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.
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