On the foreshore east of Luc-sur-Mer, Normandy, an Upper Bathonian hard-ground which caps a sponge biostrome has been thrown into a series of troughs and ridges, the latter sometimes with very steep flanks. Faunal orientation and encrustation of extensive undercuts demonstrate that these structures are of a syn-sedimentary origin. They are interpreted as submarine pseudo-anticlines which have a composite origin. The following three factors are thought to have contributed to their formation: (a) a primary morphological differentiation of the underlying biostrome; (b) sediment expansion caused by force of crystallization during hardground formation; and (c) differential compaction, being greater where the hardground overlies mainly inter-reef mud as compared with the rigid sponge framework. The main episode of this compaction occurred at an early stage in the life of the hardground. Subsequent erosion, colonization by encrusters and borers, and burial beneath a fossiliferous clay were the final stages in the history of these pseudo-anticlines.