Geology
The main concentration of ball clay in Dorset is to the north of the Purbeck Hills centred around Norden. Ball clays are sedimentary in origin. Approximately 45 million years ago (in the Lutetian stage of the Eocene epoch) the climate was tropical and an ancient River Solent washed kaolinite (formed from decomposed granite) from its parent rock on Dartmoor. As the streams flowed from upland areas they mixed with other clay minerals, sands, gravels, and vegetation before settling in low-lying basins to form overlaying seams of ball clay. Ball clays usually contain three dominant minerals: from 20-80% kaolinite, 10-25% mica, and 6-65% quartz. In addition, there are other 'accessory' minerals and some carbonaceous material (derived from ancient plants) present.
Read more about this topic: Purbeck Ball Clay