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.
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. 

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.)

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!

aquifer
Noun

an underground layer of rock or earth which holds groundwater.
assess
Verb

to evaluate or determine the amount of.
bedrock
Noun

solid rock beneath the Earth's soil and sand.
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.
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.
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.
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.
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.
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.
porous
Adjective

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

site where stone is mined.
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.
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.

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Editor

Jeannie Evers, Emdash Editing

Producer

Caryl-Sue, National Geographic Society

Last Updated

June 7, 2013

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