Well-exposed normal fault surfaces from Dixie Valley, Nevada, provide a unique opportunity to study the contact properties of fault surfaces, because it is possible to examine both the hanging wall and footwall surfaces of a fault zone which was once seismically active. The topography of the individual surfaces, as well as the aperture, or gap between the two surfaces, was measured using surface profiling instruments and a resin replication technique. These measurements enable frictional parameters of the surfaces, most notably the characteristic decay distance, Dc, to be calculated using contact theory. Although the individual surfaces are approximately self-similar and fractal, the aperture between the opposing surfaces is a stationary quantity because the surfaces are well correlated at dimensions larger than approximately 2 mm. Consequently contact theory can be used to calculate an average size for contact spots between the surfaces, and by inference, a characteristic decay distance, Dc, for frictional slip between the surfaces. Direct inspection of resin replicas of the aperture allows an independent estimate of average contact spot size. Both approaches give contact spot diameters of ≈0.16 mm, and Dc of 0.01--0.1 mm. Because the individual surfaces of the fault are fractal and approximately self-similar, correlation between the surfaces should decrease significantly during slip events, particularly if slip rates are high (seismic events). Decreased correlation between the surfaces should lead to larger contact spot sizes and hence larger Dc. Thus critical slip distances for natural faults should evolve concurrently with seismic fault slip. ¿ American Geophysical Union 1992