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.