Stories in Stone (32 page)

Slate shingles kept the elements out of the great estates of the landed gentry and the cottages of peasants.
Court rolls and
manor accounts reveal that slate had become so popular by the fourteenth and fifteenth centuries that the more desperate stole
cartloads of it, possibly for resale, but also for covering their own homes.
Other, more industrious thieves attempted to
excavate slate illegally, usually from land owned by the local manor owner.

By the time America’s earliest colonists started to abandon England for the New World in the 1600s, slate quarrying and slate
roofing were well established in Europe.
No doubt most people who ventured across the Atlantic had seen a slate roof.
Many
may have known how to put on a slate roof and many more would have known of slate’s fire resistance and durability.

In 1662, in what may be the earliest building code in the colonies, Virginia governor William Berkeley drafted “An act for
building a towne.” It stipulated that towns should be built with thirty-two houses, each made of brick two feet thick at the
foundation, eighteen feet tall, and covered in a roof of slate or tile.
At least some Jamestown residents complied; archaeologists
have found roofing slate from buildings constructed in 1663.
Seventeen years later, after a fire destroyed eighty buildings
and seventy warehouses in Boston, the General Court enacted a similar resolution requiring slate or tile roofing.
⁷
Canadian statutes of the early 1700s also recommended the use of slate to cover buildings.

Officials forgot one little factor—few, if any, people could afford slate.
Only a handful of slate-roofed houses are known
prior to about 1750.
They include Thomas Hancock’s stone home, well-known for its early use of granite and brownstone, and
the Slate Roof House in Philadelphia, built around 1690, which achieved notoriety for its singular use of slate.
⁸
In Canada about the only group who could afford the stone were the Jesuits, who clad a church, a college, and a convent in
Quebec with French slate.

Slate for roofing in seventeenth- and eighteenth-century America generally came from Wales or England.
Old World slate had
several advantages over the domestic supply.
Welsh slate cost the same or less and builders thought it looked better and was
of higher quality.
Transporting Welsh slate was also easier because it could be shipped from seaport to seaport, whereas a
lack of trains or canals often made transport Of American slate challenging.

According to slate-roofing historians, the first commercial slate quarry in the country opened about sixty miles east of Philadelphia,
near the towns of Delta and Peach Bottom, Pennsylvania.
A Welshman discovered slate there in the 1730s, but operations didn’t
start officially until 1785.
To honor this quarry and its history, the Maryland Historical Society has erected a sign just
south of Delta, on the Maryland side of the Mason-Dixon Line, highlighting this eminent fact.
The sign also reports that Peach
Bottom Slate was judged “Best in the World” at the “London Crystal Palace Exposition of 1850.” Never mind that the London
exposition took place in 1851 and the only slates to win an award in London in 1851 came from Wales and Sardinia.
⁹

After the initial Pennsylvania discovery, as well as ones in Virginia, New York,Vermont, and Maine, the American quarries
lumbered along with little growth.
Not until the 1840s did the slate industry start to grow.
Slate historian and consultant
Jeff Levine has cited three factors in the switch from Welsh slate to American slate.
First was the spread of railroads, which
lowered the cost of shipping.
Second was the introduction of architectural pattern books by people such as Calvert Vaux and
Andrew Jackson Downing, who advocated ornate roofs as a design element and stressed the beauty and colorfulness of slate.
Vaux in particular promoted the use Of American slates over Welsh.

Levine’s research into the history of the U.S.
slate industry showed that the most significant factor was Welsh immigrants,
who became the backbone of the industry.
They began to arrive in large numbers in the 1840s, driven by poor working conditions,
poor pay, strikes, and food shortages in their home country.
The Welsh immigrated to all of the American quarry regions, which
is why Pennsylvania, Maine, and Virginia have towns with Welsh-derived names such as Pen Argyl, Bangor, Arvonia, and Bethesda.

Slate quarrying, like all other stone quarrying in the 1800s, was labor intensive and dangerous, but it required a new twist,
one related directly to the geology of slate.
All slate quarried on the East Coast originated geologically as shale that later
metamorphosed into slate.
Central to the story was the Iapetus Ocean, which spread east from North America around 550 million
years ago.
Adjacent to the continent, the shallow water teemed with life, which led to deposition of limestone.
Farther east
lay a deep marine basin, out of which rose a volcanic arc.
Fine-grained mud washed off the mountains out into the basin through
submarine canyons, periodically interrupted by earthquakes that sent coarser sediments into the water and deposited beds of
sand atop the mud.
Deposition occurred slowly, on the order of one inch every three thousand years, and lasted from about
540 to 420 million years ago.

Environmental conditions did not remain stable throughout the millions of years of deposition.
During warmer periods, sea
levels rose and the water became stagnant.
Little oxygen reached the deep sediments, which slowed decomposition of organic
matter and facilitated the accumulation of material such as plankton.
Rich in carbon, these sediments turned gray to black.
During cooler periods, ocean circulation improved.
More oxygen mixed into oceanic waters and lightly oxidized the iron in
the sediments, turning them green and purple.
Animals burrowed into and churned up the mud, leaving behind well-stirred, homogenous
beds with few depositional features.

Nor did the volcanic arc remain stationary.
Plate movement carried the islands toward North America.
As the arc approached
it created a bulge in the seafloor, analogous to what happens when you push a carpet into a wall.
Sediment deposited on the
bulge accumulated in an oxygen-rich environment, which turned the mud brick red.

Not all of the slates along the eastern seaboard followed this exact plotline.
Red slate occurs only along the New York–Vermont
border, where the best beds of green and purple slate also are located.
¹⁰
Slate quarried in Pennsylvania,Virginia, and Maine is almost exclusively black or gray, but within the thousands of feet
of these sediments further variations exist.
¹¹
Thin bands of sand, called ribbons, appear periodically.
Some layers are richer in quartz, which makes the layers harder
and less suitable to use for blackboards.
Others have richer accumulations of iron-bearing minerals, which can weather and
change color after the stone has been quarried.
(Slaters refer to these varieties as “fading” or “weathering.”)

To change to slate, the shale had to pass through the geologic equivalent of a trash compactor, getting squeezed and compressed.
The walls of the trash compactor were the North American continent and a series of volcanic island arcs.
As the arcs pushed
into the beds of shale, the sedimentary beds began to fold, like when you hold either end of a piece of paper and move your
hands together.
Arc collisions happened 450 million, 410 million, and 370 million years ago.
Squeezing not only compressed
the rock by folding, it also drove out excess space filled by water in beds of shale.
The new metamorphosed rock, slate, was
harder and more dense than the original, unmetamorphosed shale.

Again, not all of the slates experienced the same level of metamorphism.
The aforementioned “Best in the World” Peach Bottom
Slate is harder than other slates because it was metamorphosed twice and at lower pressure and temperature, sort of like what
happens when you slow cook bread and it becomes dense and tough.
Its hardness led to its downfall because it cost too much
to cut and shape.
Quarriers determined that the best way to sell Peach Bottom slate was to grind it and use the granules on
asphalt roofing shingles.
You can easily determine hardness by wrapping a slate shingle with your knuckle; higher quality
shingles ring instead of thud.

Squeezing also generated two additional and critical features of slate.
Most important is the alignment of flat minerals such
as mica within the beds.
As the vise flattened the beds of shale, micas that were askew to each other began to rotate and
align, perpendicular to the direction of squeezing.
In addition, the heat and pressure from metamorphism dissolved parts of
minerals, so that the grains’ longest axes now ran perpendicular to compression.
Think of mineral realignment as creating
a rock with an internal structure akin to a deck of cards.
Geologists refer to this alignment of minerals as slaty cleavage.

Cleavage makes slate an unusual building stone.
For other sedimentary rocks, such as limestone and sandstone, the critical
factor is bedding, the layers of sediments that formed during deposition, because quarrymen exploit bedding to split or cut
stone.
In contrast, quarrymen rely on cleavage for giving slate its principal quality, the ability to be split into sheets.
Cleavage also gives slate its name; slate is a corruption of the French word
esclater
, to split.
No other rock relies on cleavage for splitting.

Bedding and cleavage affect the quarrying of rock in a similar way: Quarrymen exploit a zone of weakness to fashion a block
of stone.
The central difference between bedding and cleavage is that cleavage generates an almost unlimited number of planes
with which to split rock, because the split occurs between aligned minerals, whereas with bedding planes the split occurs
between layers, which can vary greatly in thickness.
If the bed is four inches thick, for example, it is hard to split the
rock into two-inch layers.
A quarryman can make a two-inch-thick layer but it requires cutting, a more machine-intensive process
than splitting.

Bedding and cleavage reflect a fundamental geologic difference.
Bedding is a first-order process.
It develops during the original
deposition of the rock.
Cleavage is a second-order process.
It develops after the rock originally formed, during a subsequent
change induced by pressure and temperature.
Metamorphosis also generates a secondary mineral alignment that affects quarrying.

During folding, elongate minerals such as quartz get reoriented and become aligned in the direction of the tectonic push,
comparable to what happens if you slide your hand into a pile of toothpicks and they get pushed and turned parallel to your
fingers.
Known as sculp, this alignment gives the quarrymen a plane of weakness running roughly perpendicular to cleavage,
as well as at an angle to the bedding planes of slate.

When the Welsh arrived in America in the 1840s, they knew better than anyone how to take advantage of cleavage, sculp, and
bedding to work slate.
They knew that in tightly folded rock such as slate, they needed to find a good bed, called a clear
run, and follow it.
Because the tectonic trash compactor pushed horizontally, beds folded vertically and clear runs often
dove steeply into the ground, resulting in the narrow, vertical holes that characterize slate quarrying.
To reach the rock,
the Welsh introduced a hoist and trolley system that would allow deeper penetration than the prevailing dredge and crane.
A trolley with a hoist ran along thick steel cables stretched between derricks on either side of the quarry.
Men would be
lowered into the vertical quarry on a wooden platform.
The deepest known quarry plunged nine hundred feet into the ground
at Pen Argyl, Pennsylvania.
An apocryphal story goes that a quarryman once said that the pit was so deep that “you could see
stars during the day from the bottom.”

Once down in the hole, the men had three planes of weakness to exploit.
They used a plug and feather system to break slate
along sculp.
They also could plug and feather along bedding.
Sculp and bedding basically defined the length and width of a
block.
After forming these two faces, quarrymen cleaved the slate into a block by driving wedges into the stone.
Whacking
and wedging, they would raise the block just enough to slide a chain under it.
Finally, they would lift out the block with
the trolley and hoist that had lowered them into the quarry.

Once out of the quarry, slate was sculped and split by hand.
After a block had been cut to a manageable size, a sculper took
his block of slate and chiseled a perpendicular notch into one end, in the direction of the grain.
He then placed a blade,
called a sculping chisel, into the notch and hammered it with a mallet.
Several hits later, a fissure propagated down grain
and the block cracked into two pieces, which advanced to the splitter.
Tilting his slab of slate vertically, so that it looked
like he was facing the short end of a book, the splitter placed a broad thin chisel parallel to the top and bottom sides and
tapped it with a mallet.
He continued to split each block in half along a cleavage plane until he formed sheets of the necessary
thickness, which has generally shrunk from about one inch to a quarter inch for roofing shingles.
A third man trimmed the
shingle with what looked like a dull paper cutter, notching and cutting, notching and cutting.
Depending upon its size and
where the trimmer worked, a shingle could be described as a “marchioness,” “double double double doubles,” “mumfat,” or “Rogue-why-winkest-thou.”
¹²