone and

sarsen, has been recognized in designation of the group as a Hertfordshire RIGS site. The

leader explained that puddingstone and sarsen originated by silicification of pebble beds or

sand in either the Upnor Formation (late Palaeocene) or Reading Formation (early Eocene).

As these deposits extended up the sub-Palaeogene erosion surface now exposed as the

http://picasaweb.google.com/john

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dipslope of the Chilterns, the blocks had moved down the slope of Castle Hill from the

plateau surface north of the Bourne Valley. The silcretes had probably formed within soil

profiles developed in humid subtropical conditions on these formations in the Palaeogene, i.e.

soon after their deposition.

The final site of the afternoon was the small gravel pit at Little Heath (TL 017082). This

exposes about 2 m of red sand over a coarse gravel composed of rounded flint cobbles. The

sand had originally been dug during World War I to fill sand bags for protection of London’s

houses from aerial bombing. Only 0.3 m of the topmost gravel could be seen, but the leader

explained that re-excavation of the pit in 1978 had exposed over 6 m of gravel, resting on

thin representatives of the Reading and Upnor Formations. Chalk had been proved by

augering about 10 m below the ground surface, which is here approximately 160 m OD. In

addition to flint, the gravel contains numerous small white quartz pebbles and occasional

cobbles of Hertfordshire Puddingstone, the latter suggesting that the gravel is younger than

the Reading Formation. The source of the quartz pebbles is unknown, though it has been

suggested that they are derived from the Lower Greensand west of the Chalk scarp.

As the deposits occur no more than 15 m above the projected level of the highest proto-

Thames terrace gravels on the Chiltern dipslope, they could be only a little older than this

terrace. However, the leader explained that the gravel is marine in origin, because it contains

glauconite and the coarse but well-sorted nature of the gravel suggests deposition on a sea

beach. At present the deposit is thought to be late Pliocene or earliest Pleistocene in age

(about 2.6 Ma). The most likely equivalent is the Red Crag, which is known to extend inland

from the North Sea coast in Suffolk and Essex, rising in height from near OD on the coast to

about 90 m OD at Stansted Mountfitchet (near the Essex-Hertfordshire border) and 130 m

OD at Rothamsted near Harpenden, Hertfordshire.

DUNSTABLE DOWNS, JUNE 14, 2009

By John Catt

This excursion followed a conference on the Chalk of Hertfordshire, held at Verulamium

Museum on Saturday June 13, and was led by the conference speakers: Drs Haydon Bailey,

Trevor James, Stuart Bryant, Isobel Thompson and John Catt. After assembling at the

National Trust car park (TL 008198) at 11 am, we walked through the woods to an old part of

Kensworth Quarry (TL 011199) where the Bedfordshire and Luton Geological Society have

preserved a section across the boundary between the Middle and Upper Chalk. The section

exposes the very fossiliferous Chalk Rock and a marl band (the Caburn Marl) in the upper

part of the Middle Chalk beneath. The quarry owners (Rugby Portland Cement) had kindly

provided nearby a large dump of Chalk Rock from another part of the working quarry,

allowing people to collect fossils without damaging the preserved face. Trevor James

pointed out how the vegetation growing on the preserved face is becoming concentrated

along the marl band. John Catt explained that Plateau Drift derived from a thin cover of

Reading or Upnor Formation sediments is preserved in solution pipes and cavities extending

down from the sub-Palaeogene erosion surface, and Trevor suggested that the non-calcareous

pipe deposits account for dispersed specimens of oak trees growing in younger parts of the

woodland on slopes above the preserved section.

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Fig. 3. The Hertfordshire Bourne looking up valley towards Bottom Farm

Fig. 4. The Five Knolls, Dunstable Downs

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After lunch at the NT Chiltern Gateway Centre, Haydon used a bronze model outside the

centre to demonstrate the influence of Lower Cretaceous bedrock geology on the landscape to

the northwest as seen from the crest of the Chalk escarpment. The group then walked north-

eastwards along the scarp crest towards Dunstable to view geomorphological and

archaeological features. John Catt outlined the various theories put forward to account for

the steep scarp-slope dry valleys, and drew attention to the small wooded Well Head valley

(SP 999204) at the scarp foot close to the Dunstable Gliding Club’s airfield. This valley has

been formed by the process of spring sapping, which may partly account also for the larger

scarp slope valleys. The springs rising in the valley are a source of the River Ouzel, which

flows through the town of Leighton Buzzard. Stuart Bryant and Isobel Thompson spoke

about evidence for the prehistoric occupation of the Downs, and showed us the group of late

Neolithic or Bronze Age round burial mounds known as the Five Knolls (TL 006210) (Fig.

4). From this location, we were also able to view Totternhoe Knoll (SP 979221), the site of a

Norman Motte and Bailey castle, and the Iron Age hillfort site of Maiden Bower (SP

997224). Both occur on outliers of the Melbourne Rock at the base of the Middle Chalk. An

extensive quarry at a lower level to the northwest was the source of Totternhoe Stone, a

freestone occurring within the Lower Chalk, which has been quarried probably since Roman

times. The stone was used for carved window and door frames in many Hertfordshire

churches, and for this reason the quarries had been owned by St Albans Abbey in the later

Middle Ages.

JOINT MEETING WITH EAST MIDLANDS GEOLOGICAL SOCIETY, JULY 11/12

By John Catt

In preparation for two days fieldwork in Hertfordshire and Bedfordshire, short talks were

given to members of both societies at Verulamium Museum by Haydon Bailey, Peter

Banham and John Catt on the evening of Friday July 10. The following morning, we

assembled again at Dunstable Downs to visit the Kensworth section, and then visited Little

Heath before lunch. After lunch we revisited the puddingstone/sarsenstone blocks in Castle

Hill, Berkhamsted and walked along part of the Hertfordshire Bourne valley. All these sites

are described above.

On Sunday July 12, we assembled at 10 am at the National Trust Pitstone Hill car park (SP

955150), and walked across the meadows to the edge of Steps Coombe or Incombe Hole (SP

959156), a spectacular Chiltern scarp face dry valley near Ivinghoe (Fig. 5). Here John Catt

again outlined the various theories advanced to explain such coombes, namely erosion by

meltwater from snow and ice on the Chiltern plateau surface, spring sapping, late-glacial

gelifluction and nivation. He suggested that they could best be explained by a combination

of spring sapping and gelifluction. Like the Ravensburgh and Barton valleys in northeast

Hertfordshire, the course of Steps Coombe shows right-angle bends, which have been

attributed to structural control of erosion by the pattern of joints in the Chalk. However, Eric

Brown (1969) found no correspondence between the directions of the various segments of

Steps Coombe and the joint patterns in nearby Chalk quarries, so the exact mode of origin

and development of the valley and others like it remains obscure.

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Fig. 5. Incombe Hole near Ivinghoe

Fig. 6. View of Denbies Vineyard near Dorking

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We then drove to the car park beside Wilstone Reservoir (SP 904135), and walked round the

western side of the reservoir to visit the dry course of the Wendover Arm of the Grand Union

Canal. On the way Peter Banham pointed out several springs that feed the reservoir and rise

from the Totternhoe Stone and other hard fractured layers within the Grey (Lower) Chalk.

One spring (at SP 908127) occurs in the floor of a deep narrow valley that must have

originated by the process of spring sapping. Peter outlined the history of the Wendover

Arm, intended as a feeder for the main Grand Union Canal, and explained that it had leaked

because it crossed the axis of a minor syncline, so that a section of it was cut in the fissured

upper part of the Lower Chalk instead of the much less permeable Chalk Marl. After walking

along a short section of the canal to view the results of attempts to seal and reinstate it, we

returned to the cars and went for lunch at Startop’s End.

After lunch we drove to Stone Lane Quarry at Heath and Reach (SP 927290) to see an

abandoned and graded section in the Woburn Sands overlain by Gault Clay and Chalky

Boulder Clay. In front of the explanatory board erected at the entrance to the quarry by the

Bedfordshire and Luton Geological Society, Peter demonstrated the various parts of the

Woburn Sands previously exposed here, and explained the problems of conservation of the

numerous geological sections in the Leighton Buzzard area.

For the final site of the day, we drove to Husborne Crawley (SP 955362), to examine a bright

green, partially silicified sandstone in the walls and tower of the church. The sandstone is

thought to have been quarried on the northern side of the steep-sided mound on which the

church is built. This site is close to the base of the Woburn Sands, where they overlie

Ampthill and Oxford Clay. John Catt said he had found blocks of a similar bright green

sandstone and uncemented green glauconitic sand near the base of the Lower Greensand

while mapping the soils on part of Woburn Experimental Farm a short distance to the east of

the church. He thought that the green colour of unoxidized glauconite had been preserved

where the basal sand was waterlogged just above the clay. This situation could also explain

redeposition of silica from water percolating through the sand to form the sandstone.

Through the kindness of the churchwardens, we were able to enter the church and Mike

Howgate explained how weak foundations had caused the church walls to crack so badly that

much of it had to be rebuilt in the early 20th

century. Stages in the rebuilding were illustrated

by early photographs in a small exhibition inside the church.

Dr Ian Sutton proposed a vote of thanks to all the leaders of the weekend meeting, and from

Husborne Crawley our visitors from the East Midlands left for home by way of the nearby

M1.

VISIT TO DORKING AND DENBIES VINEYARD, SEPTEMBER 5

By Linda Hamling

We met our leader, Professor Dick Selley of Imperial College London and self-styled

notorious local geologist, in the car park of Denbies Wine Estate, which lies in the lee of Box

Hill. From here, we drove into Dorking to visit Dorking Caves, the entrance to which is well

hidden behind an obscure steel door at the side of the town’s war memorial. They are dug in

Folkestone Sands, which are soft and easily excavated, yet strong enough to support tunnels

without collapsing. In the cave walls, we could see prominent cross-bedding, indicating

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deposition in a shallow sea. As part of the Lower Greensand, the sands were originally

glauconitic and green, but they have been oxidized, releasing iron from the glauconite, which

has been reprecipitated to form hardened concretions and layers (carstone) and create

Leisegang rings in the sand. The caves are probably late mediaeval, and consist of well

shafts leading to tunnels dating from the 17th

and 19th

centuries. Previously they were used as

wine vaults, and some of the tunnels allow access to stalls, which are still labelled with their

former contents. Dick showed us some old graffiti before we descended the 20-metre

staircase leading to a mystery chamber, which is a circular cave with a carved bench around

its wall. The purpose of the chamber is unknown. Chalybeate water rich in iron must have

flooded the cave at one time, leaving a ‘tidemark’ around the wall. We left the caves to

emerge back into the bustle of the town, and collected the cars to drive up to Ranmore

Common, where we sat in the sunshine to eat our lunch. From this picnic spot, we had a

view below of the Mole Valley and the Vale of Holmesdale along the outcrop of the Gault

Clay, with the northward-dipping Lower Greensand forming Leith Hill on the other side of

the valley.

The Dorking area has been used for vine growing for many centuries. Mediaeval vineyards

were abandoned early in the Little Ice Age, but in the 1600s the Duke of Norfolk re-

established one on the Chalk at Albury Park, and another was planted at Painshill on Bagshot

Beds in the 1700s. After lunch, we walked past St Barnabas Church, Ranmore, which was

designed by Gilbert Scott for the Cubitt family, who owned the Denbies Estate. Now the

estate is owned by Adrian White of Biwater Water Treatment. From a track descending the

Chalk scarp slope, we looked through a gap in the hedge across the vineyards in a dry valley

towards Box Hill. Dick had been responsible for suggesting to Adrian White that the south-

facing valley and lower scarp slope of the North Downs would be an ideal site for growing

vines, as they had soils similar to those of the Champagne region of France, and offered

maximum sunshine and protection from cold winds. Most of the present vineyard is planted

at lower levels on a raised Pleistocene terrace of the River Mole (Fig. 6). As we walked

down the hillside, we admired harebells, thyme, scabious, knapweed and St Johns wort on the

banks of the track, and looked down on a house built entirely of Lego for a television

programme by James May.

On reaching the main estate buildings again, we enjoyed an Imax multiscreen film about

establishment of the vineyard, which featured Dick, and then had a short tour by train of the

wine-making process. This was followed by a tutored tasting of several different wines, and

the opportunity to buy the wide range of wines in the excellent vineyard shop. Our cars were

distinctly heavier for the return to Hertfordshire! We are especially grateful to Dick for a

wonderful and convivial day and for introducing us to an unusual and important aspect of

applied geology.

VISIT TO BRIGHTON, OCTOBER 10

By John Catt

Following Colin Whiteman’s talk on October 8th

, four of us visited Brighton to see the

magnificent interglacial buried cliff and raised beach section at Black Rock (TQ 336033).

On a brilliant sunny autumn morning, we assembled on the top floor of the multistory car

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park at Brighton Marina. From this viewpoint, we could take in the whole cliff section

showing the relationship of the buried cliff to the raised beach and overlying chalky slope

deposits on the eastern side of the Sheepcote dry valley. Recent attention had been focused

on the site following a cliff fall that had damaged part of the supermarket. Although the

original intention was to cover the whole of the high cliff in concrete, Natural England and

local geologists had persuaded Brighton Council to preserve this important exposure for

visitors like ourselves to view. Part of the section has been stabilized with hundreds of rock

bolts, and rock falls from another part are prevented by a cover of wire netting.

We then walked through the supermarket car park to get a closer view of the deposits

associated with the buried cliff. The raised beach deposit, which rests on a wave-cut platform

at about 7 m OD, consists mainly of rounded flint pebbles, but Colin had also found over 200

erratics in it, mainly greywackes and pink granites. He suggested that most came from NW

France, and had been carried eastwards up the English Channel by longshore drift. A few

bones and mollusc shells had also been found in the beach deposit, including bones of a small

horse (Equus ferus), which became extinct about 200,000 years ago at the end of the

penultimate interglacial (Marine Isotope Stage 7). This means the beach cannot be Last

Interglacial (MIS 5e), as previously thought. It was probably cut when the sea level was 15

m below present, and its present height can be attributed to tectonic uplift. The relatively

high level of the beach has probably resulted from tectonic uplift. Closer to the buried cliff,

the beach shingle is interbedded with masses of large chalk boulders, which probably

represent falls from the almost vertical interglacial chalk cliff.

Colin explained that the very thick chalky slope deposits that bury the beach and cliff were

deposited mainly by fluvial erosion processes on the slopes of the dry valley rather than mass

movement by gelifluction as previously thought. They contain a few cold-tolerant terrestrial

molluscs. Recent optically stimulated thermoluminescence dates on sandy layers within the

slope deposits indicate a late MIS 7 age for most it, so the underlying beach could be early

MIS 7 or older. Further west there is a sequence of buried cliffs and raised beaches at

increasing heights, which have been dated to earlier interglacials (MISs 9, 11 and 13).

Walking east, we examined the nature of the Newhaven Chalk, which forms the high chalk

cliffs for several miles east of Brighton. It contains lines of flint nodules and also thin marl

seams, which probably originated by weathering of volcanic ashfalls into the Chalk sea.

There are numerous horizontal and steeply inclined tabular or sheet flints, which result from

mobilization of the silica of flint nodules and its redeposition in fissures created by joints and

fractures in the Chalk. Periglacial disturbance of the uppermost few metres of the cliff during

the Devensian had deformed one of the horizontal tabular flints, and broken it into small

angular fragments, forming a gravel disturbed by flask-shaped involutions penetrating

downwards into frost-shattered upper layers of the Chalk.

After lunch, we walked on to Ovingdean, where we all squeezed into Colin’s car, and were

driven to Newhaven to look at the coastal section (TQ 447001) of basal Palaeogene (Upnor

and Reading Formation) sands and gravels overlying Newhaven Chalk Formation. The

section here showed iron-pan layers in the sands and gravels, fragments of which had been

observed in the raised beach gravel and overlying slope deposits at Black Rock, where they

are probably derived from small outliers of Reading Beds on the South Downs close to

Brighton. Higher in the Newhaven exposure were compacted peaty layers with tree roots and

clays containing crystals of gypsum, which had formed by oxidation of pyrite in the clay to

form sulphuric acid, and then reaction of the acid with calcium carbonate.

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