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.
Comments
- SueB -
29 Feb 2008
Add A CommentVery interesting and readable. Geology always seems complicated so it is good to have it explained in layman's terms.
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