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THANET, SATURDAY JUNE 11, 2011By Linda Hamling Having
enjoyed a hot drink at Sam’s Bar after our journey from Hertfordshire, we
joined Haydon, our leader for the day, on the beach at Dumpton Gap (TR
397665) near Broadstairs. Only
two type sections (‘golden spikes’) have been internationally accepted
for defining the various Upper Cretaceous stage boundaries.
The Chalk exposed in the cliffs at Dumpton Gap is mainly of Early
Santonian age (83-86 Ma), and has been proposed as the type section for the
base of the Santonian Stage (= top of the Coniacian Stage).
However, the relevant international committee decided that sections
near Haydon
recounted the large amount of research on the English Chalk during the
1980s. In particular, Rory
Mortimore studied the sequence of the South Downs and Nick Robinson that of
the We
began by looking at the major marker band known as the Chartham Flint, which
is equivalent to the Mitcheldean Flint of the
Fig. 1
Haydon with
a good example of the Bedwell Columnar Flint Band. [Photo: The
calcareous foraminifera in the Chalk indicate the depth of water in which they
lived. With increasing depth of
water, there was a decrease in temperature and carbonate became more soluble, so
that less was available for ornamentation of their tests.
With an increase in water depth during flooding events, the foraminifera
are thus smoother and less ornamented. Subtle
changes in ornamentation of the Chalk foraminifera can therefore be used to
indicate changes in level of the Chalk sea.
Foraminifera are also as widely distributed as the Inoceramids, and are
as useful as stratigraphic markers. From
Dumpton Gap we drove to At
about the same level as the Peake Sponge Bed we first began to find the
stemless crinoid Uintacrinus. Also
in the cliff 10 metres or more above our heads Val spied a very impressive
specimen of the large ammonite Parapuzosia (fig.
2).
Fig.
2 Large
ammonite Parapuzosia coming out of the cliff. [Photos: As
we walked northwestwards into We
then returned to our cars at
THE CHALK OF NORMANDY, SEPTEMBER 22-26 2011 By Linda Hamling For
the first night (Thursday, September 22nd) of our long weekend
studying the Chalk of the Dieppe-Entretat region in After
a very calm crossing accompanied by the odd gannet, we drove to the western end
of the bay in The
Bray Axis extends from the bay at Moving further west we found the Seven Sisters Flint Band. This is less permeable than the Chalk above, and as a result karst cavities have formed above it to produce an extensive cave system, often with fluting on the cave walls and roves (fig. 3). Water penetrating down fissures above has carried Palaeogene sands and loess from near the surface, sorting and redepositing them as colourful sediments in the cavities above the flint band. On the wavecut platform, we searched for fossils in the hardground. The occurrence of Echinocorys and Micraster suggested that it is close to the boundary between the
Fig. 3 Sediment-filled
karst features along the impermeable Seven Sisters Flint Band. On the following morning (Saturday, September 24th) we drove through the pretty countryside of the Caux Plateau of Normandy to Fécamp, passing a sarsen on the way. In this town there are many houses with walls built of flints beautifully dressed into perfectly squared blocks. This local industry is celebrated on a roundabout with a sculpture of a man and his carthorse collecting flint from the beach. Such collection is now forbidden, as the flinty beach protects the Chalk cliffs from erosion. At
the base of the cliff at Port de Havre-Antifer petroleum port, we saw pebbles
with green glauconitic coatings within the basal Chalk.
This is the Gaize Rock, the French equivalent of the Glauconitic Marl at
the base of the Chalk in Three
sarsen menhirs guarded the entrance to the path down to Le Tilleul Plage, north
of the petroleum port. Here we found
Mammites nodosoides in the chalk at the base of the cliff.
This ammonite indicates the Lower Turonian Stage.
We also found Mytiloides, indicative of a less oxidizing
environment, in a zone characterized by large foraminifera (Globigerina),
indicating Chalk deposition in deeper water as a result of a global rise in sea
level. Intraclasts were scattered
within the Chalk, a feature that can be traced at this horizon across Europe
into eastern Moving
on we found the Antifer Hardgrounds, which replace the Plenus Marls deposited in
the deeper water of the main Chalk basin, as in southern After lunch we examined a horizon with trace fossils including Zoophycos feeding traces picked out by flint deposited in burrows. However, progress through an arch into the next bay was then thwarted by another (real) air display about to begin. But the official who barred our way did allow us to go a little further to see the dolomitized chalk in the spectacular cliff. Here we could see the toe of a synsedimentary slump structure along which a cave system had developed and the Chalk had been hardened. The slumping had resulted from irregular uplift and tilting of the Caux Block. We were also attracted by pretty pink manganiferous flints. Unable
to continue because of the air display, some of us walked over the cliff top to
Entretat to see the cliffs made famous in paintings by Monet.
On the way we were able to observe the wonderful display, which was
performed by, amongst others, the Blue Diamonds, the French
equivalent to the Red Arrows here. In
a very crowded Entretat, we saw another arch in this superb cliff.
That evening we returned to the restaurant on the On
Sunday (September 25th) we drove to Puys, which is in the main
Fig.
4. Examining flints in the Lewes Chalk Formation and on the beach. [Photo: As we walked on to St Martin Plage, we could see the Penly Nuclear Power Station, one of four along the north French coast. The Plenus Marls occur beneath it, overlain by a much expanded sequence of Holywell Chalk with no flints and intraclasts without borings indicating a low diversity fauna. However, there were abundant Mytiloides shells and we saw a giant Mammites nodosoides. There were numerous fractures, which occur right across the basin, though with different styles in each Chalk formation. The cliff at Criel Plage has Lewes Tabular Flints at its base, with the Lewes Marl above. The flints are overgrown by a second phase of silicification, and in the marl we found Mytiloides and Micaster spp. We lunched al fresco at the excellent Le Coq Hardi in Criel, a seaside village lying in a river valley exploiting a fault. After lunch we were able to paddle through freshwater streams bubbling up from impressive springs in the wave-cut platform formed of Lewes Chalk. Some were brave enough to taste the water and declared it very good. A team led by Rory has monitored the cliffs here for their stability using geophysical methods, which have shown that each Chalk formation has its own characteristic system of fracturing. Wave energy transmitted through the wave-cut platform exacerbates cliff collapse, and can be monitored using geophones, which measure acoustic emissions. Using this system one cliff collapse was predicted 48 hours before it happened. Sadly it was now time to catch the ferry home. But as we left
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KENSWORTH AND TOTTERNHOE, SATURDAY, JULY 16, 2011by
Haydon Bailey On a typical summer day, i.e. horizontal rain driven by a south-westerly gale, a hardy group of stalwart members gathered at the National Trust Centre on Dunstable Downs. It was so wet outside that we decided to stay indoors for another coffee – and then another. Just as we were thinking it wasn’t really a good day for fieldwork, a hint of blue came through the otherwise grey skies, and we decided to chance it. So we trooped off down to the nature reserve within the bounds of Kensworth Quarry (TL 011200). But our optimism proved to be unfounded, as the rain returned with renewed vigour for the rest of the morning. The Chalk face in the nature reserve had recently been thoroughly cleared of vegetation by Anne Williams and her colleagues of the Bedfordshire and Luton Geology Society, and the results were clear to see. The Caburn Marl (= Reed Marl) was clearly visible at the eastern end of the face, and its disappearance near the western end was explicable at last as the result of minor faulting. Only with the face completely cleaned was it possible to see this for the first time – a clear example of what a little site geo-conservation can achieve. We spent the rest of a very wet morning searching for fossils in the Chalk Rock above the marl. Having had our fill of the Chalk Rock, most of us retired to our bolt hole (the National Trust Centre) for a good lunch, very proud of ourselves for persevering through the weather, however John went home to St Albans to change his wet trousers. A little more drizzle after lunch wasn’t going to put us off now, and we set off for Totternhoe Quarry (SP 9822), which is now managed by the local wildlife trust. At this site, the Totternhoe Stone within the Lower Chalk is well known as a submarine channel deposit. As the sun came out we spent some while searching for fossils among the many blocks of clunch and stone in the old quarry. Numerous ammonite fragments and their aptychi were found. So the day proved a success despite our initial misgivings about the weather.
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CHAIRMAN’S
CONCLUDING REMARKS
by Haydon Bailey The
day at Kensworth and Totternhoe clearly illustrated the impact of a little
coordinated work cleaning up sites and the added value this can bring to
understanding their geology.
It is always much better if you can actually see the rocks!
In Hertfordshire we are not blessed with large expanses of geological
exposure, and those we do have rapidly become overgrown and degraded.
Currently many of our important RIGS sites are under the management of
the Hertfordshire & Middlesex Wildlife Trust, although the original reason
for their conservation was geological (e.g. Hitch Wood and Barkway Chalk pits).
If the Trust works on these sites at all, they only touch the vegetation
and don’t clean the faces for fear of damaging an important aspect of the
geology.
Conversely, we don’t clear the vegetation, as we regard this as the
Trust’s responsibility!
There is a clear need for a cooperative approach, in which the Trust
attack the undergrowth and we spend time cleaning and re-exposing what makes
these sites important in the first place – the geology.
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