Silurian | Upper Devonian | Lower Carboniferous | Triassic | Lower to Middle Jurassic
Apart from microscopically sized fossil spores, few fossils occur in the Portishead Formation. An exception is a plant and fish-bearing conglomerate in the upper part of the unit at Burrington Combe, on the north side of Blackdown. This bed is a possible correlative of the Sneyd Park Fish Bed of the Bristol District, but unlike the latter, it does not occur at the base of the Avon Group, suggesting that the shallow marine conditions of the early Carboniferous took slightly longer to become established in the Mendips.