On 14 January, UKAFH commenced its packed 2018 schedule with a long-awaited return to Aust in Gloucestershire.
Aust is a small village preceding the Severn River Crossing into Wales. From here, you can access the River Severn foreshore, and the iconic red and grey cliffs visible to commuters travelling from the Principality. The slipway from which access is gained to the foreshore was under maintenance for much of 2016 and 2017, so UKAFH had not led a hunt here for over two years – it was good finally to be back. And, as if we were all sticking to our new year’s resolutions, the hunt was perfect – great finds, great weather and a great group of intrepid fossil hunters.
We began with an explanation of the geology and were shown examples of likely finds by hunt leader, Lee-Anne Collins, and support leader, Sam Caethoven.
The cliffs either side of the Severn Crossing’s huge concrete plinths are visually impressive and illustrate a period of coastal transition, some 221-201myrs ago during the late Triassic. The towering red mudstone of the Branscombe Mudstone Formation represents a 15myr period of seasonal lacustrine deposition in which the depth and salinity of an ephemeral lake fluctuated dramatically. This is evident in the streaks of white evaporite gypsum that stutter throughout this section of the cliff. The environment was hypersaline and inhospitable, that is, an environment not suited to macrofauna and, as such, is unfossiliferous.
Above the red mudstone sits the Blue Anchor Formation. Although this green-grey mudstone represents a time of better sustained water level in a lake or lagoonal environment – evidence of an advancing shoreline – it was still hypersaline and so devoid of macrofossils. It is above these strata, at the base of the Westbury formation, where the fossils we hoped to find exist. At the base of the shale and limestones of the Westbury Formation is the Rhaetic Bonebed – a green-grey calcareous siltstone conglomerate about 205myrs old, which, in contrast to the underlying strata, is incredibly fossiliferous. The Rhaetic Bonebed represents a marine environment, which was shallow, brackish and subject to strong tidal currents and/or storm events. This is evident in the conglomerate nature of the deposit and disarticulation of vertebrate fossils, which commonly include the teeth and bones of fish, sharks and marine reptiles, such as ichthyosaurs and plesiosaurs. Although marine life was clearly profuse, the seafloor was anoxic (without oxygen), aiding the preservation of fossils, which is most notable in the accumulation of coprolites. The bonebed is about 15cm thick, so represents a relatively quick but dramatic coastal transition. Later in the Westbury Formation, the sediment becomes more homogenous, representing a less turbulent marine environment, while the seafloor becomes more hospitable, shown by the abundance of bivalves and the bioturbation of sediment dwelling organisms.
The bonebed is, however, inaccessibly high in the cliff and so we had to hope for recent rockfalls to have bought this highly fossiliferous layer to the foreshore. On this occasion, we were in luck. Some large boulders had recently fallen, while smaller pieces and isolated fossils from the bonebed were easily found amongst the shingle.
As we headed east along the foreshore, it was not long before finds were made. Mary Bite began proceedings by finding a remarkably large Severnichthys tooth – Severnichthys is a large, predatory fish named after the River Severn along which its teeth are often found. Soon, most attendees were finding examples of bonebed, which contained a plethora of fossils, including tiny fish and shark teeth, isolated bones and copious coprolites. As we approached the concrete plinth of the Severn Crossing, Mike Greaves found an Hybodus shark’s tooth, again, of remarkable size.
We carefully made our way around the concrete plinth to the east side of the bridge, where some large sections of bonebed had recently fallen. By breaking up these blocks, many
fantastic fossils were found, including several Hybodus fin spines and beautifully ornamented fish scales. It is possible to treat rocks from the bonebed with vinegar to extract the fossils they contain – placing the rock in a sealed jar containing white vinegar of 6-7% and replacing the vinegar every few days will dissolve the rock. Sieving the sediment at the bottom of the jar will yield dozens of isolated teeth and scales, which you can observe in detail under a microscope or magnifying lens. However, it is worth noting that coprolites and bone often become fragmentary during this process. As the hunt drew to a conclusion, Mike Greaves made a rare and exceptional find in the form of a large Severnichthys jaw section, which clearly contained a number of large teeth. Mike is in the process of very carefully prepping and preserving this fragile specimen, and we hope to keep you updated about his progress on our Facebook page.
Thank you to everyone who joined us for our first fossil hunt of 2018. It was a truly great start to the year and it was great to meet so many new fossil enthusiasts. We hope to see you all again soon on another fantastic fossil foray.