These marine mudstones are fossiliferous representatives of the Lower Jurassic or Lias.
The Polden Hills are capped by the Blue Lias Limestone, which in places forms a steep scarp.
The Permian may be represented, but the Trias is absent, and in general the older Palaeozoic rocks are overlaid directly by the Rhaetic and Lias.
It is, however, not impossible that the metamorphic series includes also some of the Lias.
Another well-known bed, formerly known as the "Bristol" or "Lias" Bone Bed, exists in the form of several thin layers of micaceous sandstone, with the remains of fish and saurians, which occur in the Rhaetic Black Paper Shales that lie above the Keuper marls in the south-west of England.