Using ground-penetrating radar (GPR), we studied an entire 2.2 km long rock glacier (3780-4350 m asl) in the dry Andes of Chile with the aim of inferring its composition. In the high-quality, unmigrated data, we identified the active layer base and the rock glacier floor. In between, hyperbolae generated by diffracting boulders were inventoried; the ones along the rock glacier floor (n = 51) allowed determination of the average electromagnetic (EM) velocity in the rock glacier, the latter being further used for migration. Within the rock glacier (16-39 m thick), the EM velocity varies between 0.076 and 0.167 m.ns(-1); the main stratigraphic features observed are upward-dipping reflectors. The low EM velocities (<0.10 m.ns(-1)) found at some locations suggest the presence of significant unfrozen water fractions. A strong (R-2 = 0.77), inverse linear relationship was also found between the diffracting point density and the EM velocity, and was used to indicate the ice content in the rock glacier. The fraction of ice in the rock glacier was estimated to vary between 0.22 and 0.83, with an average of 0.66; these results were tested by recalculation of the EM velocity. The relationships between rock glacier development and glacial processes are questioned. Copyright (C) 2015 John Wiley & Sons, Ltd.