A very basic geological map of England showing the limestone belt highlighted in the darker yellow running from Dorset in the south to Yorkshire in the north.
Bath Stone is an Oolitic Limestone that was lain down as sediment in a warm shallow sea 170,000,000 years ago during the Jurassic Period. Oolitic Limestone consists of minute spherical particles called Ooids, which usually consist of small fragments of shell coated with layers of calcite. Over time these have become cemented together to form the stone we find today.
The majority of limestone quarried in England comes from a belt of stone that runs from Portland and Purbeck on the south coast, -northwards along the Cotswolds and into Yorkshire. This belt of limestone has been extensively quarried for centuries along its length and many of these workings have supplied the stones used in most of our famous buildings. Although the stone comes from the same belt all have different compositions and can vary greatly in colour and quality. Bath stone is a member of the Great Oolite Series and comes from a small area at the southern end of the Cotswolds, to the south and east of Bath. The layer of stone which is on average twenty five feet thick is found at depths of between 30 to 120 feet below the tops of the hills lining the By Brook and the River Avon. Bath Stone is divided into beds varying greatly in thickness, also the quality of the stone can vary from bed to bed in the same location. Another feature affecting the size of the quarried blocks are Faults or Joints, which run vertically through the rock and are the result of earth movement during continental drift and subsidence of the slopes on the escarpments. Finally Bath Stone is classed as a Freestone, - a stone that can be cut or squared in any direction, but to get the best use out of it, it must be laid on bed, i.e. it must sit in the building as it did in the ground.
FOR FULL DETAILS ON THE GEOLOGY OF THIS AREA REFER TO: - BRITISH REGIONAL GEOLOGY, No. 15, THE HAMPSHIRE BASIN AND No. 16, BRISTOL AND GLOUCESTER DISTRICT.
THE GREAT OOLITE GROUP SUBDIVIDES INTO THE FOLLOWING.
NOTED AT FARLEIGH DOWN.
6.5m...FOREST MARBLE.-----HARD BROWN SHELLY LIMESTONE.
7m...UPPER RAGS.----BROWN SHELLY LIMESTONE...3m, WHITE OOLITE...2.5m, ROOF BED...1.5m.
4m...TWINHOE BEDS.----MARLY TO HARD LIMESTONE.
9m...COMBE DOWN OOLITE.----SHELLY OOLITE.
PHYSICAL PROPERTIES OF MONKS PARK STONE.
DENSITY...140 lbs. per cu. ft.
FAILING STRESS.... 3520 lbs. per sq. in.
ALUMINA Al203 AND FERRIC OXIDE...1.6%
CALCIUM CARBONATE CaCO3...97.2%
MAGNESIUM CARBONATE MgCO3... .2%
POROSITY % BY VOLUME....19.8
WATER UPTAKE % BY MASS....6.8
COMPRESSIVE STRENGTH MN/M3....21.6
AVERAGE MASS LOSS %....20.2
GUARANTEED LIFE 150 YEARS (IF LAID ON CORRECT BED)