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"The term slate, in ordinary usage, denotes a rock which has more or less perfect cleavage, and therefore adaptable to various commercial uses, and in which the constituent particles, with very few exceptions; cannot be distinguished except in thin section under a microscope."
"The term slate, in ordinary usage, denotes a rock which has more or less perfect cleavage, and therefore adaptable to various commercial uses, and in which the constituent particles, with very few exceptions; cannot be distinguished except in thin section under a microscope."
T. Nelson Dale, Slate in the United States, 1914
Slate Defined
Slates vary greatly in color from black through various shades of gray to greens, reds, and purples of different hues. They vary also in luster from having none to being almost as bright as mica itself. They vary greatly not only in grade of fissility (the property of rocks to split along planes of weakness into thin sheets) but in surface texture, as seen by the unaided eye or determined by touch, and still more in microscopic texture, as seen in thin section. They differ also in their mineral and chemical composition and in their physical properties.
Both slates and schists may have originated in deposits of identical character, but they have undergone different processes. Slate however, should not to be confused with schist which is a rock that may be of identical chemical and mineral composition but is either made up of coarser particles, possesses a wavy structure, or else is marked by both of these features.
For more information about the geology of Slate CLICK HERE.
Since the most extensive use of slate is for roofing purposes, the foremost point of concern is durability. A roofing material is obviously exposed to the most severe weather conditions. Roof temperatures may often reach 140° F., and the exposure offers no protection against the lowest winter temperatures. Moisture, ice, snow, and hail all contribute to the severe conditions. Wind storms deflect the rafters, and where slates are nailed down too tightly, they may strain to the breaking point. Besides the normal weather conditions, slate is exposed to flue gases and the leaching from deposits of soot. Evidently the durability of slate in such exposure depends upon the resistance of the mineral component; hence a study of the composition is important. Strength, elasticity, and density are also important characteristics.
Slate is found in sections of the United States other than the Slate Belt of Pennsylvania, but these locations where slate is quarried is obviously dependent on the specific geology that is necessary to produce slate. When looking at a map of slate locations across the United States, the vast majority of them are found along the eastern edge of the Appalachian Mountains in a region known as the Great Valley.
In addition to the locations for slate quarrying across the United States are also slate regions throughout the world. Some of the most famous slate quarries in the world are found in Wales, a region that produced many of the slaters that worked the slate quarries of the Slate Belt in Pennsylvania and from where the name Bangor originated.
The Map above is interactive and can be expanded to full screen by clicking on the FULL SCREEN symbol in the top right corner of the view. Clicking on any of the pins will provide the name of the location.
Slate Producing Regions of the World
The earliest dates of the slate industry throughout the world have been lost in time but solid historic evidence can be linked back to the Delabole quarries in Wales as early as the 16th century.
Many of the locations listed below are hot linked. Clicking on the links will show the location of these quarries in Google Maps.
United Kingdom: Cornwall (famously the town of Delabole) and Wales.
Portugal: especially around Valongo in the north of the country.
Germany: Mosel River-region, Hunsrück, Eifel, Westerwald Thuringia, north-Bavaria.
Canada: Quebec, and the east coast of Newfoundland.
United States: the Slate Belt of Eastern Pennsylvania, and the Slate Valley of Vermont and New York. Granville, New York, claims to be the colored slate capital of the world.
Slate is also found in the Arctic and was used by the Inuit to make blades for ulus.
China: has vast slate deposits; in recent years, its export of finished and unfinished slate has increased.
Source: "Slate." New World Encyclopedia, . 23 Sep 2015, 15:23 UTC. 13 May 2016, 15:51
The map to the left is taken from T. Nelson Dale's book and shows the location of all slate sites within the country. Since slate quarrying is a function of geology, note that the majority of slate operations exist along the edge of the Appalachian mountains. CLICK HERE to see this map larger
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