Campanian (Upper Cretaceous) Chalk exposed in a quarry near Portsmouth is unusual in the occurrence of a contemporaneous anticlinal fold, probably diapiric in origin. Hardgrounds and local slumps developed over the structure, and part of the chalk succession thins considerably over its crest. Erosional channels, sometimes containing bioclastic lag deposits, formed at two levels. Movement on the anticline occurred three times during the part of the Lower Campanian represented in the section; the third phase resulted in extensive fracturing of the crestal region of the fold. Burial of the structure and the onset of normal chalk deposition concluded the Cretaceous phase of its history.