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ABSTRACT

Sand ridges along the coast of northern Brittany contain alternating beds of coarse and fine to medium sand 4–10 m above normal reach of highest spring tides. They were deposited in the Holocene, largely during the last centuries, by sand charged storm surges. Deformation structures at many levels in these beds have amplitudes of 2–25 cm. Fine-grained beds were folded in a manner that suggests relative competency of transmitting horizontally directed stress. Coarse-grained beds, by yielding, permitted folding to die out vertically. It is suggested that a transitional zone existed between the storm surge and the underlying sediment. This zone consisted of a mixture of water and sand. Deformation occurred at the moment movement in this zone was arrested. Comparison with published data on deformed bedding indicates that the described structures may be environment specific.