Sir,

The observations of Reference LliboutrySchubert (1973) in the Venezuelan Andes take me back 20 years ago, when I observed the same features in the Andes near Santiago. Although at that time I was aware of Troll’s work, I had no doubt that the wind had nothing to do with it. It was a consequence of the formation of micropenitentes (very minute penitentes) in frozen soil, covered with pipkrakes at sunrise. Penitentes (Reference LliboutryLliboutry, 1954, Reference Lliboutry1956, Reference Lliboutry1964–65, tom. 1) are the consequence of an instability in the ablation process by the Sun’s rays in a cold atmosphere, and follow the direction of the Sun’s path, i.e. an east–west direction (which may be altered on steep slopes facing east or west).

Nothing in Schubert’s article compels me to change my opinion. Even if the correlation with the direction of the wind was very good (and it is not at all the case), a simple correlation is by no means a proof. Troll’s confusion came from the fact that microstriae are also seen on bare soil after gusts of wind. But they have quite different features (cf. figure in Reference LliboutryLliboutry, 1964–65, tom. 1, plate 29).

14 November 1973