Experimental evidence demonstrating gravity-driven wetting front instability in an initially dry natural facture is presented. An experimental approach is developed using a transparent analog rough-walled fracture to explore gravity-driven instability. Three different boundary conditions were observed to produce unstable fronts in the analog fracture: application of fluid at less than the imbibition capacity, inversion of a density-stratified system, and redistribution of flow at the cessation of stable infiltration. The redistribution boundary condition (analogous to the cessation of ponded infiltration) is considered in a series of systematic experiments. Gravitational gradient and magnitude of the fluid input were varied during experimentation. Qualitative observations imply that finger development is strongly correlated to the structure of the imbibition front at the onset of flow redistribution. Measurements of fingertip velocity are used to develop a first-order relationship with fingertip length. Measured finger width is compared to theoretical predictions based on linear stability theory. ¿ American Geophysical Union 1994