EDUCATION CENTRE 

A Geological Phenomenon

Why is Ball Clay found in Devon?

The Ball clays of Devon are derived from the granites which form the backbone of South-west England. Today the granite uplands are seen in areas including Dartmoor and Bodmin Moor.

Granite is an igneous rock, made up of three principal minerals: upto 65% feldspar; upto 30% quartz, and 5-7% mica.

Some 250 million years ago the climate in the region was sub-tropical. In the warm wet conditions some of the granites slowly decomposed as a result of natural processes. The feldspar minerals within the rock were eventually converted into kaolinite (one of the key minerals in ceramic clays).

In decomposing, the granite changed from a hard rock to a relatively crumbly material. Some of this was gradually weathered and eroded away, and the clay minerals within the rock were washed out and transported by rivers and streams towards the sea.

About 30 million years ago, the "Sticklepath fault system", which runs approximately Northwest- Southeast through Devon, became active, and the movements of the earth's surface created areas of low-lying flat ground. The streams running through these areas were slowed, so that the sediments they carried could settle out.

The rivers and streams fed into short-lived lakes which formed on the low lying ground, and deposited the coarser, heavier materials they were carrying - gravels and sands - first. The very fine grains of kaolinitic clay remained caught up in the water until acids produced by decaying vegetation caused the particles to clump together and settle out. They formed seams of ball clay which are between 0.5 and 1.5 metres thick.

The upland areas around the valley were at that time thickly forested with Sequoia (similar to the Giant Redwood trees of North California). These huge trees were the major source of the vast floating mats of vegetation and timber which accumulated in the lakes. As these became buried and crushed, the vegetation was formed into a low-grade type of coal, known as lignite.