The Formation of Slate Deposits

A look at Slate Deposits in the Lehigh Valley

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Developed and Produced by
The Delaware & Lehigh National Heritage Corridor

Written And Illustrated By
Lance Leonhardt

e-book Publishing software provided by Killer Interactive, LLC.
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Slate is ancient sea mud that was "cooked" by forces of the Earth long ago and turned into a hard rock that can be split and cut into large or small pieces that become products such as school blackboards, roof shingles, fence posts, and landscaping pavers.

Some of the world's largest deposits of slate are found in Lehigh and Northampton counties near the towns of Slatington, Chapman's, Pen Argyl, Bangor and Roseto.

Large quarries dug near these towns provided jobs for thousands of immigrants who helped build the region into the world's largest slate producer in the late 1800s.

Slate still is quarried today outside Slatington and Pen Argyl. Large mounds of discarded slate called "tailings" surround the towns and remind people of the role that slate played in the growth of the region.

Slate Deposits

Slate is ancient sea mud that was "cooked" by forces of the Earth long ago and turned into a hard rock that can be split and cut into large or small pieces that become products such as school blackboards, roof shingles, fence posts, and landscaping pavers.

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The story of slate begins about 470 million years ago, a time in Earth's history when Pennsylvania and the continent of North America were located near the equator. Great marine reefs located in shallow seas along the continent's eastern edge had been growing for millions of years.

Now they were threatened by a large arc of offshore volcanic islands that were slowly being pushed westward by tectonic forces generated deep below the Earth’s surface. The islands were on a collision course with the reefs and the North American continent.

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Moving land masses, called tectonic plates, move very, very slowly. The tectonic plate you live on is moving right now, but the movement is so slow that you will never detect it in your lifetime. The collision between North America and the volcanic arc of islands lasted 20 million years. The "shoving match" caused the volcanic islands to be pushed upward into a tall mountain chain; the edge of North America crumpled into a deep marine basin covered by seawater.

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Rainwater eroded the hard mountain rock into smaller rocks of all sizes called sediments. Sand, silt, and clay are three of the smallest sediments. Sometimes the silt and clay mixed with fine soil particles to make a "mud." Rivers that formed during this time carried the sediments and mud into the sea and deposited them on the bottom of the marine basin.

Formation of Slate

The story of slate begins about 470 million years ago, a time in Earth's history when Pennsylvania and the continent of North America were located near the equator. Great marine reefs located in shallow seas along the continent's eastern edge had been growing for millions of years.

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Marine Reef (Limestone Rock)
Volcanic Island Arc
Oceanic Crust
Mudclay
Continental Crust
Volcanic Island Arc Rock
Mudclay
Limestone Rock
Marine Basin
Eroded Sediment
Rivers & Streams
Rain
Eroding Mountains
Deposited Layers of Sediment (mud+clay)
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Sea currents also carried the sediments and deposited them far offshore in deeper water. Over time the sediments formed a continental shelf up and down the length of the continent. The side of the shelf that sloped steeply toward the deep ocean was called the continental slope. The area where the steep slope bottomed out and met the ocean floor was called the continental rise.

Sediments on the slope often accumulated and grew to great heights; portions of them sometimes broke away and slid down the slope, causing an avalanche. Sediments carried by the avalanches dug into the slope and rise and created submarine canyons and submarine fans of sediment. Over time these sediments accumulated into thick layers on the ocean floor.

Marine Basin

Sea currents also carried the sediments and deposited them far offshore in deeper water. Over time the sediments formed a continental shelf up and down the length of the continent. The side of the shelf that sloped steeply toward the deep ocean was called the continental slope. The area where the steep slope bottomed out and met the ocean floor was called the continental rise.

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Eroding Mountains
Shoreline
Submarine Canyon
Deposited Layers of Mud & Clay
Submarine Sediment Fan
Continental Rise
Continental Slope
Continental Shelf
Rivers & Streams
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The deep water of the continental shelf provided a dark and cold environment. Animals that lived there were bottom dwellers and fed mostly on food particles that floated down from the ocean surface. This undersea environment was dominated by creatures called brachiopods, trilobites, and crinoids.

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As the sediments collected into thick layers, the weight of the top layers squeezed - or compressed - the bottom layers into sedimentary rocks called mudstone and shale. Many of the sea creatures that lived on the continental shelf drifted down the slope when they died and became buried in the sediments. Some eventually became fossils when the sediments surrounding them hardened into rock.

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Some fossils in the mudstone and shale are made from tiny animals called graptolites, which lived together in colonies that floated on the ocean surface. Colonies of graptolites are one form of plankton. When graptolites died, they sank to the ocean bottom, became buried in the mud and clay, and fossilized as time passed.

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Over the next 200 million years, two major continental collisions occurred between North America and Africa that folded the layers of the deeply submerged shale. The second collision of the continents created the Appalachian Mountains some 280 million years ago.

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As the continental collisions occurred, some layers of shale were compressed, folded, and heated enough to be changed into a metamorphic rock called slate.

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The heat and pressure created during the folding process rearranged the clay particles in the shale. This rearrangement caused layering, or foliation, in the slate. Because it is foliated, slate can be split into hard, thin, water tight sheets that are used as roof "shingles" and floor tiles, and many other building materials.

Continental Shelf

The deep water of the continental shelf provided a dark and cold environment. Animals that lived there were bottom dwellers and fed mostly on food particles that floated down from the ocean surface. This undersea environment was dominated by creatures called brachiopods, trilobites, and crinoids.

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Trilobite
Crinoid
Brachiopod
Deposited Layers of Mud & Clay
Mudstone
Shale
Floating Graptolite Colonies
Graptolites
North America
Africa
Appalachian Mountains
Intense Folding of Shale Layers
Shale Metamorphosed into Slate
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Foliation
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A belt of slate and shale runs along the northern Lehigh Valley (the Great Valley section) toward the base of the Blue Mountain, also called Kittatinny Ridge. The Kittatinny Ridge is the southernmost ridge of the Appalachian Mountain chain in Pennsylvania.

The slate quarrying business in the Lehigh Valley began in the early 1800s. By World War II, the Lehigh Valley provided nearly half of the slate produced in the United States.

Lehigh Valley Slate

A belt of slate and shale runs along the northern Lehigh Valley (the Great Valley section) toward the base of the Blue Mountain, also called Kittatinny Ridge. The Kittatinny Ridge is the southernmost ridge of the Appalachian Mountain chain in Pennsylvania.

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South Mountain
Limestone
Shale
Slate
Blue (Kittatinny) Mountain
The Great Valley
  • Glossary

    Glossary

    • Avalanche (submarine landslide): A large amount of sediment falling, sliding, or flowing rapidly downhill, pulled by gravity.

    • Brachiopods: Marine animals (phylum Brachiopoda) having a soft-body enclosed within two connected shells on a fleshy stalk that attaches to the seabed.

    • Continental margin: The area from the shoreline to the deep flat, ocean floor, or abyssal plain, and includes the continental shelf, continental slope, and continental rise.

    • Continental rise: A section of the continental margin between the continental slope and the abyssal plain having a generally gentle incline and smooth topography.

    • Continental shelf: A section of the continental margin between the shoreline and the continental slope, typically having a gentle incline.

    • Continental slope: A section of the continental margin that is between the continental shelf and the continental rise, typically having a relatively steep incline.

    • Crinoids: Marine animals (phylum Echinodermata) characterized by a cup-like structure called a calyx (containing the mouth, digestive, and reproductive organs) surrounded by feeding arms on top of a stem attaching the animal to the bottom.

    • Foliation: The arrangement of parallel, sheet-like layers that lie at right angles to the flattened plane in metamorphic rocks like slate which have been compressed under great pressure.

    • Fossil: Any remains, trace, or imprint of a plant or animal that has been preserved in the earth’s crust since some past geologic or prehistoric time.

    • Graptolites: Now extinct marine animals (phylum Hemichordata) that formed branch-like, floating, planktonic colonies that became well-preserved fossils as they died, sank, and settled within the sea-bottom sediments that formed shale rock.

    • Marine (oceanic) basin: A large depression in the earth’s crust that is below sea level and covered by seawater.

    • Metamorphic rock: Any pre-existing solid rock that has been chemically and physically changed into another type of solid rock due to extreme heat and pressure.

    • Mud: A mixture of water and some combination of soil, silt, and clay, and often used to describe a sticky, fine-grained marine sediment.

    • Mudstone: A fine-grained sedimentary rock made from clay or mud.

    • Plankton: Organisms (animals, plants, protozoans, bacteria) that float or drift in water, especially at or near the surface.

    • Plate Tectonics: A scientific theory describing the movement of the Earth’s rigid, outermost layer of crust and mantle, the lithosphere. Large blocks or sections of the lithosphere, called tectonic plates, are moved horizontally in different directions over time by the asthenosphere, a layer of flowing molten rock below the lithosphere. The moving tectonic plates collide, spread apart, or slide by each other, and the boundaries between the moving plates are active areas for mountain building, sea floor spreading, volcanoes, and earthquakes.

    • Sedimentary rock: A layered rock resulting from the settling and packing of solid particles (such as sand and mud) that have been weathered, transported, and deposited at the earth’s surface and within bodies of water by wind, water, or ice.

    • Sediments: Solid fragments of materials of different sizes (mud particles to boulders) that have been broken down, transported, and deposited by wind, water, or ice.

    • Shale: A fine-grained, gray, red, brown or black-colored sedimentary rock formed by the compaction of mud, clay, and silt sediment deposits.

    • Slate: A compact, fine-grained, often gray-colored metamorphic rock formed under pressure from shale, a sedimentary rock. Slate is foliated, having thin layers that can be cut into smooth, flat sheets making it useful for roofing and floor tiles.

    • Submarine fan: A fan, or coned-shaped accumulation of sediment flowing out the seaward opening of a submarine canyon.

    • Submarine canyon: A steep-sided, trench or valley cut into the sea-floor of the continental shelf and continental slope. Submarine canyons may form where a large river enters the sea.

    • Trilobite: Small, extinct, marine arthropods having jointed-legs and an oval-shaped outer skeleton (exoskeleton) divided into three length-wise lobes with many horizontal segments. Among the most successful of all early animals, Trilobites were mostly bottom-dwelling predators, scavengers, and filter-feeders that lived in the oceans from about 520 million years ago to their extinction 250 million years ago.

    • Volcanic island arc: An arc-shaped chain of islands formed from volcanoes along the boundary between two colliding tectonic plates.

  • CC

    Lesson Text

  • Standards

    Pennsylvania Academic Standards

    7.1. Geography

    7.1.4.B.
    Describe and locate places and regions s defined by physical and human features.

    Geography Glossary

    Place – an area with distinctive human and physical characteristics; these characteristics give it meaning and character and distinguish it from other areas.

    Region – an area with one or more common characteristics or features that gives it a measure of consistency and makes it different from surrounding areas.

    3.2. Physical Sciences: Chemistry and Physics

    3.2.4.A4.
    Recognize that combining two or more substances may make new materials with different properties.

    3.3. Earth and Space Sciences

    3.3.4.A1.
    Describe basic landforms.

    Recognize that the surface of the earth changes due to slow processes and rapid processes.

    3.3.4.A2.
    Identify basic properties and uses of Earth’s materials including rocks, soils, water and gases of the atmosphere.

    3.3.5.A2.
    Describe the usefulness of Earth’s physical resources as raw materials for the human made world.

    3.3.4.A3.
    Recognize that fossils provide evidence about the plants and animals that lived long ago and the nature of the environment at that time.

    3.3.4.A4.
    Recognize Earth’s different water resources, including both fresh and saltwater.

    3.3.4.A6.
    CONSTANCY/CHANGE – Identify simple changes in the earth system as air, water, soil and rock interact.

    Earth and Space Sciences Glossary

    Atmosphere – the gaseous mass or envelope surrounding a celestial body, especially the one surrounding the Earth, and retained by the celestial body’s gravitational field.

    System – a set of interacting or interdependent entities, real or abstract, forming an integrated whole. An open system usually interacts with some entities in their environment. A closed system is isolated from its environment.

    4.3. Environment and Ecology

    4.3.4.A.
    Identify ways humans depend on natural resources for survival.

    Identify resources used to provide humans with energy, food, employment, housing and water.

    4.3.4.B.
    Identify the geographic origins of various natural resources.

    Environment and Ecology Glossary

    Natural resources – any materials produced by nature that can be used to produce goods or provide services.

  • My Map

    • Magnetite & Hematite ore Deposits
    • Limonite ore Deposits
    • Anthracite Coal Deposits
    • Slate Deposits
    • Surface Carbonate Rock Deposits
      (Limestone and/or Dolomite)
    • Limestone mined for cement
      (manufacture & cement plants)
    • Geography
    • Water
    • Counties
    • Cities
    Magnetite & Hematite ore Deposits
    Limonite ore Deposits
    Anthracite Coal Deposits
    Slate Deposits
    Surface Carbonate Rock Deposits
    (Limestone and/or Dolomite)
    Limestone mined for cement
    (manufacture & cement plants)
    Geography
    Water
    Counties
    County Names
    City Names

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