Banbury Stone, eastern Worcestershire – eighth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Kay Hughes writes: a walk up Bredon hill is a treat in itself. At this time of year, the wildflowers and butterflies are just magical and there is a fine view from the tower at the top, but a very special surprise is in store at the summit. A petrified elephant inhabits a hollow at the foot of the tower.
Approach the beast with care as the ground is uneven and steep in places. Once you have patted its trunk, shared a selfie with it and enjoyed the view, take a closer look at the rock itself. Find a place where the surface is freshly exposed. You will see that it is composed of many rock fragments cemented together just like a lump of coarse concrete.
The rock fragments (see inset) contain many tiny spheres called ‘ooids’, just like those found on the surrounding footpaths and in the stone-built houses of local villages. They are pieces of the local Oolitic Limestone of Jurassic age, (around 170 million years old) which underlies all of the gentle, southerly slopes of Bredon Hill.
The cement holding these fragments together is pure calcite: the same tough, resilient stuff that stalactites and stalagmites of limestone caves are made of. Geologists call this rock a limestone breccia, and it was made naturally from local materials – but how? And why?
How was the rock formed?
The breccia formed in clefts in the limestone, called ‘gulls’. Broken fragments of rock from the sides of the gull fell to the bottom, filling it up with debris. Over thousands of years, water percolating through the surrounding limestone became laden with dissolved calcite, and collected around the rock fragments. When the water evaporated the hard calcite cement was formed. The resulting rock was harder than the surrounding Oolitic limestone, which eroded away more readily. This left monoliths like the Banbury stone standing upright in the landscape. The King and Queen stones found further south on the edge of Bredon Hill are further examples of such rocks (see photo below).
But why did gulls form?
Gulls formed in the old Oolitic limestone in recent geological times (within the last few million years) as the steep-sided north western slopes of Bredon Hill were eroding away. Clefts in the limestone formed near the edge of the escarpment. Soft mudrocks which lie beneath the limestone eroded away, leaving the limestone unsupported, so it fractured under its own weight. A large slab of limestone tipped away from the hill, and the crack opened up forming a gull in which the hard breccia rock was formed.
As erosion progressed, parts of the hillside started to fall away. Initially, discrete, elongated slabs of limestone, rotated towards the slope, creating a stepped, ‘cambered’ landscape. As the pieces fractured further, landslides became more haphazard, creating the hummocky ground seen on the steep slopes below the summit.
What does cambering look like?
A cambered landform can be seen in the north east part of the hill. A small fragment of the 1:25000 Ordnance Survey map below shows where to find this field, whose extent is outlined in red. From the Banbury stone, you proceed to the north and west along the path at the edge of the steep scarp slope, it will take you through a gate into a small wood. Emerging from the wood you find yourself entering a large field that drops away from you in discrete steps, showing where the hard limestone has fractured and a slab of rock has tipped downhill. This is also a good place to view the landslides on the steepest, northerly slope of the hill.
The processes that create gulls and cambering are particularly associated with periglacial conditions, where repeated freezing and thawing accelerates erosion, so most of the effects we see today on Bredon Hill were created during the ice age, but historical records and recent monitoring of the hillside shows that the pattern of erosion continues to this day.
Two issues remain unanswered about the Banbury rock, and we look to history to help answer these questions:
- It is not a monolith, but two separate pieces of rock.
- Why does it stand in a hollow?
The first issue is simple to resolve: the original rock broke into several pieces creating the elephant shape we see now: Rev. Lloyd reports in 1967 that around 100 years previously, “the stone rolled from a higher point owing to a landslide … and broke into the stones that are there now”.
The issue of the hollow is more intriguing. So much of our landscape has been altered by man, and it is often hard to know what our ancestors have done in past centuries and millennia. This hilltop in particular is an iron age fort, subsequently inhabited by Romans, Saxons and Danes and its man-made earthworks are very much in evidence. In fact the name of the stone is derived from the name of the fort, reported successively as Baenintesburg (779 AD), Bænincgesbyrig (972 AD), 1282, Bembsbury (1727 AD), Benbury (1779 AD) and finally Banbury on modern Ordnance Survey maps.
The hollow in which the elephant now stand looks very much like an old quarry, but in fact the historical evidence points to a much more interesting natural cause, which is entirely consistent with what we know of the geology.
What history tells us
Firstly, in 1727, William Derham F.R.S. attests in his “Physico-Theology” to the existence of a cave, with stalactites that he had explored in his youth, “on the very Top of Bredon-Hill in Worcestershire near the Precipice, facing Pershore, in or near the old Fortress, called Bembsbury Camp.”
In 1781, a plan drawn by Dr Nash shows the “Bembury stone” standing near the edge of a precipice with no suggestion of a hollow or breach in the steep slope at that point.
Only 20 years later, around 1800, a major landslip occurred at the top of Bredon Hill, creating a large hollow with the Banbury Stone standing in the middle, and there are now no caves to be found in that area. Intriguingly, the outwash from the landslip was found to contain large quantities of ‘bleached’ grain, suggesting the collapse of a cave that had been used many centuries ago by the occupants of the fort for storing grain.
Caves are often found in limestone where cambering has occurred and gulls have formed. Examples can be found around Cleeve Hill and in other places along the steep edge of the Cotswold Hills, which share the same geology as Bredon HIll. Hence the collapse of a cave on Bredon Hill to create a sinkhole is highly plausible. The resilient limestone breccia of the ancient monolith survived this first dramatic earth movement, but further slippage later caused its collapse, leaving the fine elephant we see today.
Exploring the geology of Bredon Hill
The Trust publishes a trail guide in the Explore series entitled Bredon Hill (West) which documents the geology with a circular route of 8 km, starting at Kemerton. It can be purchased form the on-line shop. A similar route is followed by the trail on the Deep Time Voyager app, which can be downloaded from your Apple or Google store. Before you go, within the app, choose to download Bredon Hill. You may also like to see the text and images from the app enlarged on your PC screen – you can do this at the website deeptime.voyage.