Harrow and Hillingdon Geological Society
Caring for the Chilterns
Caring for the Chilterns – the role of the geological advisor
This presentation covered the Chalk, which forms the core of the Chilterns, how it led to the role of geological advisor and what that has involved.
What is chalk?
Chalk is a typically white, semi-lithified limestone, which gave its name (from the latin creta to the Cretaceous period, which lasted from 144 to 65 million years BP and which includes 35million years of chalk sedimentation. The Roman name for England (Albion, from the latin alba meaning white) probably derives also from the first sighting of the White Cliffs of Dover.
The chalk has been carved in historical times (eg the White horse) and it is still in use today, eg the largest active chalk quarry in Great Britain is at Kensworth in Bedfordshire.
Chalk is a biogenic rock, being formed of the skeletal remains of animals including:
What was unusual in Cretaceous times?
Northern Europe was covered in sea and there was a global greenhouse climate with no evidence of polar glaciation. Today, foraminifera with keels only occur in waters with temperatures not less than 17oC and during Cretaceous times there were keeled foraminifera right at the northern end of the North Sea. With no ice at the poles, global sea levels were very high and there was major flooding of continental shelves. There was at least 150m of seawater above southern England and fewer continental sediment sources. The clear seawater with a low Mg/Ca ratio helped the growth of coccoliths, which bloom in their millions in the right conditions. For example, one species (Emiliana huxleyii) is the most profuse single organism in the world. In these conditions, deposition of chalk, initially as a coccolith and foraminiferal ooze continued for millions of years.
At the Baldock bypass, the face excavated during construction of a cut-and-cover tunnel looks pretty uniform from a distance but in close-up there is a lot going on. For example there is iron staining from sponges in the Caburn Sponge Bed, which can be traced from Sussex to the Chilterns).
Sponge spicules and radiolarian provide potential sources for the SiO2 in flint. Interestingly, in the North Sea, radiolarian have the SiO2 replaced by CaCO3. Flints come in all shapes and sizes, of particular interest being the spiral flints (Zoophycus flints), which initially formed following burrows in the sediments, some stretching as much as 4-5m across. Their origin can be explained drawing on the PhD of Chris Clayton on the geochemistry of flints. Near the sea bed are oxygenated sediments but lower down, bacterial decay leads to the depletion of oxygen and the production of sulphides (H2S) in reducing conditions. Animal burrows cross the reduction/oxidation (Redox) boundary where SiO2 crystallises out. The periodicity of flint formation is associated with warming/cooling (Milankovich) cycles.
There are also hard grounds where the sea floor was exposed and mineralization took place. At Reed Quarry in north Hertfordshire, the Top Rock occurs at the old boundary between the Middle and Upper Chalk and the Chalk Rock just below. The Top Rock was often used by quarryment as the floor of the quarry, as at Flaunden and Redbournbury. These two beds are also present at the Aston Rowant cutting and in areas in between. Variations in thickness may be due to old Palaeozoic features in the basement beneath the Chalk.
Marl seams in the Chalk can be traced and correlated from southern England to northern Germany and these occur in the Newpit Chalk Formation at Baldock, which has very little flint.
The geological advisor
This role came about because the speaker had been a member of the Chiltern Society for a number of years and in returning his annual membership renewal he had responded to the question – Have you any skills which would be of value to the Society? – by saying he knew quite a lot about Chalk. Within a few days this was followed up by the Society asking if he could tell them anything about their disappearing river.
Chalk and the case of the disappearing Misbourne
When asked about this in September 2008, the BGS Beaconsfield map had just been published using the modern Chalk classification. It showed that the upper reaches of the river are underlain by the Newpit Chalk Formation with its marls, allowing the river to flow across it. Just downstream the Lewes Nodular Chalk Formation (which lies on top of the Newpit Chalk Formation) is exposed. This includes hard grounds with fractures between, which allow water to pass through. For example, at Kensworth Quarry there are huge pipes running through the Lewes Nodular Chalk Formation. Further downstream, the geology moves forward 85 million years to the Quaternary ice age and later sediments overlie the Chalk. These are the Beaconsfield Gravels, river gravels laid down by the proto-Thames before the last ice advance blocked its eastward flow, forming an ice-dammed lake at Rickmansworth and a big delta to the south-west before a spillway was formed down the Colne Valley to the present Thames.
Thus there is rubbly chalk with lots of holes in it, across which a major river flowed and lots of freeze-thaw action. The surprise is that the river Misbourne is there at all. Add to this the human usage of water supply from the Chalk and the result is a 500m stretch in Chalfont St Giles with no water most of the time. In high rainfall periods it flows naturally but most of the time it does not.
After reporting on this to the Society, the speaker was appointed Geological Advisor.
Two weeks later he was asked to advise on Pitstone Quarry, which developers were proposing to backfill with inert waste. Pitstone comprises a number of quarries which formerly supplied the cement works and it already has two important sites:
Backfilling the lake could have serious groundwater implications and destroy the geological site. The chalk bluff in Quarry 2 has one of the few exposures of the Plenus Marl and is a very important section. Unfortunately the quarry crosses the Buckinghamshire/Hertfordshire boundary and the geological conservation movements in both counties assumed the other was dealing with it so it was not designated as a Regionally Important Geological Site (RIGS), though this has now been resolved and it has now been so designated. Both Natural England and the Environment Agency objected to the application and it has now gone to appeal to the Secretary of State for Communities and Local Government. A revised proposal is to backfill between the chalk bluff and the lake with a clay lining and temporary lagoons for contaminated water and a containment wall comprising 60% chalk and 40% clay, which is an erodible material. Should the containment fail, any contaminated water would go into the lake. The section above water at the lake includes the Totternhoe Stone and the limestones below water level crop out in the valley below with a spring line (from which at least 8 springs and wells take their water).
Chinnor Quarry in Oxfordshire is another former quarry for the cement industry, which is bisected by the Icknield Way footpath. Housing is proposed in the quarry with the old limekilns being refurbished as a biodiversity centre for Chinnor. As geological advisor, the speaker has suggested it should also serve as a geodiversity centre. It could be used by
The Chiltern Society is not against the provision of high-speed rail to Birmingham but has seroious concerns about the choice of preferred route, which crosses the northern limb of the London Basin with a downward sequence of Seaford Chalk, Lewes Nodular Chalk, Newpit Chalk, Holywell Chalk etc.
The preferred route is the Misbourne Valley, with a tunnel from near Denham to west of old Amersham. Examination of water and borehole records shows there are up to 16m of weathered rubbly chalk at the base of the valley resting on solid chalk. The crown of the tunnel would be only 22m below ground at its shallowest beneath this, so there would be only 6m of competent chalk above it. The damage to the aquifer system could include: