Contenu du sommaire

Summary. — The work done in the western Alps by various electricity boards in the capture and control of sub-glacial torrents allowed us to study more thoroughly the different flow processes of temperate glacier and anabled us to define a série of new proposals about glacial erosion. This results from the combined and simultaneous action of three processes acting at the level of the glacier (erosion by sub-glacial waters and abrasion) and along its sides (proglacial breaking up). Only the erosion processes which occur under the glacier are discussed here. The melting of ice, a permanent and irregular phenomenon, which can be observed far upstream of the glacial front, is the origin of a concentrated and diffuse run-off. The erosion by sub-glacial water is both chemical and mechanical. Different intensities, evaluated here, produce three main shapes : torrential polish, sub-glacial channels and « gorges ». From the study of the « phreatic » sheet observed at the Argentière glacier, the idea of « glacial phreatic sheet » is assessed, giving emphasis to the hydrological, glaciological, and morphological effects. Observation of the sub-glacial sites shows that few rocks fragments (mainly sands) are dragged along by the glacier and beneath it. Splitting and plucking are a small portion of sub-glacial erosion, for it is clear that abrasion is predominant. The intensity of the abrasion depends on the quantity of rock inclusions coming into contact with the surface of the bed (maximum erosion on the lateral sides — beginning of the U shape which is characteristic of a glacial valley) and also on the velocity of bed slip, which depends itself on the sub-glacial hydraulic conditions (surgrooves and basins).

Abstract. — The Pacific piedmont of the Peruvian Andes is striking because of its forms : the coastal ridge above a littoral marked by a recent tectonic and the climatic changes of the Quaternary ; a succession of inland preandian basins, filled with tertiary and quaternary sediments ; with large furroved slopes which give access to the bed-rock of the volcanoes of the Western Cordillera. The geomorphological history must be cleared by study of the correlative sediments accumulated in the basins. The stepping disposition of the higher hills is most striking. The morphology of the coast is part of the tectonic and climatic evolution of this piedmont because of numerous raised beaches and tectonic cliffs.