The Geology of the Stones

Throughout the Jurassic period, gradual changes in the sea caused a series of contrasting sedimentary rocks to be formed. During the earliest part of the Jurassic, between 200 and 175 Ma, deposition of vast quantities of largely fine grained particles gave rise to a thick sequence of clays and limestones known as the Lias. The Lias Group is dominantly comprised of soft rocks and, apart from a band of iron-rich limestones, which have been quarried extensively around northern Oxfordshire to source the 'Hornton Stone', they have little use for building.

At around 175 Ma, the sea that covered much of England partly retreated so as to leave parts of northern and central England under terrestrial or marginal conditions such as deltas, lagoons or estuaries. In south-west England however, shallow water conditions developed and the sea became saturated with huge amounts of the mineral calcium carbonate.

Generalised stratigraphy of the Jurassic in Oxfordshire showing some of the important building stones

Generalised outcrop map for Jurassic rocks in the U.K

These rocks, of Middle Jurassic age, have been divided into twp groups: the oldest is the Inferior Oolite which is overlain by the much more substantial Great Oolite. It is from these limestones that we extract most of the best quality Jurassic building stones. Of the stones used at Filkins, Guiting Stone and Stretton (a type of Clipsham Stone) are taken from the Inferior Oolite Group whilst Stoke Ground is one of the best Bath Stones from the younger Great Oolite Group.

The limestones of the Inferior and Great Oolite Groups are such good building stones because of the particular manner in which they were formed. Within the shallow sea that occupied much of south-western England during the Middle Jurassic, the sea floor was covered in a mud composed of microscopic grains of calcium carbonate. Tiny fragments of shells and other organic remains fell to the sea floor and were gently rolled around by currents and waves. The fragments became coated in successive layers of the mud to form small round balls, around a quarter of a millimetre in diameter and known as ooids.

Within the sea, subtle variations in the type of material that was being deposited, or the size of the ooids formed means that the quality and appearance of the stone varies from layer to layer and from place to place. A certain type of stone which is found at a specific place often takes its name from the location at which the stone is best seen or quarried. Most of the best quality stone in Oxfordshire such as the Taynton Limestone (part of the Great Oolite Group) has already been worked out or is no longer economical to quarry and as such, stone used in Oxfordshire is now imported from quarries in Gloucestershire, Somerset and Dorset.

Whilst the Middle Jurassic Great and Inferior Oolite Groups provide most of England's best freestones, other good building stones are also derived from the younger parts of the Jurassic. Conditions changed after the great sequence of material that formed the Inferior and Great Oolite was deposited. The Late Jurassic saw a complex system of coral reefs beginning to be established in the subtropical sea that were present in the Oxfordshire area. Huge masses of corals and other organisms became compacted to form a very hard limestone known as the 'Coral Rag' which was packed full of fossils and popular with medieval architects.

Palaeogeographic map of England and Wales during the Middle Jurassic