Polygenetic origin of Hrad Vallis region of Mars

A variety of apparently genetically related landforms in the Hrad Vallis region of Mars is examined. It is found that the outer valley of Hrad and the neighboring valley are composite features formed by karst or thermokarst processes and the growth and coalescence of smaller fault-aligned depressions. The materials in the vicinity may be soluble carbonates or sulfates or clay-rich sediment subject to thixotropic collapse. Water circulating within fault and fissure systems removed materials or altered the stability of materials to allow formation of steep-sided depressions. After the emplacement of surface materials in early Amazonian time, water was probably discharged to the surface. Isolate topographic depressions probably formed synchronously with the early release of water. Open lakes may have existed for a time.