“These minerals constituting slate were deposited in bodies of water and erosion and deposition accounts for the different composition in successive beds. Other materials may have been deposited over the clays, and the pressure of the superimposed material may have gradually united the clays in shale. Shale has been described as ‘a laminated rock consisting essentially of clay, but it does not possess the splitting properties of slate.’ Some of the beds of shale remain as such today, but many have been subjected to the intense pressure and high temperature of the crumpling and folding of the earth’s crust. When this has been the case, the shales have been transformed into slates.This tremendous pressure changes the position of the mineral grains until they lie parallel and at a definite angle to the direction of pressure. The high temperature is at the same time acting with the pressure tending to change the constituent minerals to new minerals, such as mica, quartz, chlorite, magnetite, graphite and others. The ‘slaty cleavage’ or ease of splitting in one direction is the result of the parallel position of the mineral grains.”
The metamorphic process described above took place during the geologic time period known as the Early Paleozoic era, some 435 to 570 million years ago. As such, the vast majority of slate shingles present on our roofs today, and which continue to be quarried, originated some half-billion years before humans roamed the earth.
The purple slates and some of the green slates from Vermont and New York, and the Peach Bottom slates from south central Pennsylvania date from the early part of the Paleozoic era, known as the Cambrian period and range in age from 500 to 570 million years. The black slates of northeastern Pennsylvania, along with the Buckingham Virginia slates, and the red and remaining green slates from New York and Vermont date from the Ordovician period and are some 435 to 500 million years old.
The earth was a very different place in Paleozoic times. During the Cambrian period, the major land masses were all bunched up in the southern hemisphere and just beginning to split apart and admit the seas between them. The oceans teemed with early invertebrates, such as trilobites and brachiopods, but there was no life on land. With no plant life to hold the soil, wind and rain washed tremendous amounts of sediments off of the desolate land and into the seas. During the Ordovician period, the Appalachian Mountains rose out of the earth as the early land masses moved further apart and a volcanic island arc slammed into North America, creating massive disturbances in the earth’s crust. The climate warmed during the Ordovician period. Numerous invertebrates, such as sponges, starfish, and corals, filled the earth’s oceans, and the first, uncontested, vertebrate fish appeared in North America.
The color of slate is determined by its chemical and mineralogical composition. Since these factors differ in various localities, it is possible to obtain roofing slates in a variety of colors and shades. Upon exposure to the weather, all slate is changed slightly in color. The extent of this color change varies with different slate beds, being barely perceptible in certain slates. Those slates in which the color changes but slightly are classed as “permanent” or “unfading.” Those in which the color change is more marked and varied are known as “semi-weathering” and “weathering” slates.
North American slate deposits currently producing roofing slate are found in:
- Vermont and New York
- Quebec, Canada
- Newfoundland, Canada
VERMONT AND NEW YORK – The Taconic Overthrust, at the south end of Lake Champlain, provides all of the colored roofing slate produced in the United States. The producing region forms an ellipse about 25 miles north to south and 6 miles east to west and sits atop the Vermont / New York border. With a history of 150 years of slate production, this region contains about 400 quarry sites following the slate vein. An estimated 35 slate quarries are currently in production. The most common and commercially important colors produced in the Vermont/New York region are:
- Semi-Weathering Gray/Green, also known as Sea Green – a historically popular and high-volume product. A green or gray/green color when first quarried; after application to the roof and exposure to the elements, a percentage of these slates weather to shades of buff and brown. The degree of weathering will vary from quarry to quarry and can range from less than 10% to more than 90%. As about a dozen quarries currently produce Semi-Weathering Gray/Green roofing slate, a subtle range of color, texture and weathering characteristics can be expected.
- Semi-Weathering Vermont Gray – a traditional slate-gray color ranging from a clear appearance to having small black markings. This slate may show some weathering to buff shades depending on the quarry and position in the slate vein. This is a smooth to medium texture slate.
- Variegated Purple – a purple slate varying in shade. Some pieces are clear while some pieces have green spots and marks. A small percentage may weather to shades of buff. Some types of Variegated Purple are waxy and smooth while others are medium-textured.
- Unfading Mottled Green and Purple – green and purple combine to form unique patterns in each slate. Some pieces are mostly purple and may be called “Dark Mottle,” while at the other end of the range, green predominates and can be known as “Light Mottle” or “Reverse Mottle.” Unfading Mottled Green and Purple does not weather and is a medium-textured slate.
- Unfading Green – found mainly on the northern portion of the slate producing region. This slate exhibits shades ranging from a bright green to gray/green tones that will not weather. This also is a medium-textured slate.
- Unfading Gray – a medium-texture slate, usually with black markings. This light gray slate will not show buff and brown weathering.
- Unfading Purple – a deep purple slate, at times almost burgundy in color, which may have occasional green marks or inclusions. Unfading Purple is a less common, low volume slate type of medium texture.
- Strata Gray – a medium gray slate with black stripes also known as Mottled Gray/Black. This medium to heavy-textured slate usually shows some buff and brown weathering.
- Vermont Black and Semi-Weathering Gray/Black – includes a range of slate types from a full 1/4” thick, textured slate with a medium to dark gray color and darker linear markings to a medium-texture dark gray to black slate without lines. A small showing of buff and brown may appear. Naming and degree of weathering varies from quarry to quarry.
- Unfading Red – a bright red that will not change color, this dense and very hard slate is quarried in Washington County, New York, and is commonly found in roof applications with color patterns and designs. Unfading Red can be a drier, more brittle slate, making it more expensive to produce and contributing to its higher cost.
PENNSYLVANIA – Mined in Lehigh and Northampton Counties in Pennsylvania since the 1840s, Pennsylvania slate is blue-gray to blue-black in color and is most commonly called Pennsylvania Gray. The stone surface has a range from smooth grain to rugged texture. Over the last 150 years, an estimated 160 companies have quarried the slate veins in eastern Pennsylvania. Some quarries in this region may present as many as 70 different “beds” or color/ texture type layers. Over the years, slate produced from the various “beds” carried names including: Varitone, Colortone, Blue-Gray, Blue-Black, Cathedral Gray, Gothic Blue-Gray, Slate Gray, Storm Blue-Gray, etc. Current Pennsylvania Gray production is a blend of gray-black color shades and is a mix of S1 and S2 ratings by ASTM test standards. Weathering to brown and light gray may occur over a period of years.
VIRGINIA – THE ARVONIA-BUCKINGHAM SLATE BELT – A durable, very hard, unfading black slate continues to be produced in Buckingham County, near Arvonia, Virginia. This historically significant roofing slate has a distinct glistening mica sheen. Slate from Buckingham County does not change color and has a medium texture.
QUEBEC, CANADA – The Glendyne Quarry in St. Mark du Lac Long, Quebec, Canada, is the largest roofing-slate producing facility in North America and one of the largest in the world. Their unfading black slate is currently produced for the European and North American market and is marketed in North America as North Country Unfading Black. The first production from this vein of slate occurred in the early 1900s. This smooth-textured slate is easy to cut and has a consistent black color with subtle vertical shade markings.
NEWFOUNDLAND, CANADA – Overlooking the ocean at Burgoynes Cove, Newfoundland, is a deposit of smooth to medium-texture, unfading Cambrian slate with deep purple, green and variegated green and purple slate colors. The purple slate is very similar in color to Welsh Purple from England. Production of roofing slate from this quarry has been intermittent since the early 1900s.
North American slate deposits, once commercially quarried, but no longer in production include –
- Peach Bottom (Pennsylvania)
- Monson (Maine)
- Hard Vein (Pennsylvania)
Minor North American slate deposits quarried for short periods of time, primarily during the late nineteenth century, and which are not longer quarried include those in Georgia, Arkansas, Utah, Wisconsin, and California.