To promote a wider interest in the science of geology through organised lectures, field excursions and social activities.
To provide a link between the amateur, the student, the teacher and the professional geologist.
To foster interest in geological sites within the area with a view to their study and wise conservation.
To establish and maintain good relations with organisations that have common interests.

 

 

 

 
 
 

 

 
 Just south of Bath, if you follow the road from Peasedown St John westwards across the brook at Wellow village, as it winds up the east side the valley towards Hinton Charterhouse, you will come across a land slip. The hillside has been shored up with wire cages holding large stones, and the tarmac has cracked on the valley side. Amongst the road chippings and bitumen you will see blocks of half dried out yellow mud, and on breaking these open some will have a grey centre. This is the fuller's earth clay, and its excavation in these parts was an important industry for more than five hundred years. The mud has a slippery feel,and was used to remove grease and dirt from wool and finished woollen goods, before detergents were invented. The material is a roadmaker's nightmare, for its slipperiness will cause any road built on to shift once any weight is put on it.

This deposit is found all over the Bath area, in a division of the upper part of the Middle Jurassic stratum, known as the Fuller's Earth Rock, from the presence of a band of this clay within it. For many years it has also caused problems locally for railways, and for canal engineers. Even now road works have just restarted at at Limpley Stoke, for it is still causing land slips. The most extensive workings for fuller's earth were underground at Odd Down, on the southern outskirts of Bath, and these benefited from the very good roof of the overlying rock. Here it was mined and worked until 1980, and at one time the power for the workings was from a giant American windmill, a prominent feature on the skyline until it burnt down in 1904.

The earth is mainly a montmorillonite, a name which is preferred over the older term Smectite. This is a hydrated aluminium silicate, and was formed by the underwater breakdown of volcanic ash, after having been blown far out to sea. The windborne ash is made of fine particles of glass. If these contain less than 68 percent silica, they will break down underwater in time, though volcanic glass with more than 68 percent silica is stable for eons. Fuller's earth is rich in calcium, and poor in aluminium. The index mineral is found at Montmorillon, a town 30 miles to the east of Poitiers. There the white clay has been used in pottery since the middle ages.

A related material is Bentonite, first found at Fort Benton in Wyoming in the 1850s. There it has formed widespread thick deposits which extend into neighbouring states. Deposits in Yellowstone Park up to 50 feet thick are the result of Cretaceous volcanic activity.

 

Bentonite in Southern Utah. Taken from here.

 

Early settlers found this slippery mud a good lubricant for their wagon wheels, and as it swells up to ten times its dry volume when wetted, it is useful for sealing cracks in reservoirs and canals. Extensive investigations by the U.S. Geological Survey showed it to be mainly magnesium montmorillonite. In fact, other ions are often associated with the clay, usually calcium, sodium and iron. It is the latter in the green ferrous state which gives fresh fuller's earth at Bath its grey colour. When exposed to the air, it rapidly changes to yellowish brown, as the iron oxidises to the rust-coloured ferric ion.

The slipperiness, and the physical and chemical properties of Montmorillonite clays is explained by the structure. The smallest flakes have a 3-layer structure, like a sandwich. The outer layers are of silicon dioxide molecules, joined into one giant sheet, by sharing their oxygen atoms. Each silicon atom lies at the centre of a tetrahedron , and an oxygen atom lies at each of the four corners. The points of these four-sided pyramids face inwards, towards the filling of the sandwich. The sandwich filling is made of alumina. Each aluminium atom has an group of 8 oxygen atoms surrounding it, at the corners of an octahedron. The two oxygen atoms at the two pointed ends of each octahedron are shared with the silicon dioxide layer, but the bonding is not strong, for hydration has substituted hydroxide radicals for many of the oxygen atoms, and the binding is mainly by weak hydrogen bonding, rather than by strong covalent bonds. It is the weakness of this hydrogen bonding which allows the sandwich filling slip, and also to expand. This can be done by treating with acid, which increases the surface area from 80 square meters per gram, to 300 square meters. Such treatment greatly increases its absorptive and adsorption properties. The aluminium ions can be partially replaced by iron, or by magnesium, which gives it an overall negative charge. The silicon can be replaced by Na, K, Li, or Calcium, and the aluminium by Fe and other ions. The layers can be widened by alumina bridges, to enclose larger molecules, such as hydrocarbons.

Fuller's earth swells in water to a lesser extent, and forms a crumbly non-cohesive mass, so is easy to wash off wool once it has been fulled. Its adsorptive properties have been used in face powders, to clarify beer, to purify glycerine from soapworks, to remove ink from printed material, and in refining oils either vegetable oils for margarine,or petroleum. It is used to purify sugar and glucose, in chemical works for filtration and decolorisation, and in purifying sewage effluent. Nowadays a major use is in foundries, to give cohesion to moulding sands. Fuller's earth treated with Soda Ash has enhanced bonding properties, and also adds plasticity to the foundry sand, allowing it to be rammed tight enough to take up fine detail of the patterns. One grade of sodium-exchanged earth is used in foundations in damp sites, because of good swelling properties and low permeability to water.The major use of Bentonite is in the drilling muds of the oil industry, where it cools and lubricates the drill bits, acts as a impermeable filter cake in permeable strata, and is a supporting medium for the heavy barytes used to add weight to the mud.

Fuller's earth in Britain is found at Nutfield and Redhill, east of Reigate in Surrey, where it has been worked since Roman times, and at Woburn in Bedfordshire. Near Swindon, at Baulking, in The Vale of the White Horse, a lower Cretaceous deposit has been worked commercially. . Locally there are Bentonite deposits at Old Down between Radstock and Wells, at Usk, north of Newport where the bed is probably Silurian, at Tortworth. Further afield it is found west of the Gower at Marlow sands, at Wenlock Edge, and near Lake Bala. Worldwide, as well as the Wyoming and Montana beds, there are extensive deposits in California,Texas, Brazil, Cyprus, Africa, Siberia and Spain. The age of Bentonite deposits is usually found by Uranium-Lead isotope studies, but more recently the rare earth ratios have been found to date deposits accurately.