Tower and Town, June 2017

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Marlborough's Marine Geology

Despite being on land and near the centre of southern England now, the chalk downlands surrounding Marlborough with their fossil shells, ammonites, and sea urchins are a constant reminder that this was not always so.

Looking back 150 million years or so (see youtube.com/watch?v=5yVfJGNjok0) we see the early South Atlantic ocean opening with the northern part marked by shallow seas as the old supercontinent began to split. The oldest record of the beginnings of the North Atlantic are the Jurassic rocks visible along the Dorset coast and the Bristol Channel and extend up to the Cornbrash limestones west of Devizes. Eventually, deposition of sediments choked the shelf on which they were laid down and low-lying land emerged by the beginning of the Cretaceous period. This was a good time for dinosaurs and their footprints and bones are found in several deltaic deposits formed then.

By 100 million years ago the North Atlantic began unzipping northwards driven by plate tectonic processes and brought oceanic conditions and renewed subsidence to Britain. After periods of deltaic deposition: the Gault Clay and the Greensands, the shelf was submerged over a wide area and white “oozes” typical of fairly deep water deposition became widespread. This Chalk limestone formation extends from Ireland to Poland and is characterised by the abundance of tiny plant skeletal plates with a long name: coccolithophoridae. The extraction of carbon dioxide to form these plates by such creatures and its entrapment in chalky deposits was responsible for the some of the atmospheric cooling that ensued towards the end of the Cretaceous period and through the succeeding Tertiary.

There was a major “attempt” by rifting processes to break apart western Europe down the axis of the North Sea which must have been the locus of major earthquake activity at the time. Huge faults along the central rift valley and substantial uplift elsewhere brought chalk deposition to an end. Both were probably caused by the rising, heating and lateral movement of a hot plume of rock from the Earth’s upper mantle to the base of the Earth’s crust.

However, this rifting episode failed and activity moved westwards to western Scotland and Ireland where major volcanoes formed as a record of the successful opening of the North Atlantic west of Britain. This opening continues to the present day at a centimetre or so per year. Valleys developed on the surface of the chalk as it was variously uplifted, folded, faulted, and abandoned by the retreating sea as huge volumes of water became trapped as the Antarctic continent moved over the south polar region. Rivers sourced in the Welsh highlands deposited sands in some of these valleys. Some smaller valleys can still be seen near Lockeridge and on Fyfield Down and where the sarsen stones are silicified relics of these deposits. These streams may have drained into the London Basin which formed about 40 million years ago and whose westernmost edge is marked by the “Reading Beds” – sandstones and mudstones – reported to occur beneath the Monument in Savernake Forest.

Richard Clarke