The reason that the Y and Z holes post date the megalithic is more than just the irregularity ,there is the stratigraphic evidence which supports the belief and also the dating evidence from the antlers .That is convergent evidence with the stratigraphy alone being sufficient to confirm the belief .The irregularity is in their non -concentricity ,Thom described them as being more like a spiral than a circle . The surveyors managed fine with the Aubrey Holes why should they have suddenly have lost their abilities with the Yand Z holes ?