In addition to the global correlation lengths reported in Section 3.2.1, we also investigated correlation length scales locally in the vicinity of certain low-Σ GBs. This is accomplished by the same semivariogram process explained in Section 2.3, except that in Eq. (S3) we consider only pairs of GBs for which one of the GBs is the particular GB of interest (i.e. we fix one of the GBs). Because the resulting set of GB pairs is smaller than in the global analysis, we use larger bins with a width of 2 d • Ω for the local empirical semivariograms.