William Morris Davis in the context of "Peneplain"

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 (the geoid). 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.

There are also lesser structural base levels where erosion is delayed by resistant rocks. Examples of this include karst regions underlain by insoluble rock. Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins. An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level.

In geology and geomorphology a pediplain (from the Latin pes, genitive case pedis, meaning "foot") is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The concepts of pediplain and pediplanation were first developed by geologist Lester Charles King in his 1942 book South African Scenery. The concept gained notoriety as it was juxtaposed to peneplanation.

The coalesced pediments of the pediplains may form a series of very gentle concave slopes. Pediplains main difference to W. M. Davis’ peneplains is in the history and processes behind, and less so in the final shape. Perhaps the most notable difference in form that may be present is that of residual hills which in Davis’ peneplains are to have gentle slopes while in pediplains they ought to have the same steepness as the slopes in the early stages of erosion leading to pediplanation.