Coombs Quarry

Grid reference: Car Park SP731 332; Coombs Quarry SP 733 327.

Location: the Picnic Site Car Park for Thornborough Bridge, off the A421 Buckingham to Milton Keynes road.

Keywords: Blisworth Limestone, Blisworth Clay, Cornbrash, Jurassic, Bathonian, Medieval bridge, ridge and furrow system, Romano-British burial mounds.

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 the walk starts, the Romano-British burial mounds are worth a visit (in the field opposite the car park and A421. 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). 

   Thornborough Bridge: 

Figure 2 Cross-bedding in a block of Blisworth Limestone in the Medieval bridge near Thornborough. The inclined layers of ooliths and shell debris indicate which way the current moved a small dune in this tropical sea. In this instance the current was from right to left.

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 RIGS site for Bucks as it currently the best exposure of the Blisworth Limestone. The sequence shows 12 m of Blisworth Limestone overlain by 5 or 6 m of Blisworth Clay (often overgrown) and 2 m of Cornbrash, the latter only visible in one part of the quarry (see log and photos below).

  Figure 3 The quarry face at Coombs –Blisworth Limestone

Figure 4 Limestone beds with thin clay partings showing characteristic wavy bedding

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:

Bivalves:                                Gastropods:

Modiolus (mussel)                    Aphanoptyxis bladonensis

Liostrea (oyster)                      Aphanoptyxis langrunensis

Plagiostoma

Pleuromya                               Coral: Thamnasteria

Bakervillia                               Sea-urchin: Nucleolites

Rolierella                                 Brachiopod: Cererithyris

These are all marine forms and indicate a shallow, tropical sea. 

Secondly, there are the rocks themselves. The limestones vary between two distinct types. One is a greyer, shelly, fine-grained form that indicates a carbonate-rich muddy sea-floor with very low energy and no wave action. The other is a paler cream-coloured, shelly and oolitic limestone. This indicates a shallow, tropical sea floor which is agitated by waves. The clue here are the ooliths (‘oo’ = egg-shaped particles of carbonate; they are tiny spheres of 1 mm or less in size). These particles only form in warm, tropical waters where the carbonate content is high. They also require the gentle agitation of waves, rolling the particles to and fro, thus attaining an even coating of carbonate all round. They are very diagnostic of ancient environments - indicating a much warmer climate, very similar to the present-day Florida Keys !

The Blisworth Clay overlying the limestones is more difficult to see as it becomes overgrown with vegetation fairly rapidly. However, if an exposure is available this will show a pale-grey clay with minor silts and probably no fossils. If fossils are found these are always plant debris and rootlets. However, just one record of another type of fossil has been found - a dinosaur footprint (a theropod, see sketch below). This was found in a temporary exposure in nearby Thornborough Mill. The footprint was preserved in a slightly harder bed, and therefore is very likely to be the thin band of limestone within the clay unit. Both items of evidence indicate the close proximity of land. In fact, the Blisworth Clay represents an area of shallow water at the edge of the sea, perhaps a back lagoon. Clearly, the dinosaur was out for a paddle!

Figure 5 Sketch of the Thornborough dinosaur footprint (from a photograph appearing in the local paper)

The Cornbrash at the top of the sequence is a much harder bed of rubbly, fossiliferous limestone. The fossils are mostly broken, although a few whole specimens of bivalves and brachiopods can be found. This indicates a return to the sea, the broken fossils are evidence for wave activity.

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):

People at work maintaining the quarry:

2007 Work

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 BEHG 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 progress:

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 job!

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 access.

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 0 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.