There is an old Welsh saying that translates as follows: “Only the Welsh can quarry slate because the rocks don’t speak English.” This is quite the anthropological riddle and I won’t even attempt to unravel it, but it does put me in mind of how few people understand the vocabulary of slate. Not to be confused with the vocabulary of geology–that’s another kettle of fish—but the working language of slate, as spoken by quarriers. The following is a brief explanation of the most common slate terms.

Overburden:   unusable material burying the slate seam.

The Rock (also, The Stone):   slate.

The Bed:   the original horizontal deposits of the material that metamorphoses into slate.

Joints:   breaks in the slate formation, often consisting of a seam of different material.

The Seam:   the continuous run of the slate mass.

The Strike:  the direction of the seam, described by points of the compass.

The Dip:   the angle of the seam to the horizontal.

Blocks:   the rectangular blocks cut from the quarried stone.

The Cleave:   the plane in which the slate is split (perpendicular to the main plane of compression).

The Grain:   the plane, usually perpendicular to the cleave, in which the slate may also be split, though with much greater difficulty- a process called sculping.

The Face:   the exposed surface of the cleave.

A Book:   the slate split from the same block.

Chips:   books of slate that have not been trimmed.

Striation:  streaks or bands of material in the slate, having an excess of one or more minerals. For example, dark bands will have more carbonaceous material, lighter bands more quartz. These originate in the bedding.

Ribbons: bands in the slate with an excess of magnesium and calcium carbonates. These are much softer than the surrounding material and degrade more quickly.

Natural cleft:   the natural face of slate (before honing).

Honed surface:   natural cleft ground to a smooth surface.

Hebra slate:  Slate in which the metamorphic compression occurred primarily in one plane.

Secca slate:  Slate in which the metamorphic compression occurred in multiple planes (through folding or other disruptions).

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