  We show that a length scale xi can be extracted from the spatial correlations
of the "steep cliffs" that appear on fracture surface. Above xi, the slope
amplitudes are uncorrelated and the fracture surface is mono-affine. Below xi,
long-range spatial correlation lead to a multi-fractal behavior of the surface,
reminiscent of turbulent flows. Our results support a unifying conjecture for
the geometry of fracture surfaces: for scales > \xi the surface is the trace
left by an elastic line propagating in a random medium, while for scales < \xi
the highly correlated patterns on the surface result from the merging of
interacting damage cavities.
