A section through Harwell
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!
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