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 of loose particles.
Bedrock can extend hundreds of meters below the surface of 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 often, 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.)
