Cores from two natural joints were studied to estimate the degree of correlation between the surfaces and the distribution of apertures. Matched pairs of profiles were taken from both surfaces of each joint. These profiles were then placed together to produce a linear trace of the apertures. Power spectra of the individual surfaces were compared to that of the apertures. Correlation between the surfaces is evidenced by the lack of long wavelength power in the aperture traces relative to the individual surfaces. The correlation lengths for the two joints studied are between 0.5‐5 mm. It is clear that the correlation between the surfaces must be considered, when studying the mechanical and transport properties of rock joint