AbstractRheological properties of ice and ice/rock‐fragment mixtures relating to rock glaciers are examined. The basic criteria for flow (neglecting sliding components) are discussed with reference to published work. Even if the internal composition of a rock glacier is very variable, ranging from pure ice to an ice/rock mixture, then similar surface velocities are likely to be measured and accounted for by at least two rheological models. Under some circumstances, even when there is ice in excess of the rock debris voids ratio, flow of the frozen material may not be possible. Uncertainty in the ice/debris composition in rock glaciers allows a wide variety of temperature, density, shear strength and creep properties to account for measured velocities. Ice lenses (segregation ice) must interconnect within a surrounding mass of stronger material for substantial deformation to occur. It is suggested that a glacier ice origin can account for the measured surface flow velocities of rock glaciers as long as the ice body is thin, generally<50m. It is usually considered that low flow velocities of rock glaciers are distinctive; however, previous conditions may have shown much greater surface velocities than at present. A test for the origin of rock glaciers is given based upon flow mechanical properties. If a rock glacier had its origin as a glacier then past velocities would have been higher than at present; if as a permafrost body, then there should not have been a major decrease. Evidence is presented to show that the latter may be the case in some instance