felsbasis

Felsbasis is a geological term of German origin that denotes the surface or boundary where bedrock forms the base of a geological unit. In practice, it refers to the contact between solid rock and the overlying material, such as soil, weathered rock, or unconsolidated sediments, and is used to describe the base level at which bedrock begins to influence engineering, hydrology, and geomorphology.

In mapping, borehole logging, and geotechnical investigations, the felsbasis marks the lower limit of the surficial, often weathered or unconsolidated, material and the upper limit of intact bedrock. It may correspond to the actual exposed bedrock surface at outcrop or to an inferred depth where bedrock starts below overburden. The term can describe either the surface of bedrock reached at the ground surface or the depth at which bedrock lies beneath soils and sediments.

The concept is significant because the felsbasis affects slope stability, groundwater flow, and the load-bearing properties of the ground. It helps determine foundation conditions for buildings, infrastructure, and tunnels, as well as influences excavation, mining planning, and hydrogeological assessments. Identifying the felsbasis typically involves a combination of field observations, boreholes, and geophysical methods to locate the transition from weathered or overlying material to solid bedrock.

Regional usage of the term is common in German-language geology and engineering literature, where it is sometimes translated as bedrock surface or base of bedrock. See also bedrock, overburden, saprolite, and geotechnical engineering.