Bucks Geology Group
Grid reference: Car Park SP731 332; Coombs Quarry SP 733 327.
Ordnance Survey map:
Aylesbury and Leighton Buzzard Sheet 165.
1:50,000 series Thame sheet 237 and Aylesbury sheet 238.
Bucks County Council administration area:
Bucks County Council; managed by The Chilterns Countryside Management Project.
Area of site:
Access, location and parking:
The Picnic Site Car Park for Thornborough Bridge, off the A421 Buckingham to Milton Keynes main road.
Interest Summary: an excellent site with a blend of archaeological, geological and botanical interest. The geology covers Jurassic Blisworth Limestone, Blisworth Clay and Cornbrash.
Before walking from the car park on the A421 towards Coombs quarry the Romano-British burial mounds are worth a visit (in the field next north of the carpark). These are the burial mounds of high status Britons at the time of Roman occupation. They may have origins from the Coritani tribe (the original Iron Age tribe of the Buckingham area) or from the Catuvellauni (a Belgic group who took over a large territory with a 'headquarters' in St Albans ). The mounds were excavated in 1839 for the Duke of Buckingham and the finds were sent to the Cambridge Museum of Archaeology, but have been returned to Bucks on long-term loan and can be seen in the Old Gaol Museum, Buckingham. The finds included bronze jugs, a bronze lamp, amphorae and samian pottery. A high status collection. In addition, a temple dedicated to the Eygyptian Goddess Isis was found on the south side of the road and is also Romano-British.
The medieval bridge is visible from the car park and is a good place to start the geological visit. The 14th century bridge is in remarkably good condition. It is made from the local stone: the Blisworth Limestone. Some of the blocks within the bridge show sedimentary structures of the limestone better than they can be seen in the fresher faces of the quarry. Look out for the evidence of its formation: ooliths, shells (whole and fragments) and cross-stratification that shows the movement of water currents in the sea that deposited these particles (Figures 1 and 2 below).
Coombs Quarry: To reach the quarry you need to walk across a couple of fields. With the Medieval bridge behind you walk under the new bridge. The path splits; take the right fork over the stile. Walk directly across the field (south). Note the medieval ridge and furrow field system that can still be seen as undulations across this field. The gentle curve of the furrows is due to the large team of oxen that must have been used in this instance. Walk over the little bridge and cross the next stile. Continue on across the next field - but just before you go through the next (usually open) large gate turn immediately left through another open gate and field. The gate ahead of you is the entrance to the quarry.
The site is maintained for public use by Bucks County Council's Countryside Management Service. The quarry was used until about the
end of the 1890s for extracting building stone and rock for lime burning (note the two brick lime kilns at the northern end of the site). These kilns are 19th century, but lime-kilns have been on site since the Roman times. The quarry has been designated a LGS site for Bucks as it is the best exposure of the Blisworth Limestone.
The sequence shows 12m of Blisworth Limestone overlain by
5 or 6m of Blisworth Clay (often overgrown) and 2m of Cornbrash with a thin veneer of Kellaways Clay, the latter
only visible in one part of the quarry (see log above and photos below).
Photo above: The quarry face at Coombs - Blisworth Limestone.
There are a number of clues which inform us of the ancient environment represented by these sediments. Firstly there are the fossils. Examples of several different bivalves, gastropods and corals should be found within the rock debris often deposited in the central area and in the rock face itself. The following are common species from the limestone on this site:
Modiolus (mussel), Liostrea (oyster), Plagiostoma, Pleuromya, Bakervillia, Rolierella
Aphanoptyxis bladonensis, Aphanoptyxis langrunensis
These are all marine forms and indicate a shallow, tropical sea.
The environment of deposition was apparently quiet, low energy, as demonstarted by the fine grained lime mud, the abundant pellets (fossil faeces?) and in situ delicate corals. There is little evidence for much cross-stratification which would indicate a higher energy environment. The limestones were deposited close to exposed land as occasional sub-aerial exposure surfaces, fossil faunas typical of low salinity and the only Jurassic mammal fossil (tooth) from the county have been recorded from this site. The climate and landscape during this part of the Jurassic must have resembled the present-day Florida Keys!
The Blisworth Clay overlying the limestones is a pale-grey clay with minor
silts. Rare fossils are restricted to plant debris and rootlets, with a few beds containing small oysters. However, just one record of another type of fossil has been found - a dinosaur footprint (a theropod, see sketch below). This was found at nearby Thornborough Mill. The footprint was preserved in a thin limestone bed. Both items of evidence indicate the close proximity of land. The Blisworth Clay probably represents a sequence of strata deposited in a shallow brackish to fresh water close to the shoreline, such as back lagoon mud flats or salt marsh.
Above: Photograph of the Thornborough dinosaur footprint which is on display in Oxford University Museum. The long middle toe print is 10cm long
The Cornbrash is only seen in the quarry due to a fault
which brings this younger rock down, alongside the older Blisworth Limestone and
Clay (see below):
The Cornbrash at the top of the sequence is a massive bed of rubbly, fossiliferous limestone. The fossils are mostly broken, although a few whole specimens of bivalves, brachiopods, and sea urchins can be found. This indicates a
return to marine conditions. The ammonite Clydoniceras and abundant small bivalve Meleagrinella prove that this is Lower Cornbrash, and that all the upper Cornbrash is missing as the Kellaways Formation directly overlies this limestone.
People at work maintaining the quarry:
2011 Recording work
During November 2011 a number of members took part in the recording and measuring a lithological section at Coombs Quarry.
The wooden walkways, which had been in place for several
years, had become very dangerous and the layout of the quarry made it awkward to
maintain the site. The wet summer of 2007 had ensured luxuriant vegetation
growth and the site became very overgrown, despite efforts of the County
Council's team and the BGG volunteers. The only way forward was to take out the
rotting boardwalks and re-landscape the quarry, thus ensuring a safer route for
viewing the geology and also ensuring ease of future maintenance by moving large
rocks hidden in the undergrowth. With funding from the LHI, the site soon was
transformed from a mass of vegetation and unsafe walkways to a site worthy of
being the County's best disused quarry. The local natural history was also
taken into account during the transformation. Here are photographs to show the
Faces of Blisworth Limestone are only just visible above the vegetation cover. No
access to these faces was possible. Compare this view to photos below!
Mike standing precariously on the rotting walkway. This was not going to be an easy
This was the main face just inside the front access gate before the work commenced.
This was the same face as above soon after the work had commenced. Although still
requiring a clean-up, the face of Jurassic limestone and clay can now be seen.
Below: the same face a few metres along.
With the vegetation cleared from the main central area, the JCB could start the face
clearance and re-landscaping of the quarry floor to ensure future safety and
Here the JCB is working back into the corner of the quarry with the fault line. A
very distinct red horizon has become visible between the grey clay at the top of
the section and the underlying limestone. This has not been visible before. When
the quarry work has finished a new log will be prepared by the group.
View to the gate by January 4th 2008 - the vegetation stripped leaving the hawthorn
in place, dangerous drops re-landscaped into safe slopes
The Blisworth Limestone could now be seen for the first time in many months. Freshly
exposed it is now ready for members of the group to go in and record the new
sections, which now go deeper than they had ever been at this location.
Coombs current interpretation board.
Further reading suggestions:
Davies, L. (1994) Buckinghamshire rediscovered, Earth Heritage Magazine. EH2. pg.16-17 pdf
Ellis, N.V et al. (1996) An Introduction to the Geological Conservation Review. Chapter 6. pg.85-86 featuring quarry photo.
Eyers, J. (2002) Geological Walks in north Buckinghamshire. Thornborough Bridge and Coombs Quarry pg.18-21
Sumbler, M.G. (2002) Geology of the Buckingham district. Middle and Upper Jurassic pg.8-12
Images used with permission: © Copyright Buckinghamshire County Museum (Licence No. 44)