During falling tides the foreshores of many beaches develop steep-headed incised drainageways that resemble and may be analogous to some larger scale drainage systems of both Earth and Mars. The beach systems are formed entirely by groundwater outflow and sapping where the water table intersects the beach face. At this intersection outflow dilates the sand and moves surface grains outward into the runoff. This forms gully heads, which then advance headward up the slope in directions largely controlled by directional permeability. Activity ceases when the water table falls below the level of the gully heads.
Rather extensive, though miniature, angulate-dendritic drainage systems can result from this process, but sapping has rarely been recognized as a major factor in the formation of larger scale drainage systems on Earth. On Mars, however, the analogy of the beach models may help explain the origin of some large valleys as a result of sapping by outflow of groundwater derived from thawed ground ice. Such relict effects of sapping may be recognizable on Mars because they have not been obscured by surface processes, as they tend to be on Earth. Re-evaluation of many terrestrial fluvial systems may reveal that sapping has been a common and important valley-forming process here too.