Evaluation of high-resolution images allows a systematic characterization of the walls of Dao and Harmakhis Valles, Mars. Variations in wall morphology are classified into three archetypes (exposed, mantled, and incised). Gullies are exposed by removal of semi-competent mantling deposits, suggesting progressive development of feature-types in an idealized sequence from i) mantled walls, to ii) incised mantle, and eventually iii) walls that display exposed gullies. Analysis of latitude, elevation, and wall orientation of 420 representative sample sites reveals spatial patterns that display preferential clustering of the two end member morphologies: exposed gullies are favored at lower latitude, higher elevation, and on east facing walls; mantled walls are more prevalent at higher latitude, lower elevation, and on south and southwest facing walls. These morphologies and their patterns are consistent with the hypothesis that gullies emerge from beneath an ice-rich mantle that is degrading in response to local variations in total solar insolation.