We explain why snow pits across different sites in East Antarctica show visually similar isotopic variations. We argue that the similarity and the apparent cycles of around 20  cm in the δD and δ¹⁸O variations are the result of a seasonal cycle in isotopes, noise, for example from precipitation intermittency, and diffusion. The near constancy of the diffusion length across many ice-coring sites explains why the structure and cycle length is largely independent of the accumulation conditions.