'Depth-to-rock' is a crucial component of the GlobalSoilMap.net project, because soil attributes should only be mapped where soil is physically present. We have used a Generalized Linear Mixed Model (GLMM) to map the probability that depth-to-rock exceeds a depth of interest in the soil profile. The GLMM has fixed effects based on gamma-ray spectrometry, and random effects based on a nugget + exponential covariance function. The model predicts with high accuracy in the shallow soil layers (<30 cm), but accuracy degrades with depth. This is at least partly because gamma-ray spectrometry only measures the top 40 cm of the soil profile. Future work must address the speed of the algorithm, and deal with heteroscedastic variance as depth in the profile increases.