An aggregate grain is formed when two or more originally separate particles become cemented together by micrite. In Recent environments the best known aggregate grains are the grapestones of the Bahama Banks. These are composed of rounded particles, initially bound together by microbial filaments which later become sites of cementation. At the same time the component particles are micritised. In ancient cemented limestones, it may be

impossible in practice to distinguish between aggregate grains produced by a grapestone process and those which are reworked chunks of partly consolidated sediment.