Stonesfield Slate

Stonesfield slate is a thin, fissile limestone from around the village of Stonesfield in Oxfordshire, England, once heavily worked for roofing slates. It is one of the two main sources of traditional Cotswold stone slates and is also of great importance to palaeontology.

Geology

The slaty limestone horizons date from the Middle Jurassic, mid-Bathonian stage. Earlier work placed them in the Sharp's Hill Formation, but borehole and shaft evidence indicates that at least three horizons within the Taynton Limestone Formation were quarried. The surrounding rock is mostly ooidal grainstone, a limestone made of small rounded grains.

Production

The stone was mined in the 18th and 19th centuries. Like Collyweston slate, Stonesfield slate was split by frost: blocks were wetted, covered with earth and left until frosts froze the moisture in the stone and opened its bedding planes. Frost-split slates are thinner, lighter and more regular than hand-split slates. Cotswold guidance notes that Stonesfield slates were used on prestigious roofs such as those of Oxford colleges.

Fossils

The slate beds have yielded plants, insects, fish and vertebrates, including some of the earliest known mammals, pterosaurs and Megalosaurus, the first dinosaur ever scientifically described. The fossil plants, about 25 morphospecies, suggest a coastal, possibly seasonally dry environment with mangrove-like vegetation.

Today

Conservation guidance discourages stripping slates from one building to supply another; reclaimed slates should stay on the building or group they came from. Matching new limestone slates from other Cotswold sources, or from geologically similar sources, are used where the original cannot be found.