Background Information on Collyweston Slating

The Geology

What is Slate?

Stone roofs are generally referred to as stone slates, slate stones, stone tiles or tilestones.Stone slates are formed from sedimentary rocks (shale: composed of clay, quartz, mica and small amounts of iron, calcium and magnesium minerals) split along lines parallel to the natural bedding plane. A true slate is one which has fissility independent of the bedding. It is a metamorphic rock (one which has been changed from its original form by the effects of heat or pressure or both.) The increases in heat and pressure can be produced by earth movements; for example, during mountain-building phases as in the geological periods which formed the British slates.

(The Building Slates of The British Isles: Diane Hart and Craft Techniques for Traditional Buildings: Adela Wright)

Colour and Texture The differences in colour and texture in the sedimentary rock from which the slates are made arise from the conditions in which the beds were deposited millions of years ago. Shallow water produced a rippled stone. Deeper, calmer waters produced flatter smoother and generally larger slates. In slow moving water the sand particles are smaller and the slates' texture is finer, and less grainy. The variety of colours from pale yellow to red, which is a result of iron staining, together with the variety of texture, imparts the local distinctiveness, which can vary from village to village. Until improvements in transport made it easy to import cheaper, mass produced products, each village used its local stones, providing its buildings with a distinctive local fingerprint.

(English Heritage)

Click on the text below to view picture of a Collyweston Slate, donated by David Ellis:

Long Short'un

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