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Defining Slate

The Making of a Slate (the geology)

Slate is defined as a fine-grained metamorphic rock.


Fine-grained because the original components were just that: tiny fragments of eroded rock, minerals and dirt (pretty much everything), transported by rivers and strewn on the sea-bed. This sediment, a muddy clay, was the beginning of a slate bed.


Much slate we use began its formation in the Cambrian period. The earth as we know it would have been unrecognizable. Continents were adrift, the earth’s plates shifting and mountain ranges erupting from the surface. This activity generated enormous pressure and heat, transforming the clay sediment into shale. Further upheaval turned the shale into slate, literally a metamorphosis of materials. Mica, Silicates, Quartz, Alumina, Chlorites, Chromates, Pyrites and Iron Oxides were formed and combined to give the rock we know as slate.


The minerals and crystals contained were forced into alignment, perpendicular to the direction of pressure. It is this particular formation that gives slate its identifying feature, ‘slaty cleavage’, a plane called the ‘cleave’, along which it can be readily split.


Up North, in a Vermont quarry, a huge slate boulder is prized from the earth. A block is sawn from the rock and a chisel, expertly placed on the ‘cleave’, is struck. The slate, 500 million years in the making, explodes apart giving a thin and perfect sheet. This is trimmed and squared, two holes punched and... "We have the Making of a Slate."

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