Then & Now
65 million years of geology
2 Ice Ages
50 years of gravel extraction
Geology of the Cotswold Water Park
The underlying rocks mainly comprise sand, limestone and clay known as the Cornbrash, Kellaways Beds and Oxford Clay which were deposited during the Jurassic period between 175 and 155 million years ago, when this area was part of a warm shallow tropical sea. The gravel deposits in this area were laid down during the last Ice Ages, 350,000 and 45,000 years ago. Rock debris probably accumulated on the Cotswold hills to the North in winter by freeze-thaw action on the local bedrock and formed liquid mud slides on the frozen subsoil in spring and early summer.
During the short warm summers the tributary streams, swollen by snow and ice melt, became huge muddy torrents capable of transporting rock debris a long way down into the main valley, creating wide braided rivers and streams in the process. These were deposited as gravel spreads on the Thames Valley floor.
The gravel lies just under the surface, between 0.5m - 6m deep, within which can be found the Ice Age remains of woolly mammoth, rhino, deer and bison. These layers can reveal mammoth remains, such as teeth, parts of tusk and bones, and in 2005 a complete woolly mammoth skull was uncovered, and is now on show at the Gateway Centre, thanks to the generosity of Moreton C Cullimore Ltd, on whose land the skull was discovered.
Below the gravels, in the Oxford Clays, remains of marine reptiles and various sea creatures such as ammonites, belemnites, gastropods can be found, although they are often trapped in nodules of clay which require some hefty hammering. The CWP Trust runs organised Fossil Hunts for its members and the public several times a year.
Before mineral companies can start to extract the sand and gravel, archaeologists are called in to examine sites for any evidence of human activity. This can lead to some exciting finds, such as the Roman cemetery at Horcott, a large Romano British farmstead at Cleveland Farm, and evidence of Iron Age and Saxon occupation at Claydon Pike. With so much excavation taking place, this has resulted in the Water Park becoming, archaeologically, the most researched part of Britain. Remains from over 6,000 years of human habitation have been discovered - from the early Stone Age, through the Bronze & Iron Ages, the Roman & Medieval eras, to the present day.
Although the Romans made good use of the local supply of sand, gravel and clay, gravel extraction only really began on a large scale about 50 years ago. Because the water table is so high, the first quarries were dug 'wet'. Today, however, quarries are 'de-watered', during extraction of the sand and gravel. When extraction is complete, all pumps are switched off and the holes fill naturally with water. So far, more than 150 lakes have been created this way: almost 1,000 hectares of open water.
The gravel deposits range in depth from a few centimetres to 6 metres, and begin about 1 metre below the surface. At present, 4 mineral companies are extracting approximately 1.5 million tonnes per year from 360 hectares, with a further 370 hectares having permission for extraction. The emerging mineral plans propose allocating another 550 hectares for extraction. Beyond this, there's another 50 years supply of sand & gravel.
Once the gravel has been extracted, the sides and bottom of the pit are shaped and lined with the Oxford clay, providing ‘bumpy bottoms and crinkly edges’ which make great habitats for wildlife such as aquatic plants, fish, invertebrates and water birds. Planting of the lake edges with reeds often takes place in order to soften the profile and create more excellent places where wildlife can thrive.