2010-11 Newsletter - Hertfordshire Geological Society

NEWSLETTER, WINTER 2011
MIMRAM VALLEY, SATURDAY APRIL 17, 2010
By Linda Hamling
We met Mike Howgate, our leader for the day, at the garden centre in Codicote. He led us
first through some woodland opposite, where bluebells and wood anemones were just coming
into bloom, and we then emerged into a large field where Mike gave an introduction to the
day. During the Anglian glaciation about 450,000 years ago, ice spread across the eastern
part of the Chiltern (Chalk) scarp and diverted the River Mimram from its preglacial course,
which was probably through Codicote, into its present position to the west of the village. The
ice deposited mainly glaciofluvial sands and gravels over the Chalk. The British Geological
Survey described these as kame deposits, because they form mounds and ridges. |One side of
the field was bordered by a prominent gravel ridge, and in the opposite direction we could see
numerous mounds covered by woodland. In a small scrape in the middle of the field, we
found Bunter quartzite pebbles, which the ice had brought from the Midlands, and in the past
there have been numerous sand and gravel pits in the area. Continuing south-westwards
across Codicote Heath, and passing numerous other small exposures of the sands and gravels,
we descended the steep slope into the present Mimram valley. After joining a path running
parallel to the river, we had a view up a dry tributary valley, which was cut in gravels on the
eastern side but in Chalk to the west. Boreholes in the area have shown that the sands and
gravels are very variable in thickness, but locally reach 40 ft. Where the path met the
Mimram, we left the river’s main course to follow the mill-race to Kimpton Mill (TL
198185). North of the mill, a watercress bed is fed by the mill leat, and beside the nearby
lane, a cottage was built for the river keeper, who had the duty of ensuring fair use of the
river for all its users.
We then retraced our steps to the garden centre, where we had lunch. From there we drove
to Nine Wells Watercress Farm (TL 180213), which is run by the Sanson brothers. First we
bought bunches of delicious cress cut fresh from the beds, before a tour. A railway line
salvaged from the Western Front in WWI runs along the length of the beds, enabling the
cress to be harvested. A manicured hedge acts as a windbreak. The nine wells are boreholes
taking water from the Melbourn Rock 200 ft below to feed the beds. Mike partially covered
the head of one borehole with a brick to show the strength of the flow. Overflow from the
watercress beds provides most of the water feeding the Mimram. One of the Mr Sanson’s
told us that his great grandfather had drilled the wells. Little frogs leapt around our feet as
we talked, and we also saw our first Brimstone butterflies of the year.
We then drove on to Deacon Hill (TL 126299), which is on the north scarp of the Chilterns
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near Pegsdon. The hill is capped by basal Upper Chalk disturbed by flint workings, and
below us to the north, spurs of the escarpment were capped by outliers of Melbourn Rock and
Totternhoe Stone. Further north we could clearly see the vale formed by the Gault Clay
towards, in the middle distance, the Lower Greensand escarpment, and beyond that the
northern side of the valley of the Great Ouse. Beyond Pirton to the northeast is another small
outlier of Totternhoe Stone, from which stone was quarried for Pirton church. The Chalk
scarp in this area is dissected by several steep-sided dry valleys, the origin of which is still
debated. Spring sapping and gelifluction or nivation in periglacial conditions have all been
invoked to explain them. One valley is known as Roaring Meg, because it contains a stream
fed by a strongly-flowing spring rising probably from the Melbourn Rock.
When Pirton first became a separate parish, Ralph Lindsay Loughborough, its vicar, was
shocked at the dilapidated state of the church, and began collecting funds to repair it. He
successfully appealed for 50,000 shillings (£2500) to be raised by donations of one shilling
per person, and used the money slowly to rebuild the tower and parts of the chancel and nave.
The stone was quarried by volunteer labour from the small outlier of Totternhoe Stone, which
was part of the parish glebeland. The stone was hardened by exposure for six months before
use. In his daily diary, Ralph Loughborough recorded fossils found during the quarry work.
These included giant ammonites, Parapuzosia austeni, up to 1.5 m across, and a fossil fish
Ctenothrissa radians, which was put on display in the church but sadly stolen. However, we
were able to examine two of the ammonites, which had been built into the south wall of the
church. In the porch, a piece of the dark brown ferruginous sandstone from the Lower
Greensand had also been built into the wall. We sat in the church while Mike described the
environment in which the Totternhoe Stone fossils had lived, and then went outside to
examine the remains of the adjacent Norman Motte and Bailey castle.
We thank Mike for the really interesting day exploring the fascinating geology of this part of
Hertfordshire.
NORTHERN IRELAND, MAY 8-15, 2010
By Linda Hamling and Jean Gardner
Despite problems with delays to flights caused by Icelandic volcanic ash, we made our way
successfully from Stansted to Belfast International Airport, where we collected our transport
for the week, two luxurious Ford Galaxies. From the airport, we drove eastwards to Larne
and then northwards along the magnificent Antrim coast road to our hotel for the week in
Carnlough. H.V.Morton described this road as finer than the Corniche in the south of France,
and we were later to hear from our leader, Phil Doughty, about the geological difficulties
encountered during construction. The hotel had once belonged to Winston Churchill, who
inherited it from his second cousin, the Marchioness of Londonderry.
Sunday, May 9. On our first day, we drove north long the coast before deviating inland to
cross high moorland, where we stopped to look at Loughareema or the Vanishing Lake (fig.
1). The main depression contained a small amount of water, but at other times the whole area
is transformed into a large lake fed by streams at times of heavy rain. With no surface exit,
the lake drains rapidly into the underlying Ulster White Limestone (= Chalk), but the changes
in lake level cannot be explained by variations in the groundwater table, so it is thought the
swallow hole that drains the lake periodically becomes blocked with peaty debris. When the
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Fig. 1. Loughareema or the Vanishing Lake.
Fig.2. Carrickarade Island and rope bridge.
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hydrostatic pressure on this plug becomes too great, the plug is suddenly dispersed, thus
draining the lake into the Chalk aquifer beneath.
From Loughareema, we continued northwards into the Carey River Basin. Here Phil pointed
out a series of flat-topped terraces, which originated as deltas created by glaciofluvial gravels
deposited by outwash streams flowing into a lake ponded between the high land and the
retreating ice sheet to the north. Previously the gravels were exploited for aggregate, but
extraction had been terminated to preserve these interesting geomorphological features.
At the charming harbour of Balintoy, Phil outlined the geology of Northern Ireland for us.
Within a 20 km radius of Portadown, there is more diverse geology than in any other part of
the province, though there are no Cambrian rocks. The Lewisian forms the little island of
Inishtrahull, and in Balintoy were to see Ulster White Limestone and the Tertiary basalts that
cover a large part of N. Ireland. When the Atlantic was opening around 60 Ma, there were
volcanic outpourings of two types of lava, the Lower Basalts and the Causeway Basalts. The
‘carapace’ of Lower Basalt has preserved the Chalk, which developed a karstic landscape
before it was covered. In a subtropical to warm temperate climate, the upper surface of the
Lower Basalt was weathered to form a laterite, in which iron and aluminium were
concentrated by weathering and leaching of silica and alkali metals. This reddened horizon,
known as the Interbasaltic Formation, provides an easily recognizable reference point in the
sequence. As the ice sheet melted after the Last Glaciation, the land rose by glacio-isostatic
adjustment, but did so irregularly in relation to the eustatically rising sea level, thus
producing a series of raised beaches around the present coast.
In the middle of Balintoy harbour, the basalt has been downthrown to the north along a fault,
so that Chalk occurs to the south and basalts outcrop on the seaward side of the harbour. In
the car park, a sea cave exposed a fault which had brecciated the Chalk; we came upon
another on the beach as we walked westwards. Just offshore were seastacks of Lower Basalt,
and further out were other stacks formed of the tholeiites of the Causeway Basalt. Soon it
became boggy underfoot, where we crossed an outcrop of Liassic (Lower Jurassic) clay
emerging from beneath the Chalk, and marked by a line of springs. The springs have caused
rotational slipping, as indicated by the landward inclination of seastacks composed of Chalk;
one stack had a natural arch, whose appearance gave rise to the name Elephant Rock.
Belemnites and crab burrows were found in the fractured Chalk exposed on the shore. In
White Park Bay the Chalk is partially covered by large sand dunes.
After returning the Balintoy for lunch, we walked to see the Bendoo Plug, a subcircular
parasitic cone of dolerite about 350 m in diameter intruded into the Chalk. The nearby
Bendoo House is an idiosyncratic home built by Newton Penprase progressively from 1936.
Leaving Balintoy we travelled on to see Carrickarade Island, which is the vent and plug of
another volcano. After crossing the rope bridge (fig. 2), we were able to take a close look at
lavas, tuffs and volcanic bombs – all very apposite considering our visit to N. Ireland had
nearly been prevented by volcanic ash from Iceland. Finally, at Larrybane Quarry, we were
able to take a close look at the Ulster White Limestone. It is very hard, rich in belemnites
and affected by complex joint patterns, so that it is very different from the Chalk we are used
to in Hertfordshire. This is partly explained by the succession being younger, the oldest
Chalk in N. Ireland being younger than the youngest in Hertfordshire, but most of the
lithological features can be attributed to the physical pressures and chemical changes
resulting from the Tertiary volcanic activity.
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Monday, May 10. From our hotel in Carnlough, we first went to Kinbane Head, where the
cliffs were a perfect spring garden of primroses and orchids. On the shore a large seastack
surmounted by the remains of a castle had formed on the flank of a volcano where lavas had
pushed out from the neck through limestone, causing it to shatter. In places Chalk apparently
overlies the basalt, but it is fact a tuff composed mainly of fragmented chalk ejected from the
volcano and containing volcanic bombs. On returning to the car park at the top of the cliff,
Phil found a cobble of basaltic tuff displaying the impression of a leaf of a deciduous plant
60-62 Ma old.
On Ballycastle beach we saw Carboniferous rocks, including a seat earth with root
impressions. Salt pans once existed here, and to reach the site we crossed a wooden bridge to
an outcrop of sandstones. In the early 1700s, an enterprising gentleman, Hugh Boyd,
operated coal mines under Ballycastle. The coal was of Namurian (Millstone Grit) age, and
the mines were worked by local families. Boyd devised a pumping system for draining water
from the mines. 6-8 tons of coal was required to produce 1 ton of salt, and six gallons of
seawater had to be evaporated to produce a pound of salt.
Just along the coast is the cottage where Marconi carried out radio transmission experiments.
Above the cottage are large travertine deposits, and to the east we could see the impressive
Fair Head Sill with a crude columnar structure at the top. The scree of basalt columns below
is inhabited by feral goats. We then stopped to look at the adit of the North Star coal mine,
and view the outcrop on the beach below of the North Star Dyke, a photograph of which was
used by Arthur Holmes in his textbook on Physical Geology.
After lunch we drove along the narrow lane to Murlough Bay, where the Ulster White
Limestone is seen to rest unconformably on Triassic sandstones (fig. 3), with no intervening
Jurassic, though pebbles of Jurassic rocks have been found at the junction. After parking
near the cliff top, we walked downhill to obtain a wonderful panoramic view of the Fair Head
Sill, with large landslipped blocks of dolerite and limestone. A fault separates parts of the
cliff exposing limestone over Triassic rocks to the east and the Fair Head Sill over Namurian
sandstones to the west. Further coal mines occur along this part of the coast, including the
Twin Arch Mine, near to which are the remains of miners’ cottages. Further east we went to
see a limekiln, in which limestone from fallen blocks had been burnt with coal from
Ballycastle. Further east, the Great Gaw Fault brought Dalradian metamorphic rocks to the
surface.
Tuesday, May 11. We met Paul Lyle, our leader for the day, on the shore at Portrush. Here
he demonstrated a downward sequence of the basaltic Portrush Sill, Chalk and Lias.
Offshore, the Portrush Sill thickens seaward and dipping southwards forms the offshore
Skerries. In Portrush a fierce debate ranged in the late 18th century between the Neptunists
and Vulcanists. Werner, a Neptunist, believed that marine fossils found in the Liassic shales
indicated that the basalt had crystallized from seawater, but of course he had failed to
distinguish the two rock types. The Lias has been metamorphosed by the basalt to hornfels,
rather like a porcellanite. James Hutton disputed the Neptunist theory and maintained that
the basalt had crystallized from molten rock material, but he did not live long enough to see
his ideas confirmed by later work. Portrush is therefore an important site for the
development of geological science.
We then drove to White Rocks, where the Ulster White Limestone is at sea level and has
been altered and shattered explosively by volcanic pressures as at Kinbane. Large blocks of
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Fig. 3. Ulster White Limestone over Trias, Murlough Bay.
Fig. 4. Dunluce Castle
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basalt and limestone have been mixed as the two rocks collapsed chaotically into the neck of
the volcano. The basalt contains numerous vesicles and amygdales filled with zeolites, which
had crystallized from hot water circulating within the volcano. Margaret spotted vesicles
which were streaked out and aligned by the flow of the basalt. East of the vent, the bedded
limestone dipping southwards had an irregular surface, which was covered by the first basalt
flow.
We then drove to Dunluce Castle (fig. 4), where we walked down the cliff to see a vent
eroded by the sea, the Lower Basalt having been downfaulted against the Toe Valley Fault
between here and Portrush. The lavas here form about 12 pahoehoe flows, each about 1 m
thick, which is much less than the 30-40 m flows of the Causeway Basalt. Reddened
weathered material formed from ash is incorporated into the basal part of the subsequent
flows, enabling the different flows to be distinguished.
After lunch we visited the Giant’s Causeway. From the visitor centre, we walked down the
slope of the palaeovalley, which was filled with flows of the Causeway Basalts. The Port na
Spaniagh laterite, which was previously worked for bauxite, was exposed below the first
Causeway Basalt flow. Further down, we descended beneath the Windy Gap laterite, which
was formed on another flow of the Lower Basalt, so that the palaeovalley floor lay above us.
But as we moved lower, the palaeovalley floor descended and disappeared below the
Causeway Basalt, then rose higher again on the opposite side of the palaeovalley. When the
valley filled with lava, the river in it was temporarily blocked and formed lakes. The water
cooled the surface of the flow quickly, and slower cooling beneath this frozen cap generated
the large, regular polygonal columns for which the Giant’s Causeway is famous (fig.5).
Tomkieff named these the colonnade. Where water can penetrate the lava, perhaps down
cracks formed by minor earthquakes, it cools more quickly forming curved, less regular
columns of what Tomkieff called the entablature. After exploring the causeway, we climbed
up to look at the ‘Giant’s Organ’, where the regular colonnade is topped by an entablature of
the same flow (fig. 6). Further on, the ‘Giant’s Amphitheatre’ displays the whole sequence
of Lower Basalt, Interbasaltic Laterite and Causeway Basalt. The ‘Giant’s Eye’ consists of a
spheroidal mass of resistant, less weathered basalt within the Interbasaltic Laterite.
Wednesday, May 12. On this morning we drove westwards across N. Ireland to Marble
Arch Caves in Co. Fermanagh. This is the first International Geopark in Britain. On the way
we passed Slemish, a volcanic plug rising out of lush countryside moulded by ice into
hummocky drumlin fields. We also passed along the wide main street of Cookstown, which
once hosted a cattle market and contains attractive houses built of limestone and bricks.
Because Phil was instrumental in getting the caves declared a Geopark, we were privileged to
have a tour led personally by the director Richard Watson. It began with a trip in a boat on
the subterranean River Cladagh, formed by the confluence of three rivers. The Carboniferous
Dartry Limestone here is very pure and vulnerable to dissolution and cave formation.
Beneath is the less pure, flaggy Glencar Limestone, which impedes the downward movement
of water. After walking through the public section of the cave system to admire the waterpolished cave walls and impressive stalagmites and stalactites, we walked through the
adjacent nature reserve of Cladagh Glen to where the river emerged from the cave under the
Marble Arch. A cliff on the river bank displayed the junction between the Dartry and
Glencar Limestones, and beside the path we examined a tufa deposit formed where calcium
carbonate had been deposited from water emerging on the valley side.
Thursday, May 13. We started this day on Larne Promenade beside the monument to
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Fig. 5. Causeway basalt, Giant’s Causeway.
Fig. 6. Giant’s Organ showing multiple basalt flows with colonnade and entablature structure.
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William Bald, the builder of the magnificent Antrim Coast Road. He was a Scottish surveyor
and cartographer, who was commissioned to construct a road skirting the notoriously
unstable Antrim coast. The weight of the Ulster White Limestone and overlying basalts has
squeezed out the underlying Lias clays, leading to collapse of large blocks of the heavy hard
rocks above. Bald drained the seaward slopes to moderate the swelling and movement of the
clays, and also blasted away much of the unstable cliffs. It took five years to construct the
road from Larne to Cushendall; the cost was originally estimated at £25,000, but the final
cost was £38,000. Bald had an assistant, Charles Lanyon, architect of Queens University and
the Customs House in Belfast, who designed the ‘Big Bridge’ which crosses Glendun. The
beach platform north of Larne exposes gently dipping grey Lower Jurassic clays and
limestones of the Waterloo Mudstone Formation. Walking towards Larne, we also saw the
paler coloured upper part of the marine Triassic Penarth Group, in which we found fish scales
and teeth. There seems to have been continuous deposition from the late Triassic into the
early Jurassic, so the sequence here is a good candidate for a global stratotype for the base of
the Jurassic. The actual dividing line is marked by the first appearance of the ammonite
Psiloceras. After examining two limekilns, we drove northwards along the coast road, which
is here built on a raised beach with abandoned sea caves. Before Black Cave tunnel, we
crossed a major fault and then entered Drains Bay to reach Ballygally Head, another volcanic
plug. The massive arcuate scarp of Sallagh Braes probably originated as massive rotational
landslip, the town of Ballygally having been built on the slipped material.
A mudslide in Lias clay at Drumnagreagh has caused constant problems for the coast road.
The latest scheme has been to build a large concrete bund, behind which the mud collects and
is periodically cleared out and dumped on the beach on the opposite side of the road. Over a
period of several years, parts of an Ichthyosaur skeleton found in the dumped clay have been
refitted and are on display in the Ulster Museum. Other much larger landslips at Peaks Point
near Glenarm (fig. 7) have resulted in serious falls of limestone onto the road, the largest in
1968 closing the road with a fall of over 400 tons. The Chalk here is fractured by diagonal
joints, which open when the limestone moves over the Lias clays beneath in periods of heavy
rain. The original suggestion for resolving the problem here was to drive sheet piles into the
Lias clays to prevent their seaward movement, but initially this was judged too expensive.
Instead large blocks of Chalk were blasted off the cliff face and used to build an artificial
platform to stabilize the Lias. But the blocks were soon washed away by storms, and the
original idea had to be adopted. A trough was dug between the cliff and the road to catch any
falling rock and rip-rap was put in front of the sheet piling to dissipate the force of waves.
The entire work cost £575,000.
After lunch in Carnlough, we looked back at Straidkilly, the ‘sliding village’, which also
suffers from frequent landslips. From Glenariff Quay near Waterfoot, we had a view down
Glenariff, which is thought to be floored by Triassic deposits, with no overlying Jurassic.
The boundary between the Chalk and overlying Tertiary basalts is marked by the upper limit
of green pasture fields. Workable iron ore was found in the Interbasaltic Horizon in the 19th
century, and during WWII there were about 60 active iron ore mines in the Glen. In the cliff
beside the road there are raised beach caves cut in red Triassic sandstone.
We then drove on to Cushendun, a village designed by Clough Williams-Ellis and now
owned by the National Trust, where we met Tony Bazley. Cushendun is on the southern
margin of the mountain chain thrown up by the Grampian orogeny, which extended from
Scandinavia to Canada. Just north of the village we looked at rocks of the Glen Dun
Formation, part of the Neoproterozoic Dalradian, which are actually upside down on the
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Fig. 7. Landslip in Ulster White Limestone near Glenarm.
Fig. 8. Examining the conglomerate at Cushendum.
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underside of the Loch Tay Nappe. The psammitic rocks here are about 600 Ma old, and have
been affected by three phases of folding. The sandy country rock was intruded by the
Cushendun Microgranite, which is widely used as a roadstone. It is actually a dacite about
460 Ma old. On the southern side of the river at Cushendun, were looked back at the skyline
to see that the Dalradian rocks have a knobbly appearance, whereas the basalts form
smoother landscape. Here the cliffs are formed of a coarse red Devonian conglomerate
containing mainly well rounded quartzite boulders (fig. 8), which were probably deposited by
sudden floods in a wadi under otherwise dry desert conditions. Along planes of weakness
large sea caves had been carved out by Holocene marine erosion. At the top of the
conglomerate there are many volcanic boulders. Walking south-eastwards along the beach
from Cushendun, we crossed the outcrop of a porphyritic dacite dated to 406 Ma, then
another conglomerate (the Red Arch Conglomerate) faulted against the dacite. This
conglomerate was first mapped as Upper Old Red Sandstone, but recent palaeomagnetic
studies suggest it is Upper Carboniferous in age. Inland from Cushendun, we could see the
volcanic plug of Teivebulliagh, which has baked the Interbasaltic Laterite into a tough
porcellanite, from which stone tools were made in the Neolithic period.
Friday, May 14. On this morning we drove south to Helen’s Bay, where we met our leader
Bernard Anderson. The rocks here are part of the Upper Ordovician and lower Silurian
Longford Down Terrane, equivalent to rocks of these ages in the Scottish Southern Uplands.
The continuity with southern Scotland was originally recognized in 1852 by Reverend Bryce.
They consist of poorly sorted marine turbidites containing mainly angular sand fragments of
quartz, feldspar and other minerals and rocks. Beneath are black graptolitic shales and then
spilitic lava flows with chert between the pillows indicating extrusion on an ocean floor. The
sequence constitutes an accretionary prism, where wedges of sediment were allochthonously
accreted to the continent of Laurentia as Avalonia was being pushed beneath Scotland during
closure of the Iapetus Ocean. Pillow lavas were most evident at the headland of Horse Rock.
Braving the slippery rocks, it was possible to go to the end to see an inverted pillow (fig. 9).
Adjacent was a vent agglomerate of spilitic clasts in an ashy matrix created by an eruption
about 450 Ma. Nearby we could see black graptolitic shales with a sandstone raft or
olistolith.
We then drove to the northern margin of Strangford Lough, and walked to the summit of
Scrabo Hill, a sill-like intrusion of dolerite. From this vantage point we could see the Ards
Peninsula covered with small rounded drumlins and more elongated ridges known as Rogen
Moraines. When the ice spread across Scrabo Hill, it created a crag and tail by leaving a tail
of debris stretching down-glacier from the hill. Had it not been for the protection afforded
by the sill, the Triassic Sherwood Sandstone lying 30 m below the summit would have been
eroded away. We were able to see this pink and yellow sandstone in a nearby quarry, where
it is intruded by transgressive sills and a dyke (fig. 10). A footprint of the Triassic reptile
Chirotherium found in this quarry is now in Ulster Museum.
We then drove into Belfast to obtain view the impressive new earth science galleries of the
Ulster Museum. The afternoon closed with a reception kindly hosted by the Belfast
Geologists’ Society. On the final morning, we had a little spare time, so some of us walked
from the hotel to see the Cranny Falls and Gortin Limestone Quarry, which had supplied
Glasgow with building stone. We followed the route of the gravity railway, which had been
used to bring rock from the quarry across the Antrim coast road to the small port. We then
drove to Belfast International Airport for the flight home. We were very lucky, because the
airport was closed the next day by the volcanic ash cloud! We thank John, Peter, Phil, Paul,
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Fig. 9. Upside-down pillow at Helen’s Bay.
Fig. 10. Quarry in Triassic sandstones with transgressive sills, Scrabo Hill.
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Richard, Tony and Bernard for their contributions to a superb week, and are also very
grateful to Margaret and Linda for taking on the responsibility of driving us around so safely
and comfortably for the week.
HAMPSTEAD HEATH, JUNE 13th, 2010
By Lesley Exton
Five members and one guest met our leader Diana Clements at Whitestone Pond on top of the
heath, which is highest point in London (400 ft). Here Diana introduced the day by
explaining something of the history of the area as well as its geology. There was a race track
here in the 17th century. The heath lies within the London Syncline and is part of the outcrop
of the Bagshot Sands, with Claygate Beds and London Clay beneath. Under the syncline
there is an anticline in the Palaeozoic rocks that form the London Platform. From the pond
we walked north to Sandy Heath to examine the uniformly fine sand of the Bagshot Beds
exposed there. Diana had us collecting pebbles on our way across the heath; most were of
flint, though we also found small quartz pebbles. The area is hummocky, as it was worked
for sand in the 1860s to build St Pancras Station, and in the centre of the excavations a
shallow pond has formed probably where impermeable iron pans in the sand hold up the
water. At Dr Joad’s Hockey Pitch, the sand is coarser at the base of the Bagshot Beds, then
downslope it suddenly becomes finer where you cross onto the Claygate Beds. The different
sands are visible in the holes formed where trees were uprooted in the great storm of 1987.
Our next stop was the famous Sphagnum Moss SSSI enclosure sited on Claygate Beds. After
admiring the bog pepper and irises on the way, the bog itself was rather disappointing,
because the gate to the enclosure was locked and the enclosure itself rather overgrown. We
continued our walk past the Old Dairy, which at one time supplied milk to the city, which
was provided by cows grazing on the heath. A slight detour took us to a sculpture of Eugene
Dodeigne, which is said to be of ‘granite’, though this is unlikely because the stone contained
numerous fossils! The final stop before lunch was a quarry exposing Bagshot Beds near
Kenwood House. Diana hopes this will be preserved as an SSSI, though the gardening staff
at the house have attempted to cover the exposure by planting non-native pine trees.
After lunch at Kenwood House, we walked up to the viewpoint at Cohen’s Fields, from
where we could look across central London. At a lower level we looked at the Goodison
Fountain, which is fed by a spring emerging near the top of the Claygate Beds. As you can
taste iron in the water, it isn’t very drinkable, though local dogs, hot from running over the
heath, seemed to enjoy it. Even further downhill, we saw a series of ponds built by the New
River Company near the junction of the Claygate Beds and London Clay to transport water in
elm pipes by gravity into the city. We then crossed South Meadow, where Linda saw a grass
snake while the rest of us admired the view across Canary Wharf to Shooter’s Hill south of
the Thames. Diana explained that two old boreholes for water in the city, one at Kentish
Town (1853) and another at the Meux Brewery in Tottenham Court Road (1879) were
probably separated by a fault, as Jurassic limestone is recorded only in the latter.
Heading further south, we visited the 19th century brickfields, which have been flattened to
produce a football pitch. The London Clay dug here was left on the surface to weather and
lose its pyrite and gypsum because it could be used for making bricks. Fossil hunters (the
London Clay Club) came to search for fossils on each new load excavated. Chalk and ashes
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were mixed with the clay to make the bricks, which were then left to dry for six weeks and
fired with kindling wood in piles. Red bricks were susceptible to frost weathering, but the
yellow ones resisted the cold better. We then walked the viaduct, which had to be rebuilt
twice, as it is sited on wet ground at the junction between the Claygate Beds and London
Clay. The large pond below the viaduct was created by removal of London Clay. A railway
across the viaduct brought chalk in and took the finished bricks out.
The final stop of the day was at the Pepper Pot, now thought to have been built as an
icehouse for Kenwood House, the ice having been brought from Russia during the winter. It
is built of the local bricks, which show how they were squashed into moulds for firing. At
the Vale of Health, we said goodbye and thanked Diana for a very interesting day. It was
hard to imagine the vale was once the site of brick works and a glue factory. However,
Hampstead was a spa town, with its own water bottling plant and pump rooms, so that rich
Londoners had their own country cottages here to escape the insanitary conditions of the city.
Most of the group returned uphill to the cars, but Linda and I returned to the tube station
along Well Walk, which allowed us to examine the fountains and brick wall we had
previously seen illustrated in one of Diana’s numerous old photos of the heath area.
BURNHAM BEECHES, NORTHMOOR HILL & HAREFIELD, 10th JULY, 2010
By John Catt
We met Graham Hickman, our leader for the day, at the Burnham Beeches wildlife centre
(SU 953851) near Farnham Common. Here he had laid out a display of local rocks and had
also arranged a small experiment to illustrate the very different permeabilities of the two
main deposits of the area, the Reading Beds and terrace gravels of the proto-Thames. The
Reading Beds consist of interbedded sands and silty clays deposited in an Eocene fluvial or
deltaic environment; the clay layers make it much less permeable than the overlying terrace
gravels, which consist of current bedded sands and gravels. We spent the morning visiting
parts of the woodland where local streams incised into the Reading Beds disappear down
swallow holes into the very permeable Chalk that is locally exposed beneath. Graham also
demonstrated an array of narrow boreholes, which were used to demonstrate the depth to the
water table in the Reading Beds. While we were testing the electronic equipment on the end
of a tape for measuring the water table depth, Linda was again entranced by a snake, this time
an adder. The final site before lunch was a small sand pit exposing the Thames gravels,
which are periodically excavated to provide hard core for footpaths through the woodland.
We found flint and quartzite pebbles, and at the surface the gravels were reddened and clay
enriched by interglacial soil development.
After lunch at the wildlife centre, we drove to Northmoor Hill (TQ 035892) near Denham,
where an old Chalk quarry exposes the overlying Reading Beds on the side of the Colne
Valley. In the last few years, the Bucks Earth Heritage Group have done a large amount of
geoconservation in the quarry, including laying out a pathway and steps leading to sites of
geological importance. We first looked westwards over Denham Aerodrome, which is on
gravels of the Winter Hill Terrace of the proto-Thames, deposited in the Anglian Stage about
450,000 years ago. The Winter Hill Terrace as a whole can be divided into two parts. The
earlier part slopes gently to the east, like other proto-Thames terraces, and was deposited just
before the glaciation. The later part is almost flat at 82 m OD, because it was deposited as a
delta into the Moor Mill Lake, which was formed when the Thames was impounded by the
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Anglian ice front at Bricket Wood.
From the car park, we walked gently downhill towards the Colne valley. The first site to
admire was a beautiful funnel-shaped swallow hole, where a small stream originating a short
distance away on the Reading Beds outcrop disappears into the Chalk. From there we
walked a short distance into a large overgrown Chalk pit, where BEHG had cut steps in the
side so that you can climb up to see the basal Tertiary (Palaeogene) beds. Above the eroded
surface of the Chalk, the succession here consists of a basal glauconitic sand about 30 cm
thick and rich in small round flint pebbles (the Upnor Formation), with yellowish grey sands
and clay bands of the Reading Formation above.
Our final stop of the day was to examine a preserved section in the side of the now largely
infilled Harefield Quarry (TQ 049898). This overgrown section again shows the boundary
between the Upper Chalk and the basal Tertiary or Palaeogene beds (Upnor overlain by
Reading Formation). The site is famous for borings into the upper 50 cm of the Chalk, which
are filled with dark green glauconitic sand of the Upnor Formation. These were originally
attributed to a marine annelid worm named Terebella harefieldensis, but they are now related
to a crustacean named Glyphicnus harefieldensis. The basal part of the overlying Reading
Formation consists of grey and brown silty sands. Haydon pointed out a line of very large
flint nodules just below the eroded top of the Chalk; he thought that these may correlate with
the paramoudra flints exposed in the small Chalk pit at Potters Crouch near St Albans. The
upper part of the quarry originally exposed more Reading Beds and much of the London
Clay, but the growth of trees and bushes since the pit was closed has unfortunately obscured
these higher parts of the Palaeogene sequence.
THE PUDDINGSTONE POW-WOW, AUGUST 7th 2010
By John Catt
Our annual soiree in 2010 was extended to include a series of talks and a discussion on
Hertfordshire Puddingstone. This ‘Puddingstone Pow-Wow’ was held as usual at
Verulamium Museum, but the programme was organized by Jane Tubb of the East Herts
Geology Club as part of their 10th anniversary celebrations. Peter Banham was chairman for
the afternoon, and introduced talks by Jackie Skipper on the Lambeth Group, John Catt on
the puddingstones in the St Albans and Radlett areas, Steve Perkins on processes involved in
the formation of puddingstone, Jane Tubb on the puddingstones in the Colliers End outlier
and Diana Clements on the sarsens in the Bagshot Beds of Surrey. Bryan Lovell brought the
conference to an end with a talk on Palaeogene climate change entitled ‘Is the proof in the
puddingstone?’
Jackie Skipper gave us a regional framework for the Palaeogene deposits of SE England
using sequence stratigraphy. She suggested that the worldwide warming event at 55.8 Ma,
the Palaeocene-Eocene boundary (the PETM or Palaeocene-Eocene Thermal Maximum), was
the peak of a complex period of humid subtropical climate in the early Tertiary, which led to
strong weathering and the loss of carbonate, silica and iron from soils. These components
were subsequently redeposited as calcrete, silcrete and ferricrete in various situations within
and beneath the soil profile. John Catt described records of in situ puddingstone found in the
Reading Beds of the St Albans Palaeogene outliers at the City Hospital and Seventh Day
Adventist Church, and in the main Palaeogene outcrop at Radlett. There were also some
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early records of in situ sarsenstone (silcrete with no flint pebbles) in the Reading Beds at
various sites in Middlesex. However, in situ occurrences are rare. Most known blocks of
both puddingstone and sarsenstone occur in later deposits, such as the Plateau Drift of the
Chilterns (formed by weathering of a veneer of basal Palaeogene beds preserved over the
Chalk), slope deposits on valley sides and fluvial deposits on Chiltern valley floors. All of
these could have been derived by various processes from the basal Palaeogene sequence, but
the exact date of their origin is impossible to determine.
Steve Perkins set out hypotheses for the various pedogenetic and diagenetic processes that led
to formation of puddingstone. Making particular use of the magnificent new display of
puddingstones around the doors flanking the recently refurbished Hertford Museum, he
showed how the flint pebbles could have been weathered and recrystallized with repeated
infiltration of silica and iron in solution from water percolating through the surrounding soil
or sediment. In discussion, Jenny Huggett suggested that the processes envisaged by Steve
could be investigated by electron microscopy and isotopic analyses (e.g. oxygen isotope
analyses of the cementing materials or of fluid inclusions in modified flint pebbles).
Jane Tubb gave a detailed description of how the puddingstones were found by Bessie
Parkins in situ in the Palaeogene deposits at Sutes Farm on the Colliers End outlier. She
suggested that they originated in the Upnor Formation, but as the basal pebble beds of the
Reading Formation at some localities were deposited in channels cutting down into the Upnor
Formation, there remains the possibility that they date from the Reading Formation as
elsewhere in Hertfordshire. Haydon Bailey suggested looking for microfossils such as
dinoflagellates in the soft sediments surrounding the puddingstone.
Di Clements made an indisputable case for silicification of sands to form sarsenstones in the
Bagshot Formation, which must be much younger than the PETM. She also suggested that
the excellent detailed records of puddingstones and sarsenstones made by John Cooper,
previously at the Natural History Museum, should be digitized to clarify exactly how many
periods of silicification there were in the Palaeogene. Finally Bryan Lovell emphasized that
the PETM resulted from a massive input of carbon (as CO2 or CH4) into the atmosphere,
which caused not only the global warming but also a rise in sea level, acidification of the
oceans and extinction of many fossil groups. After the rapid release of approximately 1000
gigatonnes of carbon into the atmosphere, it was slowly removed over the next 300,000
years. This can be used as a guide to how long it might take for the present release of carbon
into the atmosphere to decrease. Bryan hopes to get the Science Museum in London to
incorporate a specimen of puddingstone in their planned Climate Change Gallery.
After the lectures, John Catt led a short ‘field excursion’ to view the numerous puddingstones
in the St Michaels area within walking distance of Verulamium Museum. The group first
examined several small blocks of puddingstone incorporated into the walls of St Michaels
church and, walking down St Michaels Street, we noted the sarsenstone setts used in the
kerbstones and gutter, which probably came from Walters Ash, where large blocks of sarsen
were found in the Plateau Drift of the Chilterns. Near St Michaels bridge the history of the
River Ver was outlined to explain the famous large puddingstone in front of Kingsbury Mill,
which was removed from the river bed in 1887. Further large and impressive puddingstones
were examined near the junction of St Michaels Street and Fishpool Street and in the gardens
of Kingsbury House and Kingsbury Lodge, where we were welcomed by Malcolm and Liz
Holliday. The garden of Kingsbury Lodge has an unusual puddingstone in which the few
pebbles are dispersed in a rather silty or clayey matrix; it is possibly a pedogenic silcrete.
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