  Soil creeps imperceptibly downhill, but also fails catastrophically to create
landslides. Despite the importance of these processes as hazards and in
sculpting landscapes, there is no agreed upon model that captures the full
range of behavior. Here we examine the granular origins of hillslope soil
transport by Discrete Element Method simulations, and re-analysis of
measurements in natural landscapes. We find creep for slopes below a critical
gradient, where average particle velocity (sediment flux) increases
exponentially with friction coefficient (gradient). At critical there is a
continuous transition to a dense-granular flow rheology. Slow earthflows and
landslides thus exhibit glassy dynamics characteristic of a wide range of
disordered materials; they are described by a two-phase flux equation that
emerges from grain-scale friction alone. This glassy model reproduces
topographic profiles of natural hillslopes, showing its promise for predicting
hillslope evolution over geologic timescales.
