Boundary Layer

[DeltaT]

There is a stationary layer of air against the surface of the wing as part of the Boundary Layer. A dirty/rough surface also has a stationary layer against it. Which would therefore mean subsequent layers of air in the Boundary Layer 'see' no difference between a smooth and rough surface.
So is the difference that the Seperation Point is further forward with a dirty surface?

[BEagle]

The transition point (not the separation point) will be further upstream if the surface is rough / dirty; such a surface will provoke turbulence and increase the depth of the boundary layer.