What is bedrock?
Bedrock is the solid layer of rock underneath soil or sediment. Sometimes it is visible at land surface. Bedrock is a general name for geologic materials that are well lithified (or cemented). It includes igneous, metamorphic, and sedimentary rock types. Bedrock in Wisconsin is very old—hundreds of millions of years to over two billion years old. Wisconsin’s bedrock came from many geologic settings, ranging from the roots of ancient volcanoes to deposits on the floor of an ancient ocean that once covered much of the Midwest. Bedrock is different from Quaternary (or surficial) geologic materials, which are usually not lithified into rock and are much younger in age.
Why do we map bedrock?
Bedrock geologic maps show where different types of solid rock occur in an area. They also give information about the physical properties of the rocks. This is useful because the physical characteristics of different bedrock units vary greatly. Through bedrock geologic mapping, geologists learn what bedrock unit is present either at land surface or beneath uncemented surficial deposits.
Bedrock maps are helpful for land use planners, local governments, farmers, businesses, private citizens, and scientists. For example, land use planners use bedrock maps to help choose locations for landfills and highway corridors. Bedrock maps also show farmers and agronomists the distribution of fractured bedrock. These fractures can allow surface contaminants like manure or pesticides to seep into the groundwater.
Bedrock is also used in many industries in Wisconsin, so mapping bedrock geology helps interested groups understand what resources are available or what potential problems they might encounter. Some units are better than others as sources of road aggregate materials and frac sand. Some geologic units have higher risk of rockfalls along roads, and some may be more likely to contain metallic mineral deposits. Groundwater wells drilled into rocks with metallic mineral deposits may also be more at risk for water contamination.