Bricks are produced in all three countries of mainland Britain, although England accounts for about 90% of production. Although other clays are used on a small-scale, the location of the industry tends to reflect the distribution of the principle brick clay resources. In approximate order of tonnage used in brick making these are;

Carboniferous mudstones in Northern England and central Scotland: Variable in quality, with only a small proportion suitable for brick manufacture for brick manufacture (most too high in carbon and sulphur). Despite this, they are the most important resource, accounting for almost 30% of consumption. Almost all bricks made from Carboniferous mudstones are formed by extrusion. Most clay drainage pipes are made from these clays.

Etruria Formation or Etruria Marl (Carboniferous age): High Quality clay is close in composition to the "ideal" brick clay. Extracted and used almost entirely within the West Midlands. Bricks are made by both extrusion and soft-mud methods. Often used to 'sweeten' poorer-quality clays. Most clay roof tile makers use this material. Outcrop area is very restricted.

Mercia Mudstone Group or 'Keuper Marl' (Triassic age): in the Midlands: Extraction and use of these clays confined to the Midlands. Mineralogy of this clay gives rise to a distinctive pale-bodied brick. Extrusion is commonest manufacturing method.

Peterborough Member or 'Lower Oxford Clay' (Jurassic age): Extraction of these clays confined to Cambridgeshire and Bedfordshire. High carbon content requires an unusual manufacturing process (pressed bricks fired in chambered kilns). There has been a reduction in usage of this clay over the last 30 years and manufacture of bricks is now restricted to three large production units.

Weald and Wadhurst clays (Cretaceous age): Principle brick clay resource in southeast England. Most production takes place south of London. Bricks are mostly manufactured using 'soft mud' process.

Fireclays (from the Carboniferous coalfields in the Midlands and the North): Associated with coal seams and produced almost exclusively as a by-product of opencast coal extraction. Although comprising less than 7 per cent of total consumption, they are important clays used extensively in high-value extruded buff bricks which are manufactured at sites across the Midlands, the North and Scotland.

Good quality brick clays consist predominately of the clay minerals Kaolinite and illite. These impart desirable properties which are important in forming and firing the brick. Quantity and particle size of the quartz (silica) component of the clay are also critical in determining forming and firing behaviour. Carbon and sulphur can have a major influence on firing performance, with low levels preferred. The familiar red/brown colours of most bricks are due to the presence of iron minerals in almost all clays. However, the presence of carbonate minerals such as calcite and dolomite can produce paler-coloured bricks. Production of very pale buff/cream 'through-colour' bricks is presently only made possible by using fire-clays with low iron contents. Fully-durable yellow bricks (such as London 'stocks') are made from a mixture of clay and chalk.

 http://www.ibstockancestry.co.uk/swanage.htm