A plateau finish is one that closely resembles a broken-in cylinder bore. When the bore is honed, the surface of the metal will have microscopic peaks and valleys. Peaks don’t provide much ring support, so as soon as the engine is started the piston rings start to scrub up and down and shear off the tallest peaks. As the engine continues to run, the peaks will be gradually shaved down until the cylinder bores are relatively smooth and flat (except for the valleys in the crosshatch that must be there to hold oil).
http://www.enginebuildermag.com/2002/...