Groundwater sapping

Groundwater sapping is a geomorphic erosion process that results in the headward migration of channels in response to near constant fluid discharge at a fixed point.

Small fans form at the apex of the rhombic features, which are eventually fed by the remaining backflow of water traveling downslope.

This contrasts with the more common branching or dendritic pattern of V-shaped valleys produced by overland flows that become wider with distance from their source.

[5] Weakened basal rocks are unable to support more resistant upper layers, causing valley head and sidewalls to collapse inwards.

[5] Theater-shaped channel heads are characterized by overhanging sidewalls that are relatively dry compared to the lower level rocks below the zone of seepage.

Scientists believe that groundwater sapping created these gullies in Noachis Terra on Mars. [ citation needed ] NASA image.
The two canyons at center-left in this photo from Canyonlands National Park in Utah have the theater-shaped heads typical of valley features shaped by groundwater sapping. The Green River is at right.