Preservation of old palaeosurfaces in glaciated areas: examples from the French western Alps

Abstract

Abstract Remnants of deeply-weathered rock have been recorded in many regions which were ice-covered during the Pleistocene. They generally belong to shields and Palaeozoic massifs. This paper presents an example from the French Alps which testifies to the good preservation of very old (> 200 Ma) landforms in a high relief area situated on the western side of the ‘Grandes Rousses’ which was entirely covered by ice 30 000 years ago. Sedimentary structures, such as ripples on sandstone bedding planes and other erosional near-shore features, dating back to the Late Triassic before the development of the north, passive, margin of the Tethys ocean, are found in this high relief area. These are remarkably well preserved and occur in close proximity to typical glacial landforms. More generally, widespread elements of a pre-Triassic erosional surface are still recognized in the present landscape. Evidently, ice was unable to obliterate these 200 Ma old features in a high mountain area which was strongly glaciated 30 000 years ago.