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1 Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore BEDRO CK For the complete encyclopedic entry with media resources, visit: 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 1 of 5

2 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 (mountainbuilding), 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. 2 of 5

3 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.) Vocabulary aquifer Term 3 of 5 an underground layer of rock or earth which holds groundwater. assess verb to evaluate or determine the amount of. bedrock solid rock beneath the Earth's soil and sand. chemical weathering civil engineer process changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. person who works in the design and construction of buildings, roads, and other public facilities.

4 Term crust rocky outermost layer of Earth or other planet. drill verb to make a hole using a rotating digging tool. erosion act in which earth is worn away, often by water, wind, or ice. expose verb to uncover. geologic map representation of spatial information displaying data about rocks and minerals. geology study of the physical history of the Earth, its composition, its structure, and the processes that form and change it. glacier mass of ice that moves slowly over land. granite type of hard, igneous rock. gravel small stones or pebbles. Ice Age last glacial period, which peaked about 20,000 years ago. landscape the geographic features of a region. mineral inorganic material that has a characteristic chemical composition and specific crystal structure. natural gas type of fossil fuel made up mostly of the gas methane. orogenic event process of a specific mountain range or ranges being formed. outcropping layer of rock visible above the surface of the Earth. particle small piece of material. petroleum fossil fuel formed from the remains of ancient organisms. Also called crude oil. plateau 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 site where stone is mined. rock natural substance composed of solid mineral matter. 4 of 5

5 Term rockhead adjective, upper boundary of a bedrock formation. root part of a plant that secures it in the soil, obtains water and nutrients, and often stores food made by leaves. rubble rough pieces of stone. sand small, loose grains of disintegrated rocks. sandstone rock formed by grains of sand. saprolite weathered or decomposed bedrock that essentially remains in its original site. sediment solid material transported and deposited by water, ice, and wind. soil top layer of the Earth's surface where plants can grow. stratigraphy study of rock layers and layering. movement of plates beneath the Earth's surface that tectonic uplift causes one part of the landscape to rise higher than the surrounding area. till rock, earth, and gravel left behind by a retreating or melting glacier. unconsolidated minerals or sediments in the form of loose particles, rock such as sand or gravel. weathering the breaking down or dissolving of the Earth's surface rocks and minerals. Articles & Profiles National Geographic News: Oldest Rocks on Earth Discovered? Worksheets & Handouts Maine Geological Survey: Virtual Tour of Maine's Bedrock Geology National Geographic Society. All rights reserved. 5 of 5