Fossil Beach (Sedbury Cliffs)

Sedbury Cliffs and the foreshore contain similar geological layering to the more famous and visible Aust Cliff on the opposite bank of the River Severn. However, at Sedbury the cliffs are wooded at the top and overgrown with bushes and trees at the base, with fallen blocks and old trunks amongst them. Farther along the beach from the grass, sandstone blocks become more common.

The highest rock layer is the brown rocks of the Blue Lias formation, which was deposited between the Rhaetian Age (which closed the Triassic Period 201.3 million years ago (mya)) and the Ages of the Hettangian (201.3 to 199.3 mya) and Sinemurian (199.3 to 190.8 mya), the first two ages of the Jurassic Period. ^[3] The Blue Lias contains limestone, mudstone and siltstone, and this formation comprises the lower boundary of the Lias Group. The lias here can be thicker than at Aust. ^[4] Lias contains clays highly prone to expansion and contraction which aid landslip and collapse, therefore helping to facilitating the release of accessible fossils. ^[2]

The lias lies upon the pale grey Cotham member of the Lilstock Formation, which itself lies on the darker grey Westbury Formation. The Lilstock and Westbury formations, containing shale, limestone and sandstone, are included in the parent Penarth Group and they were deposited in the Rhaetian Age (208.5 to 201.3 mya) of the Triassic Period.

The next layer down is made up of the pale greenish- or bluish-grey mudstones and siltstones of the Blue Anchor Formation which, at the boundary, can share some of the same features with the overlying Westbury rocks. ^[5] The Blue Anchor rocks lie atop the Branscombe Formation which comprises the substantial reddish-brown mudstone and siltstone with some greenish-grey 'reduced' patches similar in appearance to the Blue Anchor rock. ^[6] The Blue Anchor and Branscome Formations, both laid down between the Norian (227 to 208.5 mya) and the Rhaetian ages, are included in the parent Mercia Mudstone Group (deposited from the early Triassic Period to the Rhaetian Age) which underlies much of the area, colouring the soil, and extending far north-east into Northern England. ^[7]

The layers of rock in the cliff were formed in shallow, sub-tropical seas when this region was closer to the equator. The fossils to be found on the foreshore are largely from the beds of Blue Lias, Lilstone and Westbury formations from the top of the cliff and reflect the marine nature of the deposits. Jurassic ammonites, brachiopods, bivalves and other sea organisms with shells are found in the collapsed rock. It is possible, although rarer, to find Triassic bones and sharks' teeth embedded in suitable 'bone rock'. The base of the Westbury formation is the Westbury Bone Bed, known for containing fossils of fish, reptiles and of course bivalves ^[8] .^[9]

Often overlooked is the fine sandy material amongst the pebbles on the foreshore, particularly further along the beach round the headland. This material, when sifted through, is an excellent source of tiny crinoid fossils, with the familiar sorting in this location into tubes and star-shaped segments - sometimes in stacks of several at a time - and can be easily found by the keen eyes of children or by using a hand lens.