Present status and development of rock glacier complexes in south-faced valleys (45°N, French Alps)

The landscapes of the Vallon de la Route and Vallon de Pradieu (France) display typical geomorphological features of the Southern French Alps, with very few or no glaciers but a wide periglacial belt that extends from 2500 to 3100 m a.s.l. These valleys are unusual in that they contain several generations of rock glaciers that, from their rooting zone to their front, have developed in a topoclimatic setting characterised by high mean insolation (southerly aspect) and relatively low altitude. In this work, we determined the present status of these landforms, and more precisely the characteristics of the icy layers within the rock glaciers, via electrical soundings and thermal measurements, which we then combined with field observations. The permafrost zones in both areas are highly fragmented, whereas ground-ice can be present in landforms previously assumed as relict on the basis of their geomorphological characteristics alone. We used an empirical relationship between rock glacier flow velocity and terrain slope to estimate the time needed for both rock glacier assemblages to reach their present size. Our analyses therefore provide at the same time a broad relative chronological framework of the landscape setting up together with an overview of the spatial patterns of ice-rich permafrost features. It also suggests a number of hypotheses for the development of these landforms; however, further work involving more accurate dating methods is required to constrain these hypotheses.