The Geology of Harwell Parish

by Martin Ricketts, February 2007

• The solid stuff
• A section through Harwell
• Sarsens
• Surface Drift Deposits
• The Bargain Stone

The solid stuff

You probably know a bit about the local geology – that the Ridgeway is chalk, that in northwest Oxfordshire there is limestone, and Harwell is on greensand, but how did it all come about?

If you were to drill down into the ground you would eventually come to red sandstone. This dates from when Oxfordshire was part of a desert in Pangea, the super-continent, about 500 million years ago (My), and 60º south of the equator. England moved most of the way north to where we are now over the following 100 million years; it adds up to a large distance but was in fact only about 4cm per year.

Nothing of the following periods remains visible in Oxfordshire until one gets to the Jurassic period, starting about 200 My ago. The limestone from the Jurassic is visible in north Oxfordshire, but in Harwell and the rest of the south of the county more recent layers lie on top. This is because later earth movements tilted the layers so they slope down from northwest to southeast at about 1° from the horizontal, and then erosion in more recent times sliced off the top.

The period when what is visible around Harwell was being formed is known as the Cretaceous, which lasted from 145 My to 65 My ago. The sands and clays were formed in seas of different depths, sands close to the river mouths, and clays further away, reached by only the finer particles.

Somewhere about 110 to 100 My ago southern England was under water apart from Cornwall. For a long time fine particles of Gault Clay settled in a band from Wiltshire to Bedfordshire, including this area. This is the earliest level to be found in Harwell, in the north of the Parish. Mostly it is covered by a thin layer of recent drift deposits.

Later, while Gault continued to be deposited east of a line from Dunstable to Eastbourne, the mass of high ground over Cornwall supplied sand to the west of the basin and this became the Upper Greensand on which the village sits. It is the greenish mineral glauconite that gives it its name. This layer is up to 60m thick in places but about 20m in thick Harwell.

A continued sea-level rise in the later Cretaceous, 100 My and 65 My ago, meant little sand and clay sediment, but deposition of the skeletons of mainly algae but including other sea creatures. Only the lower chalk is found in Harwell Parish and this has a significant clay content, hence its grey colour. It does not contain flint. In contrast the middle and upper layers of chalk are much whiter, and the upper layer, forming the Ridgeway, contains flint.

At the end of the Cretaceous period, pressures from surrounding areas, caused by Africa colliding with Europe, caused gentle folding over much of southern England. The level dipped to the southeast in Oxfordshire and into what is known as the London basin. Later erosion sliced through the layers, so the chalk forming the Ridgeway extends southwest on the surface into Hampshire and Dorset, but to the southeast dips under London and comes up as the North Downs. And in Harwell, as one moves southwards, each layer disappears under the next one.

There is a bulge to the north in the eroded layers of chalk in the Hendred to Harwell area altering the general contours. So the steepest slope in Harwell is NE-SW rather than the more general NW-SE.

 

A section through Harwell

The map shows the different layers that are at the surface, ignoring drift for the moment. Let us follow a line roughly from north to south though the Parish on the line of the High Street; from A to B the map, with North at the right-hand side. Starting at the boundary with Sutton Courtenay the land is about 50 m above sea level; the underlying material is Gault clay, but there is a shallow layer of drifted material on top.

Walking south, we come to a rise just before we reach the A34, where the Upper Greensand starts. The High Street is fairly flat, at a height of 80m, and then the land rises again before the A417 is reached. This is due to a thin layer of sand and grit and then the chalk. By the Reading Road we are onto the Lower Chalk, which extends to the Southern edge of the Parish.

The Cretaceous period, which saw the deposition of Gault Clay, Greensand, and Chalk, came to an end about 65 Million years ago. The Hampshire and London basins continued as sea for some time, while the rest of southern England became land.

The period from 65 to about 2 Million years ago is known as the Tertiary period, although the Primary and Secondary Periods are terms no longer used. As mentioned before, the gentle folding in southern England during the Tertiary period is possibly related to the formation of the Alps, but it could also be the result of the Atlantic sea-floor spreading and associated changes. The amount of material removed from Britain since it became land is quite staggering – up to 2 km depth in the north of England, and although this reduces to a hundred metres or less here in the south, the total volume is thought to be possibly 200,000 cubic kilometres!

Sarsens

At some time in this period, after maybe 5 million of years of erosion, there was some further deposition, producing the Reading beds, of sand and gravels. On some of the Chalk Downs there was a thin layer of sand and gravel that later became hardened by silica. This is thought to be due to evaporation of groundwater containing dissolved silica – the mean annual temperatures were higher than 20C for long periods. This hard layer was broken up during the tundra conditions that existed during parts of the last 2 million years, and all that were left were scattered blocks called Sarsens, for example near Ashdown House, West Oxfordshire.

Here and there in the parish you can see light brown stones which are very hard, for example on the corner by the White Hart, by the barn in Townsend, and outside Princes Manor Farm. These are sarsens, and have survived although all the chalk they formed on has long since disappeared. As evidence they have not been brought here, there are still examples to be seen in the hedges and fields. The photo shows the group outside Bishop’s Manor Farm Barn, Townsend.

 

Surface Drift Deposits

On the 1802 Parish Award map a gravel pit is shown near where the Harwell Site main gate is today. What were they digging? And generally one thinks of going up when one goes south towards the Ridgeway – but from the top of the Winnaway through Chilton and Compton to the river Pang is downhill all the way! How did this come about?

The map below is basically the same as before but two areas have been shaded. In the north of the Parish extending up the valleys there are what is known as ‘Head’ deposits, forming a layer of up to a few metres on top of Greensand and the Gault clay. And overlapping the Parish boundary in the south are ‘Coombe’ deposits.

For the remaining 50 million years or so of the Tertiary period, Oxfordshire had a warm climate, subject to slow weathering. The significant feature of the last 2 million years (known as the Quaternary Period) was the colder climate, with several periods when ice-sheets spread south into England. There was only one when the ice came close to Oxfordshire, called the Anglian glaciation, which happened about ½ million years ago. During these periods, the annual freezing and thawing cycles would have shattered surface rocks, and large flows of water would have removed much material from its source and deposited it elsewhere as sand and gravel where the flow rate was lower. The oldest known Thames deposit is at Nettlebed, 170 metres above the present river, so you can see a lot of erosion has taken place in the last million years or so!

In the local context, the shattered surface material would have been carried down-slope by the spring melt-water into the valleys and the lower-lying areas. This is the so-called ‘Head’ material in the north of the Parish, and consists mainly of clay, sand and flint. The Greensand strip below the Downs extends almost continuously west to Ashbury, but getting narrower all the time. Likewise, there is a strip of ‘Head’ material. It is mostly only 100 to 200 metres wide, but Harwell is unusual in having more than a kilometre north to south, as it covers the shallow valley between Milton Heights and Didcot.

Where the drift material is in dry valleys it consists of Chalk fragments with Flint, and is known as ‘Coombe Deposit’. This is what is in the south of Harwell Parish, and explains why there is a gravel pit on the Enclosure Map near the Harwell site main gate. The area is at the head of the dry valley that runs down to Compton and thence becomes the Pang.

We now have this miles-wide strip south of the Parish – the Ridgeway – where there is no surface water due to the chalk being permeable. But during those colder periods the ground would have been frozen, tundra-like and not permeable. There would have been flows over the chalk, and hence erosion and the formation of the valleys that we see today.

It is thought that long before all the Chalk over Harwell eroded, in the early Quaternary Period, there must have been a significant flow of water through Harwell and Chilton to Compton to create the Pang valley by erosion. This water may have come from melting ice to the north and west of Oxfordshire, before the Goring gap formed, or the Thames even existed.

 

The Bargain Stone and others like it

The Bargain Stone as it is known is in the High Street; the Harwell Book just says “the stone outside Winterbrook House, known as the bargain stone, is said to have been used at these horse-fairs for paying the money; some people believe that it may have been an earlier altar-stone.” Left is the view from the footpath side:

There are a number of similar stones in Harwell, for example on the corner opposite The White Hart, and in the footings of the Cherry Barn, and of the barn at the corner of High Street and Grove Road, shown in the photo to the right.

They are all about 60cm high and some have clearly been worked. For example, viewing the Bargain Stone from the road, below, it has a large indentation on the underside; maybe it was thought not good enough for barn footings. Other bargain stones are known with holes in where the bargain was struck by shaking hands through the hole. Perhaps with this one they shook hands underneath?

Regarding their origin, these stones are Coral Rag. Examples of the fossil coral can be seen in the Bargain Stone, and more clearly in a stone in the wall of the Cherry Barn, beside the entrance steps, with remains of the coral as well as the imprints, as shown in the photo:

So where has the stone come from? It probably came from Abingdon Abbey. A lot of Coral Rag can still be seen in what remains. The Abbot and monks signed a deed of surrender to the Crown on February 9th 1538, after being assured of decent pensions. It’s been estimated that the Abbey was the sixth wealthiest in England, and John Wellsbourne, the King’s man in charge of winding up Abbey affairs, found the Abbey had been stripped of all its furnishings . The community had made careful preparations for the surrender! Within weeks the Church was dismantled and all the good quality building stone barged down the Thames. So it is likely that the pieces in Harwell were among the left-overs and came soon after this date.

To find out more, get the book The Geology of Oxfordshire by Philip Powell; the library has copies. If you have comments, corrections (I’ve only done an evening class in the subject) or more information please contact me, Martin Ricketts.