In geology and geomorphology a base level is the lower limit for the vertical position of an erosion process. The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory. The "ultimate base level" is the surface that results from horizontal projection of the sea level under landmasses. It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.