The rocks of Sherston parish were laid down in a shallow sea during the Jurassic Period, between 165 and 160 million years ago. We now know the Earth’s crust consists of a number of “tectonic plates” – relatively rigid structures which move slowly over the Earth’s surface. In the Jurassic Period, the plate bearing what is now the British Isles was about to start moving northward again after a long pause, and Sherston was about 30°N – roughly the latitude the Canary Islands are today. The seas were warm with strong currents – similar to the modern Bahamas. This environment gives rise to ooliths, which are tiny egg-shaped bodies formed by precipitation of lime (calcium carbonate) on a minute nucleus of sand. Deposition of these in the sea, and compression by sediments deposited on top, has given rise to the rock (oolitic limestone), which underlies Sherston.

Oolitic limestone is a good building stone, used extensively in Bath and the Oxford colleges as well as Cotswold villages like Sherston. The old houses in the village are largely built of, and on, a hard oolitic limestone called the Forest Marble. This rock underlies the whole parish and can be seen in small disused quarries and road cuttings (see image 1). It is not strictly speaking a marble – it is named after Charnwood Forest in Oxfordshire where it was once used for making ornamental fireplaces. The name was given in about 1799 by William Smith, who made the first national geological map and who started his researches in the Bath area. “Forest Marble” is thus one of the earliest geological names.

Forest Marble outcrop

1. Forest Marble outcrop, Tanners Hill, Sherston

The seas in which the Forest Marble was deposited were inhabited by a variety of creatures, largely shellfish. Common fossil remains in Sherston parish include belemnites, which are extinct relatives of modern squids and cuttlefish, and the broken remains of sea urchins and their relatives the crinoids. The most common fossil remains are however shell debris (see picture 2), the remains of long-ago beaches and sandbanks. There were larger creatures in the seas of the time, but they stayed further out to sea where there was more to eat – hence their remains are not found here. On the adjacent land masses the dinosaurs held sway, including Megalosaurus, the first dinosaur to be named (see picture 3) whose remains have been found in rocks of the same age near Oxford. The first birds (now the only surviving relatives of the dinosaurs) were beginning to take to the air. The vegetation would have looked odd to our eyes – there were no flowering plants as these had not yet appeared, but there were swampy forests of such plants as conifers, cycads and the maidenhair fern tree Ginkgo. A specimen of this “living fossil” species, apparently unchanged since the Jurassic, is planted outside Sherston Post Office – a living link to the time when its ancestral relatives grew on the islands round about the present site of Sherston.

Fossil shell debris

2. Fossil shell debris in oolitic limestone on a
Sherston dry stone wall – the remains of
a beach of 160 million years ago.

Dinosaurs explore modern Sherston

3. Two Megalosaurus dinosaurs explore modern Sherston.
The dinosaur images (from Wikipedia*) have been
superimposed at approximately the correct size.

After the Forest Marble had been deposited, sea level began to rise, and a succession of other rocks was deposited on top, culminating in a thick layer of chalk in the succeeding Cretaceous Period. All this has gone: eroded away in the succeeding 60 million years of the Tertiary Period when Britain once again rose above the sea. Another legacy of the Tertiary Period is the 3°NW-SE slope of the Parish, reflecting the slope of the underlying rocks, which were horizontal when laid down. This may have been caused by the uplift of the Alps far to the south, which happened at the same time, or it may be due to more local uplift. The tilt, however, preserves for us the rock types which were once above us and which can now be seen to the southeast (see diagram 4). Boreholes drilled for water supply confirm the rocks underneath the Parish – one at Carriers Farm on the Luckington Road reached almost to sea level.

Rock strata in North Wiltshire

4. Rock strata in North Wiltshire. The rocks were originally
laid down horizontally (top) and after tilting and erosion
gave rise to the present landscape (bottom). Diagram
not to scale and vertical scale greatly exaggerated.

Another striking feature of the area is the trench-like valleys which hold the River Avon and its tributaries (see picture 5). The modern river is not large enough to have cut them – almost certainly they largely date from the end of the last ice age, about 10,000 years ago. The last ice age (known in Britain as the Devensian Glaciation) lasted about 100,000 years, and a large icecap accumulated over most of Britain. Though the main icecap did not quite reach Sherston, the landscape would have been tundra-like, with accumulations of ice and snow on higher ground. We now know that at the end of the ice age the temperature rose remarkably quickly and this would have generated large volumes of meltwater, roaring down the existing valleys and excavating them close to their present depth. The excavated material was deposited on the plains beyond Malmesbury, or swept out to sea.

The Cliff, Sherston

5. The Cliff, Sherston - trench-like valley
cut by glacial meltwaters.

Another legacy of the ice age is the large amount of broken stone (brash) in the soils which appears whenever a garden is dug or a field is ploughed. This was mostly produced by weathering of the exposed surface rock by frost, freeze-thaw, and other similar processes. The soils of the area have benefited, however, by being south of the main icecap. Most areas of Britain are covered in a layer of sticky boulder clay derived from the moving ice: in Sherston parish the soils are free of this, and thus relatively well-drained and easy to work, though there are wetter patches with more clay. The predominant soil types in the Parish are a brown calcareous soil of the Sherbourne series, and more clayey brown earths of the Shippon and Tetbury series. The wettest soils are surface water gleys of the Denchworth series. These soil types give the landscape some of its character, and support the predominantly arable agriculture of the Parish.
Richard Skeffington
Professor of Geography at Reading University
* The dinosaurs are a copyright-free image from Wikipedia.

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