Formation and composition of a graded estuarine shell bed

The surface bed of a mudflat in the Ythan Estuary, Aberdeenshire, consists of mud and sandy mud overlying broken-shell material, the whole bed being graded. Production of shell fragments by bird predation and their subsequent incorporation in the bed due to the activities of a burrowing fauna are considered. The graded bed represents a stable situation in the mudflat area. Diagenesis of shells within the bed varies with species, Mytillus being prone to disintegration into fine needles of carbonate.