Geology of the Bradford Hundred
Bradford on Avon, Wiltshire
The geology of Bradford is relatively simple. Sedimentary rocks dip gently from west to east with only a few faults to upset them. They were all formed in warm shallow sea during the late part of the early Jurassic and the middle of the Jurassic Period, 165 to 150 million years ago, as the area that is now Britain slowly moved northwards through the tropics towards its present position.
> Larger map from British Geological Survey, from the Geology of Britain Viewer, centred on the Museum
The oldest rocks exposed in the Bradford Hundred area, the Upper Lias clay and Midford Sand, are found in the valley bottom in the west. The clay was laid down in a marine environment and contains fossils of ammonites. The sand, overlying the clay, seems to have been part of a huge bar, rather like a long submarine sand-dune.
They are succeeded by the Inferior Oolite, Fuller’s Earth and Great Oolite limestones and clays which form the Cotswold Hills, of which some beds have been exploited for building materials, especially those that go under the name of Bath Stone.
The top of the hills, which slopes towards the east, is made up of an extremely variable series of thin sandstone, limestone and clay beds which are known as Forest Marble. The variability reflects the changing environments in which the sediments were laid down- environments that were rather like the Bahamas and Florida are today, but populated by dinosaurs.
A thin and widespread limestone called the Cornbrash comes next, followed by the Kellaways Beds (named after a place near Chippenham), a fossil-rich sandy limestone and then the lowest beds of the Oxford Clay. Later strata are outside the Hundred.
In the valley bottoms are much more recent deposits- gravel and alluvium that date from the Ice Age and later.