© 2020-2022 Paul Mobbs; released under the Creative Commons license.
Created: 30th January 2020;
updated: 9th July 2022
Length: ~2,000 words
The chain of hills that emerge from the flat plain near Rugby, and slowly transform into the Cotswolds beyond the Evenlode valley in Northwest Oxfordshire, have a unique geology. That gives rise to a particular ecology. But of far greater significance is that as a barrier between the Midlands and the South East of England, these hills are a relatively empty and under-developed stretch of land. And in modern Britain, any ‘under-developed’ area has an innately greater natural value.
For those who still eat locally grown food these hills are, quite literally, in our blood!
The orange-brown soil comes from a hard, iron-rich sandy limestone; the Marlstone Rock Bed (shown in brown on the map) and the Dyrham Formation (shown in dark orange). This sits in a thick slab across these hills; thickest around Banbury, and then tapering away either side towards Chipping Norton and Northampton. Before Britain imported iron ore, it was the source of a small proportion of Britain's iron for half a century.
Stand at one end of the slab near Sibford Heath, and on a clear day you can see to the other end near Daventry, 20 miles away. Look directly east from this same point, and the next highest hilltop is 460 miles away in the Teutoburg Forest in Germany.
There are ancient north to south routes of travel in this area that provide excellent ways to explore this landscape. In part that’s because pretty much every modern route passes north-west to south east, following the ‘passes’ along the river valleys that cut the hills. This leaves large areas of relatively undeveloped countryside in between.
During the Medieval period, ‘The Portway’ ran over the drier high ground from the Thames valley into Leicestershire. And in ancient pre-history ‘The Jurassic Way’ ran down this escarpment of hills from Lincolnshire to the great stone circles of Avebury and Stonehenge.
Few tourists venture deep into these hills, where in places – except for the overhead drone of aircraft – it’s still possible to escape the noise of modern life (and even mobile phone coverage!). The more spectacular attractions of the nearby walking honey-pots of The Cotswolds, The Chilterns, and The Peak District, ensure that we get few visitors here.
These sandy hills and their quagmire clay valleys have always been an obstruction; which is why this land has always been on the fringe of civil administration rather than a vital part of it. They were an obstacle to easy movement until the creation of railways and the M40; which is in part why the boundaries of the local Iron Age tribes still persist in the modern county boundaries today.
In short, ‘The Irondowns’ are at the centre of England, on the edge of everywhere, and yet they are nowhere – and that has been the case for almost three thousand years!
As I used to tell my kids when walking past the old ironstone buildings in the centre of town, pointing out the myriad of large fossils they contain, “this is ‘Jurassic Park’”. The rocks locally were created in the Jurassic Era; but more specifically, a short section at the very beginning of the Jurassic, called the ‘Lias’ – roughly 180 to 200 million years ago.
Lias group rocks form a band of clays, limestones, and sandstones across England (see map below); from the famous ‘fossil coastline’ of Dorset, to the dramatically eroding coastline of North Yorkshire. Quite apart from the fossils, and their historic importance as a source of iron, they are also the basis for Britain’s oil and gas resources – concentrated from the organic ooze from all the life that existed in the shallow seas when these rocks were laid down, slowly cooked by heat and time to make oil and gas.
From 280 to 300 million years ago a huge continental landmass (which eventually became Africa and South America) collided with the another continent (that today is North America, Europe, and North Asia). Called the Variscan orogeny, it created the hills and mountains across much of the middle of Europe. With that came volcanoes, which added mineral-rich basalts to the surface and deep granite intrusions, across nearby areas such as the Clee Hills, Charnwood Forest, and the Derbyshire Peak District.
Over millions of years the basalts and deeper granites were eroded. About 200 million years ago, Banburyshire lay on the shoreline of a land mass which stretched from South Wales up to the Peak District, made of the rocks pushed up by the Variscan orogeny. To the south, Southern England was a shallow sea, where limestone was formed from the calcium minerals flushed out from all that eroded rock.
Being on that ancient shoreline, our local rocks are a mixture of both – a sand-rich limestone; and preserved within it can be found the wealth of sea life that existed on that shoreline environment.
Volcanic basalt and granite rocks also bring with them a lot of metal minerals. Over millions of years, as those metal minerals weathered on the surface, they formed minerals such as chamosite and siderite which flowed down and settled along the shallow shoreline. Over time those minerals were trapped in the layers of sandy limestone.
That was not the end of this process. As the chemistry of the layers changed, as they were buried and squeezed over millions of years, the chamosite degraded creating minerals like limonite, and the calcium compounds were slowly dissolved and flushed away, concentrating the iron minerals in the rock even more. This created what is called the ‘Marlstone Rock Bed’ – a thin layer of sandy limestone that can be as much as 20% to 30% iron-based minerals.
This is the reason why, from the Iron Age, the rocks of this area have been excavated for the production of iron. It was expanded during the Roman era, with local production sites at Swalcliffe Lea and possibly Kings Sutton; and carried on in local villages to satisfy their needs until at least the Nineteenth Century. Then from the early Twentieth Century until the late 1960s millions of tonnes of rock were excavated from around Banbury for iron and steel production at Corby.
Below these layers is the clay-rich Charmouth Mudstone Formation. These are the rocks that provide all the fossils along the Dorset coast – though locally they are not so prolific. Above the Marlstone are the sticky clays of the Whitby Mudstone Formation – which also has many fossils. Both of these layers are also high in iron minerals because they are next to the Marlstone.
Older buildings in Banbury, made with clay from either of these layers, have a very reddish brick because of the high levels of iron. When fired with often sulphourous coal to make bricks these minerals react to create iron pyrite. As a result Banbury bricks are very soft, will spall if they get wet, and tend not to weather well – which is why a large part of Banbury’s poorly built Victorian buildings have been demolished.
Of course, it’s not just iron minerals that these rocks contain. There are also variable levels of magnesium, titanium, manganese, vanadium, cobalt and nickel. In places there are significant levels of arsenic (where it has been dissolved and moved from the rock by groundwater, then concentrated where that groundwater has evaporated). And across the whole area, there are higher than usual levels of uranium and thorium, which creates a problem with radon levels in buildings made of ironstone, or more recent buildings which are erected directly above the ironstone bed.
Finally, the sandy-limestone rocks, and how they sit in relation to the clays above and below, creates a very characteristic hydrogeology. The constituents of the soils these rocks produce in turn creates a characteristic soil ecology. And the combination of the water-cycle and soils gives rise to a characteristic natural landscape ecology (classified nationally by Natural England as ‘The Cotswolds’ and ‘The Northamptonshire Uplands’ National Character Areas – which overlap immediately around the Banbury area).
Many years ago I was walking from Thirsk to York along the foot of The Hambleton Hills. Suddenly I realised, as I looked around, ‘I was home’: The landscape looked so similar! Later I discovered that the geology of this area is an extension of the geology around the Banbury area.
This is why ‘The Irondowns’ represents a singular landscape: A convergence of various geological accidents over ‘deep time’, interacting with the ‘deep history’ of human settlement for 12,000 years, creating the landscape we experience today. By understanding that deeper narrative, for how these hills arose and how we have lived upon them, the signposts of that story are far more easily seen within the seeming mayhem of the modern world.
So often today, people complain that ‘all towns are alike’; with the same shops, an identical urban landscapes, all projecting the same lifestyle. If you want to experience something unique, all you need do is leave the town and experience the local countryside around, to find a new way of looking at our lives today.
Rocks are only one part of this story. Rocks, and the soils they create, are in turn shaped by water and the confluence of water into streams and rivers. Just as with the local rocks, to understand the topography and ecology of the area it’s important to understand its watercourses.
Carry on along the escarpment a little further north from Banbury, beyond where the three counties meet, to the flat boggy fields above the village of Hellidon. Here you stand at the southern watershed of England – at the source of:
Alternately follow the ridgeline from Charlton, through Newbottle, and Farthinghoe to Greatworth – which divides the catchment of the River Great Ouse from the River Cherwell. Or go west for a short walk from the Sibfords along the ridge through Swalcliffe Grange to Tadmarton Heath – which divides the River Cherwell from the River Stour.
This many sound all a bit dry and abstracted, but once you understand rocks, soils, and the water that flows around them, you understand the roots of ecology: Different soil types encourage different types of plants, and whether soils are wet or well drained creates completely different plant communities; if you are interested in foraging, moisture and soils have a big impact not only on what plants you find, but also how well they grow or fruit; and, if you are interested in spotting wildlife, then knowing the type of plants or ground cover they favour will allow you to find them more easily.