The origins of slate go back 450 to 600 million years when very fine clay silts were deposited at the bottom of riverbeds, lakes and seas. Over millions of years these sediments were compressed and heated, altering the arrangement and alignment of the minerals. The resulting stone has a unique parallel alignment of the minerals that allows the stone to be separated into layers (split) – a distinctive characteristic of slate.
Inhabitants of northern Wales are believed to have been among the first to take advantage of slate’s natural cleavage for the production of roofing slate around 1300. Over several centuries the Welsh developed a proud tradition of slate production, one that continues to this day at the famous Penrhyn Quarry operated by Welsh//Slate. In the mid-1800s Welsh immigrants brought their slate working experience to the northeastern US, starting what was to become a major industry in the area.
The first slate quarry to operate in Vermont began in 1839, however it wasn’t until the mid 1800’s that slate quarries became commercially viable. Welsh immigrants experienced in making roofing were a valuable work force for the Vermont quarries and catapulted this fledgling industry to the national and international stage. Slate was a common roofing material in the late 1800’s, and the early 1900’s marked the high point for slate production in Vermont and other slate quarries in the United States. Today natural roofing slate competes against many other man-made products but it continues to be the standard for most colleges, universities, churches, institutional buildings and custom homes.
If you are interested in learning more about the history of slate, the Slate Valley Museum in Granville, NY, the National Slate Museum in Wales and The Slate Roof Bible by Joseph Jenkins are all great resources.
In Vermont and New York there are many different colors of slate—greens, greys, black, purples and red. There are also slates that are mottled with two or more colors—green and purple, grey and black for example. Neighboring slate quarries can yield stone of completely different colors—a grey/black quarry is next to a weathering green quarry—and even within a single quarry different colors can be extracted. There are no certainties with regard to color, quantity, quality or direction of the vein when quarrying slate.