• Bedrock is the hard, solid rock beneath surface materials such as soil and gravel. Bedrock also underlies sand and other sediments on the ocean floor. Bedrock is consolidated rock, meaning it is solid and tightly bound. Overlying material is often unconsolidated rock, which is made up of loose particles.
     
    Bedrock can extend hundreds of meters below the surface of the Earth, toward the base of Earth's crust. The upper boundary of bedrock is called its rockhead. 
     
    Above the rockhead, bedrock may be overlain with saprolite. Saprolite is bedrock that has undergone intense weathering, or wearing away. Saprolite has actually undergone the process of chemical weathering. This means saprolite is not just less-consolidated bedrock, it has a different chemical composition. Flowing water or ice has interacted with minerals in the bedrock to change its chemical make-up.
     
    Above the saprolite may be layers of soil, sand, or sediment. These are usually ofter, younger, and unconsolidated rocks.
     
    Exposed bedrock can be seen on some mountaintops, along rocky coastlines, in stone quarries, and on plateaus. Often, these visible exposures of bedrock are called outcroppings or outcrops. Outcrops can be exposed through natural processes such as erosion or tectonic uplift. Outcrops can also be reached through deliberate drilling.
     
    People and Bedrock
     
    Identifying bedrock is an important part of geology, stratigraphy, and civil engineering. 
     
    Science
    Geology is the study of rocks and minerals. Stratigraphy is the study of rock layers (stratification). Stratigraphers study the way rocks, and their relationships to each other, change over time.
     
    Determining the depth and type of bedrock helps geologists and stratigraphers describe the natural history of a region. 
     
    For instance, the southern part of the U.S. state of Indiana has exposed bedrock. The northern part of the state is covered by meters of soil and unconsolidated rock. This landscape offers geologists a clue about how far glaciers extended during the Ice Age. The thick soil of northern Indiana was in part created as giant glaciers carved across the region's rockhead, grinding it into unconsolidated gravel. The bedrock of the southern part of the state experienced less weathering and erosion, and was left with less glacial till as the glaciers retreated.
     
    Bedrock also helps geologists identify rock formations. Rock formations, sometimes called geological or lithostratigraphic units, are sections of rock that share a common origin and range. 
     
    Rock formations help geologists create geologic maps. Geologic maps often display bedrock formations, usually in bright colors. Sandstone bedrock may be colored orange, while granite bedrock may be purple. 
     
    Geologic maps help scientists identify sites of orogenic events (mountain-building), for instance. A geologic map of the United States reveals a continuous bedrock formation, more than 400 million years old, stretching from northern Georgia all the way through Maine. This helps geologists identify the extent of the ancient Appalachian Mountain range.
     
    Engineering
    Civil engineers rely on accurate measurements and assessments of bedrock to build safe, stable buildings, bridges, and wells. 
     
    Aquifers, underground pockets of water, exist in porous bedrock formations, such as sandstone. Deposits of petroleum and natural gas can also be found and accessed by drilling through bedrock.
     
    Building foundations are sometimes secured by drilling to the rockhead. Soil and unconsolidated rock often cannot support the weight of a building, and the building may sag or sink. 
     
    Engineers also rely on bedrock to make sure bridges are safe and secure. To erect the Brooklyn Bridge, for instance, engineers created airtight cylinders to transport workers deep below the bed of the East River in New York, New York. These workers could then secure the bridge's towers directly to the bedrock. (One tower, at least! The Brooklyn tower is anchored in bedrock, while the Manhattan tower is anchored in the sand of the riverbed.)
    bedrock
    A tree grows in bedrock.

    Earth's Oldest Rocks
    In 2008, geologists announced that a swath of exposed bedrock in the Canadian province of Quebec was the oldest place on the Earths surface. The crust on the Nuvvuagittuq greenstone belt, on the eastern shore of Hudson Bay, is 4.28 billion years old, dating to when the Earth was still cooling from its formation!

    Meet the Flintstones
    The television comedy The Flintstones is set in the town of Bedrock. Geologic names are easy to spot in the cartoon: the Flintstones' neighbors are the Rubbles, Fred Flintstone works for Mr. Slate at Slate Rock and Gravel, Pebbles Flintstone works at Pyrite Advertising Agency, and the local newspapers are the Daily Granite and the Daily Slab. Fred Flintstone's lookalike is even named J.P. Gotrox. (Sound it out.)

    Bedrock Bacteria?
    In 2013, scientists discovered water that had been trapped in bedrock for more than a billion years. The water might contain microbes that evolved independently from the surface world—a finding that gives new hope to the search for life on other planets. 
  • Term Part of Speech Definition Encyclopedic Entry
    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    		 Encyclopedic Entry: aquifer
    assess Verb

    to evaluate or determine the amount of.
    bedrock Noun

    solid rock beneath the Earth's soil and sand.

    		 Encyclopedic Entry: bedrock
    chemical weathering Noun

    process changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions.
    civil engineer Noun

    person who works in the design and construction of buildings, roads, and other public facilities.
    crust Noun

    rocky outermost layer of Earth or other planet.

    		 Encyclopedic Entry: crust
    drill Verb

    to make a hole using a rotating digging tool.
    erosion Noun

    act in which earth is worn away, often by water, wind, or ice.

    		 Encyclopedic Entry: erosion
    expose Verb

    to uncover.
    geologic map Noun

    representation of spatial information displaying data about rocks and minerals.
    geology Noun

    study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.
    glacier Noun

    mass of ice that moves slowly over land.

    		 Encyclopedic Entry: glacier
    granite Noun

    type of hard, igneous rock.
    gravel Noun

    small stones or pebbles.
    Ice Age Noun

    last glacial period, which peaked about 20,000 years ago.
    landscape Noun

    the geographic features of a region.

    		 Encyclopedic Entry: landscape
    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.
    natural gas Noun

    type of fossil fuel made up mostly of the gas methane.

    		 Encyclopedic Entry: natural gas
    orogenic event Noun

    process of a specific mountain range or ranges being formed.
    outcropping Noun

    layer of rock visible above the surface of the Earth.
    particle Noun

    small piece of material.
    petroleum Noun

    fossil fuel formed from the remains of ancient organisms. Also called crude oil.
    plateau Noun

    large region that is higher than the surrounding area and relatively flat.

    		 Encyclopedic Entry: plateau
    porous Adjective

    full of tiny holes, or able to be permeated by water.
    quarry Noun

    site where stone is mined.

    		 Encyclopedic Entry: quarry
    rock Noun

    natural substance composed of solid mineral matter.
    rockhead adjective, noun

    upper boundary of a bedrock formation.
    root Noun

    part of a plant that secures it in the soil, obtains water and nutrients, and often stores food made by leaves.
    rubble Noun

    rough pieces of stone.
    sand Noun

    small, loose grains of disintegrated rocks.
    sandstone Noun

    common sedimentary rock formed by grains of sand compacted or cemented with material such as clay.
    saprolite Noun

    weathered or decomposed bedrock that essentially remains in its original site.
    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    		 Encyclopedic Entry: sediment
    soil Noun

    top layer of the Earth's surface where plants can grow.
    stratigraphy Noun

    study of rock layers and layering.
    tectonic uplift Noun

    movement of plates beneath the Earth's surface that causes one part of the landscape to rise higher than the surrounding area.
    till Noun

    rock, earth, and gravel left behind by a retreating or melting glacier.
    unconsolidated rock Noun

    minerals or sediments in the form of loose particles, such as sand or gravel.
    weathering Noun

    the breaking down or dissolving of the Earth's surface rocks and minerals.

    		 Encyclopedic Entry: weathering

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Jeannie Evers, Emdash Editing

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Caryl-Sue, National Geographic Society

Last Updated

June 7, 2013

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