Evidence is presented for the origin of high-resistivity barriers at certain interfaces in high-field varistor materials. The barriers give rise to terrace contrast which can be observed using remote electron beam induced current imaging in the SEM. It is proposed that these interfaces, which are often associated with a thick intergranular layer of a compositionally distinct bismuth rich oxide, are relicts of the powder structure and correspond to agglomerate surfaces. It is suggested that the presence of these resistive surfaces ultimately limits the current that can flow at breakdown.