Slate from Southwestern Vermont and across the border
into New York State is touted as the best quality slate in the country and
sometimes the world. Its uses range from roofing slate to floor tiles, flagging
stones for patios, cladding for buildings, fireplace mantles, stairs, counter
tops, windowsills, monuments, statues, tubs and sinks, and no doubt a few other
uses. Slate is a metamorphic rock, typically a fine-grained shale or mudstone
that was metamorphosed, or changed, by intense heat and pressure so that very
distinct fracture cleavages exist at regular intervals that can be easily split
into thin sheets. The straight planar surfaces of the individual pieces after
splitting remain strong and firm. The shale/mudstone beds were originally laid
down horizontally in quiet waters over millions of years. Later they were folded,
tilted, and subjected to intense heat and pressure.
The story of
how the Vermont Slate was formed is a complex one that involved many dedicated
and keen geologists to unravel over many years. To convey the whole story, one
needs to understand and communicate quite a few different geological concepts
and processes. The story begins out into the ocean, what is called the Iapetus
Ocean that first formed about 700 million years ago. About 160 million years
after that, or 540 million years ago, that ocean switched directions and began
to close, creating what is known as an Island Arc System. At the mid-ocean
ridge where the ocean had first spread apart making a spreading or divergent boundary
of two tectonic plates, the stress forces acting on the plates gradually changed.
The plates began moving together now making a convergent plate boundary. The
plate to the west began to subduct or push under the plate to the east. As the
lower plate’s oceanic crust was pushed down further into the mantle the rock
cooked and removed volatiles. This caused sections of the overriding mantle to melt,
and the magma moved upward. These new hot spots in the mantle formed volcanos. When
more of the ocean crust moved over the hot spots new volcanos formed which made
a series of volcanic islands. This new chain of islands was in the shape of an
arc. The Hawaiian Islands and the Aleutian Islands are examples today of island
arc systems with new islands being created by volcanic activity as the plates
move over hotspots.
For the next
70 million years or so the island arc system named Ammonusuc carried an island
continental mass toward what was then the east coast of North America about 60
miles or so inland what it is now. Geologists now call the ancient North American
continent Laurentia. The shales and mudstones that make up the Vermont Slates
were originally deposited on the continental shelf, slope, and rise which proceeded
further offshore of the passive eastern continental margin of Laurentia. Rocks
further offshore were deposited slowly in small amounts in low energy
conditions in quiet water. As the island arc system moved closer to Laurentia it
began to push onto Laurentia causing its rocks to buckle and deform. This was
the beginning of the Taconic Orogeny, or mountain-building event, which pushed
up these rocks onto the eastern edge of Laurentia. The mudstone rocks that make
up the slates would later be folded intensely again by thrust faulting and
undergo low-grade regional metamorphism which changes the crystal structure of
the rocks and creates cleavages aligned perpendicular to the compressive forces
acting on the rocks. Thus, since the compression is oriented more or less east-west,
the cleavage planes are oriented more or less north-south at different angles.
Metamorphic rocks have a characteristic called foliation or repetitive layering
which is caused by differential pressure or shearing forces acting on the
rocks. The Vermont slates have millimeter-scale foliation. The “slaty cleavage”
is caused by fine-grained flakes of clays regrowing in places perpendicular to
compression. This cleavage allows the slate to be split into thin sheets
parallel to the foliation, a rock property known as fissility.
Image Source: Wikipedia.
Entry: Taconic Orogeny
Those Cambrian
and Ordovician mudstones and shales would be pushed about 100km westward as Ammonusuc
system collided with Laurentia and later by a series of thrust faults. The island
arc system, or micro-continent as they can also be called, accreted or joined the
ancient North American continent adding new land to Laurentia to the east of a
line known as Cameron’s Line running north-south through central Connecticut,
Central Massachusetts, and central Vermont. Modern day New Hampshire and Maine were
once part of the Ammonusuc island arc system. This line marks the surficial
part of the suture zone of the two continents. They would also later be intensely
folded and buckled by a reactivation event, and then metamorphosed by heat and
pressure in one or possibly two metamorphism events. Finally, after rocks at
the surface were eroded the metamorphic slates were exposed at the surface.
In 1972 E-An
Zen proposed a three-deformation chronology of the slates of Western Vermont-
dates are tentative: Event 1) at Trenton time, Middle-Ordovician
- involved large-scale soft rock submarine gravity sliding. Turbidites slid to
the west into a geosyncline that began to develop from the Taconic Phase. Event
2) Late-Ordovician to Early Silurian – isoclinal recumbent folding.
Formation of slaty (early) cleavage, emplacement of the Sudbury slice and the
Florence nappe, and low-grade regional metamorphism. Event 3)
Devonian Acadian – small-scale (tens to hundred meters) upright or overturned
recumbent folds with shallow plunging, north-trending axes. These folds
refolded the early slaty cleavage and there was possibly a second metamorphic
event in higher-grade areas along the Taconic Range.
The Vermont Slate Belt on the Taconic Allochthon,
It’s Extent and Characteristics
The Taconic
allochthon is about 200km long and 25km wide and encompasses part of
Southwestern Vermont, Northeastern NY, and Western Massachusetts. This is where
the Vermont Slates are deposited, or rather have been re-deposited after
movement. The area is just west of and alongside the north-south trending Green
Mountains, or Green Mountain anticlinorium. The outcrops consist of rocks of
two different ages. An allochthon is a section of rock that was moved from its
original position via a thrust fault, a type of reverse fault associated with
compressive forces where one body of rock faults and moves at low angle
laterally across another body of rock. Allochthonous rocks may also include
beds that are steeply folded or overturned, which can complicate unraveling geologic
history. The cleavage of the Vermont Slates is high angle, from 30 to 90 deg.
90 deg cleavage is equivalent to being stacked vertically, on their sides like
books on a bookshelf. The cleavage planes are generally parallel to the dip of
the rocks, which is to the east. However, I believe some of the overturned
folds may lean or dip westward. The rocks of the Cambrian St. Catherine formation
include the Mettawee Slate, also known as the Bull Formation. This rock occurs
at surface at the center of the slate belt making up about half of the slate
outcrops. It is reported as being 49-312 ft thick. Purple and greenish gray slates
alternate. The greenish gray slate thickness range is reported at 98-197 ft and
the purple slate at 39-49 ft. There are occasional dark red slates as well. The
Mettawee/Bull Slate has well developed cleavage and makes good roofing slate.
The Ordovician aged Indian River and the Poultney formations, both slates, are younger
than the St. Catherine Formation. They outcrop on both sides of the St.
Catherine/Mettawee/Bull and make up the other half of the slate outcrop belt. The
color of the Indian River ranges from red to green to blue green. It also has
well developed fracture cleavage. It ranges from about 82-180 ft thick. The
Poultney Slate occurs above the Mettawee and below the Indian River. Its
thickness varies quite a bit due an angular unconformity which allowed much of
it to be eroded away. Unconformities often involve sections where rocks are
missing due to erosion so that time is also missing. The range of thickness of
the Poultney Formation in this area is between 0 and 700ft.
The Vermont
Slate quarry region that is active today is quite small and well-defined. Its
extent is about 1.5 miles wide and 12 miles long and occurs in the Poultney
Valley in Rutland County, Vermont. The region makes up 1.8% of Rutland County
and 0.18% of the State of Vermont. According to a paper prepared for that
Vermont House Committee on Natural resources, Fish, and Wildlife in 2019, there
is “no risk of slate quarries emerging elsewhere.” I really do not know
how they arrived at that conclusion. It could be economics or slate quality and
ease of extraction within the general area. I do know that in some places in
the world slate is mined rather than quarried, which likely involves more waste
rock and environmental impact. The steep dips of the Vermont deposits may lend
them more suitable to quarrying than mining.
Geologic Map Showing
the Outcrops Along the Taconic Allochthon. Old Vermont Geological Survey Map.
The Area to the West of the Taconic Klippe Trace Makes up the Taconic Allochthon.
Areas of the Slate Belt Quarried Historically.
Source: Slate deposits of Vermont & New York. Roofing Slate Dictionary.
Area Quarried Today. Source: Vermont’s Slate Industry and Act 250. Prepared for House Committee on Natural Resources, Fish & Wildlife. January 14, 2020
A Surface Geological
Map from the Mobile App, Rockd. Pink is Bull Formation - Mettawee Slate Facies.
Dark Green is Ordovician aged Poultney Formation Slate and Phyllite. Olive
Green is Cambrian aged West Castleton Formation – dark gray and black slate and
phyllite interbedded with limestone. Lightest Green is Late Ordovician aged Pawlett
Formation – carbonaceous slate interbedded with quartz-plagioclase wacke. Gray
is Bomoseen Graywacke member of Cambrian/Precambrian Bull Formation.
Color Variations Due to Mineral Content and
Oxidation
As noted,
there are several different color variations in the Vermont Slates including
gray, gray green, blue green, purple, red, black, and mottled. Some sections of
the slates are deemed ‘fading; and some deemed ‘unfading.’ This refers to the
likelihood of color changes through oxidation due to weathering. The fading
slates will typically turn to a more buff or tan color. Sometimes only part of
the slates will fade, making a two-tone roof that people find aesthetically
pleasing. These would be fading mottled slates. There is also an unfading
mottled slate that has stunning variations of green and purple that make each
tile look like a unique work of art. A few of the fading grades are prone to excessive
oxidation and will have shorter lives as roofing slate. The unfading grades do
not oxidize enough to make color changes. The slates that will weather the most
have certain minerals like calcite and pyrite that will be more affected by oxidation
when exposed to the weather.
Mineral
content in the rocks is responsible for the colors. Red and purple slates are
colored by hematite, an iron oxide. Green slates are colored by chlorite, a
green clay mineral. Black and gray slates are colored by carbon. As noted, the
different color grades are typically in different beds within the sequence and
represent varying mineral contents of the different beds when the beds were
formed. Geologically, the variations may have to do with different water depths,
depositional rates, and biological content.
Most slates around the world, including the highest quality slates in Wales and Spain, are black. The unique colors of Vermont slate give them an aesthetic advantage over other slates. Slates in general can be quarried and processed without the use of chemicals and too much energy expenditure in processing. They are considered environmentally advantageous in this regard. They are a little bit heavier than asphalt, but they last 6 times longer than asphalt. They have become popular with “green” builders. Although the highest quality roofing slates are considered to be those from Wales and Spain, the Vermont Slates have much better color variation and arguable aesthetic qualities that could confer them as the world’s best slate.
Unfading Mottled Green
and Purple Slate from a Quarry in Operation for 150 years by Vermont Structural
Slate Company. Favored for indoor uses. Source: Vermont Structural Slate Company
Website. Custom Architectural Stone
Projects | Vermont Structural Slate Co.
History of the Vermont Slate Industry
The 24 mile
long Slate Valley that runs through Poultney is famous for slate quarries.
Quarrying began in the 1840’s. The first slate-roofed barn built in 1848 is
still standing today, slate roof intact. There are many other examples of the
old slate roofs still holding out. Immigrants from Wales, Poland, Austria,
Italy, worked the slate quarries in the past. As the slate roofing industry began
in the 1850’s immigrants from Wales came where slate was quarried since 1399. The
Vermont Slate was first used for school slates and slate pencils. A quarry in
the 1840’s was producing 600 school slates per day. According to the Poultney
Historical Society The current Vermont Slate industry is worth about $40
million, quite small by mining and materials standards. They also note that a
requirement in California for non-flammable roofs has increased demand. There
is also a demand center for slate in the U.S. Southeast.
The local
industry once employed 5000 people before the advent of asphalt shingles in the
1920’s began competing with slate. The production boom of Vermont Slate
occurred between 1880 and 1920 when production was Other lower-grade slates as
well as other roofing materials also compete. In 2000 there were 38 companies
operating but now there are 20.
10 quarries
currently produce slate in Vermont and New York. Quarrying and milling employ
about 300 people. Most are small family businesses with the exception of the
Vermont Structural Slate Company which runs the Eureka quarrying and milling operation
in Fair Haven. This is in contrast to Vermont’s granite and marble quarries
which are owned by large corporations. Vermont’s family slate companies have
been compared to family farms, getting low returns on investment and being barely
profitable. The largest company, Vermont Slate Company, was recently acquired
by Spanish Slate company Cupa Pizzaras S.A., I believe in 2021.
The process of
producing the slate involves drilling and blasting followed by manual
splitting. There is some difficulty removing the blocks from the quarry due to
the tendency of the rocks to split along cleavage planes. After drilling and
blasting rocks are removed by power shovel, crane, or power boom, loaded onto a
bucket, and moved to the milling area. Quarrying stops in the winter but
milling becomes most active in winter. Different milling techniques are used
for roofing slate than for flooring tiles and flagging. Both are cut by diamond
saw and split to desired thickness. Floor tiles are run through a planer to get
to uniform thickness. All other products are considered structural slate,
including fireplace mantles, electrical panels, and windowsills.
Some slate
quarries are held in reserve as resources to be tapped if demand rises. While
there have been some issues with opposition to slate quarries, those have been
few. Some have opposed blasting but there have been no recorded or reported local
damages from it. Others don’t like the conical piles of waste rocks. These rocks
can be used for road aggregate and other uses, and are often donated. Some say
the quarries lower property values, but it is one of the main historical
industries in the area and the quarries do not take up that much space. One quarry,
in operation since 1852 encompasses about 10 acres. It is a small industry that
does keep some resources reserved. As a valuable and very functional building
and aesthetic resource with environmental benefits compared to alternatives,
those reserves should be protected as needed but also continuously developed.
Image from Vermont
Structural Slate Company Website Showing Work at a Slate Quarry. Custom Architectural Stone
Projects | Vermont Structural Slate Co.
Image from Vermont
Structural Slate Company Website Showing Manually Splitting Slate.
Custom Architectural Stone
Projects | Vermont Structural Slate Co.
General References (uncited)
Taconic Orogeny.
Wikipedia. Taconic
orogeny - Wikipedia
Slate. Vermont Department
of environmental Conservation. Slate
| Department of Environmental Conservation (vermont.gov)
Vermont’s Slate
Industry and Act 250. Prepared for House Committee on Natural Resources, Fish
&
Wildlife. January
14, 2020. Vermont
Slate Industry and Act 250
Vermont Slate
acquisition by the world leader in natural slate. Cupa Pizzaras.
Vermont
Slate acquisition by the world leader in natural slate | Cupa Pizarras
Slate Deposits and
Slate industry of the United States. T .Nelson Dale. Series A, Economic Geology
63, Bulletin No. 275. U.S, Geological Survey. 1906. Slate
deposits and slate industry of the United States | U.S. Geological Survey
(usgs.gov)
Slate Colors and
Characteristics ICS. National Slate Association. Slate Colors |
National Slate Association
Slate deposits of
Vermont & New York. Roofing Slate Dictionary. roofing
slate dictionary - Slate deposits of Vermont & New York
(schieferlexikon.de)
Cameron’s Line. Infographics,
Maps, Music, and More. Blog by Rich Coffey. 2016.
Some Revisions in
the Interpretation of the Taconic Allochthon in West-Central Vermont. E-An Zen.
Geological Society of America Bulletin. 1972. 83 (9): 2573-2588. Some
Revisions in the Interpretation of the Taconic Allochthon in West-Central
Vermont | GSA Bulletin | GeoScienceWorld
Slate is Great:
Geology of Poultney, Vermont. Gabrielle Lucci. STEM Fair Project 2020. Slate
is Great: Geology of Poultney, Vermont (arcgis.com)
Geology of the western
boundary of the Taconic Allochthon near Troy and the anastomosing cleavage in the
Taconic Melange. Zong-Guo Xia. 1983. Unpublished Master Thesis. Abstract. University
at Albany, State University of New York. "Geology
of the western boundary of the Taconic Allochthon near Troy an" by
Zong-Guo Xia (albany.edu)
Bedrock Geology of
the Pawlet Quadrangle, Vermont. Robert C. Shumaker. Vermont Geological Survey.
Bulletin No. 30. 1967. https://anrweb.vt.gov/PubDocs/DEC/GEO/Bulletins/Shumaker1967All.pdf
Slate. GeologyScience.
Slate Rock |
Properties, Composition, Formation, Uses (geologyscience.com)
Vermont’s Slate
Industry. Poultney Historical Society. Vermont's
Slate Industry - Poultney Vermont Historical Society
(poultneyhistoricalsociety.org)
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