Nakul Deshpande, David Furbish, Paulo Arratia, Douglas J Jerolmack 

Soil-mantled hillslopes owe their smooth, convex shape to creep; the slow and persistent, gravity-driven motion of grains on slopes below the angle of repose. Existing models presume that soil creep occurs via mechanical displacement of grains by (bio)physical disturbances. Recent simulations, however, suggest that soil can creep without these disturbances, due to internal relaxation dynamics characteristic of disordered and fragile solids such as glass. Here we report experimental observations of creeping motion in an undisturbed sandpile, at micron resolution over timescales of 10^0-10^6 s, for a variety of natural and synthetic granular materials. We observe two behaviors typically associated with creeping glass: strain occurs as localized and spatially-heterogeneous grain motions; and creep rates decay as a power-law function of time. Further, creep can be accelerated or suppressed by thermal cycles and shaking, respectively. Averaged strain profiles decay exponentially with depth, in agreement with field observations of creeping hillslope soils. Our findings demonstrate that soil is fragile in terms of sensitivity to disturbances, but that creep dynamics are robust across grains and glasses. Mapping soil creep to the more generic glass problem provides a new framework for modeling hillslope sediment transport, and new insights on the nature of yield and failure.