Abstract
Ground-water sapping is an erosional process that produces landforms with unique characteristics. Sapped drainage systems differ in morphology, pattern, network spatial evolution, rate of erosion, and degree of structural control from their fluvial counterparts. Investigation of deeply entrenched theater-headed valleys in the Glen Canyon region of the Colorado Plateau indicates that ground-water sapping is the predominant mechanism of growth. The canyons occur in the Navajo Sandstone, a highly transmissive aquifer underlain by essentially impermeable rocks. Within this formation, two populations of valleys with markedly different features are identified. The first group exhibits theater heads: longitudinal profiles with high, step-like discontinuities and commonly asymmetric, structurally controlled patterns. The second group is characterized by tapered terminations; a relatively smooth, concave-up profile; and a more arborescent network. Because the valleys have developed under the same lithologic, stratigraphic, and climatic conditions, the differences in form are attributed primarily to structural constraints that determine the relative effectiveness of overland-flow and ground-water (sapping) processes. Of particular importance is the dip direction of the beds relative to that of valley growth, inasmuch as this relationship controls the occurrence and distribution of ground-water seepage at valley walls. Laterally flowing ground water also exploits fractures at depth, so that the drainage pattern of theater-headed valleys reflects that of the regional jointing pattern.
Martian valleys exhibit numerous morphologic similarities to canyons formed in the Navajo Sandstone. These include theater-shaped heads, nearly constant width from source to outlet, high and steep sidewalls, hanging outlets, and a large degree of structural control. Although the constituent materials, scale, climate, structure, and ground-water conditions of Mars cannot be replicated in any Earth analog, the striking similarities in form suggest that the gross geomorphic processes may be similar and that sapping processes have operated to create the Martian valleys.
