Aust Cliff is half a hill, sliced open like a birthday cake by the River Severn’s slow knife, exposing two ornamental layers of pink and blue-green mudstone. Right here, about 200m years ago, a red desert was overwhelmed by a balmy ocean. Today, a cold northern sea mingles with a slowly churning river of mud, crossed at this point by the old Severn Bridge, once the longest single-span bridge in the world. The scale of this place, in time, space and ambition, is magnificent.
The estuarine sky is brewing rain. A rush of pebbles falls down the cliff with a sinister hiss – a reminder to keep our distance. Motley rocks jostle at our feet: pieces of the coloured mudstones (from the ancient desert) mingle with the rarer limestones and other marine deposits fallen from the top of the cliff.
We are after fossils from these upper strata, inaccessible until chunks are torn off by winter’s teeth – each storm brings down a little more. We are hoping for a piece of the elusive Westbury bone bed – a mixture of small pebbles and fossils stirred together by the sea and bound into a sandy matrix. It reminds me of Christmas pudding, generously studded with dark nuggets like ancient raisins. In fact, they are coprolites – small lumps of fossilised excrement from the ichthyosaurs and plesiosaurs of the late Triassic ocean. There are scales and teeth too, from reptiles, sharks and bony fish (including Severnichthys – the Severn fish) and sometimes larger fossils such as dinosaur bones.
We collect slices of pink satin spar, a form of gypsum in ribbon-like layers and veins; two tiny broken geodes, their crystal cavities split open; and a fossilised oyster shell, freed from its limestone bed. Then the rain comes, hard and fast. The skeetering stone-falls become continuous, louder than the rain, and we retreat, disappointed.
Back at home I pick up the best piece of bone bed we have collected to date. It is full of coprolites with dainty fish teeth as small as fennel seeds, as black as coal, as polished as pearls: a 200m-year-old piece of perfection in the palm of my hand. Next time…
