Glacier ice in rock glaciers: a case study in the Vanoise Massif, Northern French Alps

Abstract. We investigated the Sachette rock glacier, Vanoise Massif, Northern French Alps, using former equilibrium line altitude reconstruction from glacial deposits, aerial photograph analysis, and ground-penetrating radar (GPR). The rock glacier is a young (probably <6000 yr) and active landform. The GPR survey consisted of two CMP measurements and four constant-offset profiles. From CMP measurements, the radar wave velocity in exposed shallow massive ice is 0.165–0.17 m ns⁻¹. The constant-offset GPR data was processed and analysed in order to reconstruct the stratigraphy and model the radar wave velocity in two dimensions. The integration of the morphology, the velocity models, and the stratigraphy emphasized, in the upper half of the rock glacier, the good correspondence between high radar wave velocities (>0.15–0.16 m ns⁻¹) and reflectors having a dipping-syncline structure, typical of true glaciers. Consequently, the rock glacier structure is described as being constituted of a glacial massive ice core embedded into diamictons. Our study of the Sachette rock glacier highlights possible significance of rock glaciers and interactions between glacier and permafrost in alpine environments.