Understanding canyon incision on the margins of the Andes illuminates the uplift history of the range. Cotahuasi-Ocoña Canyon is one of the deeply incised canyons along the western edge of the Andes. Schildgen et al. have just published a study of canyon incision and knickpoint migration, using apatite 3He/4He thermochronometry. While the technique, based on the spatial distribution of 4He within an apatite grain, is a bit of a head-scratcher, especially to non-geochemists, it is extremely powerful, giving a detailed cooling/exhumation history from a single grain, rather than the multiple samples that more traditional thermochronological techniques require. Schildgen et al. deduce that the incision of the Cotahuasi-Ocoña canyon took place by knickpoint migration, rather than the uniform onset of rapid incision.