In southern Aeolis Quadrangle in eastern Mars, parallel slope valleys, flat-floored branching valleys, V-shaped branching valleys, and flat-floored straight canyons dissect the heavily cratered plateau sequence. Associated knife-like ridges are interpreted as fissure eruption vents, and thin, dark, stratiform outcrops are interpreted as exhumed igneous sills or lava flows. Ridged lava plains are also common but are not themselves modified by fluvial processes. I mapped 56 asymmetric scarps or ridges that are probable thrust faults. These faults exhibit an orientation vector mean of N63¿W¿11¿ (95% confidence interval), and they transect the lava plains and the older plateau sequence units. By comparison, the vector mean for the 264 valleys mapped is N48¿W¿12¿, with a larger dispersion about the mean. The similar orientations displayed by thrust fault and valley axes suggest that valley locations are partly controlled by preexisting thrust faults and related fracture systems. Most valleys are also arranged orthogonally to, and along the perimeter of, the ridged plains. A possible model for valley development is: (1) freshly outgassed water became entombed as frost, snow, and ice within the cratered terrains during heavy bombardment and the accompanying deposition of impact ejecta, volcanic ash, and eolian materials, (2) effusive volcanism and lava sill emplacement heated subsurface ice in the vicinity of the ridge plains, and faults and fractures provided zones of increased permeability for water transport to the surface, and (3) headward sapping at thermal springs, thermokarst subsidence, and limited downvalley fluid flows then carved and modified the valleys. ¿ American Geophysical Union 1990