On 15th September 21 UKAFH members set out to explore the geology and fossils of Doniford Bay in Somerset. Our group assembled at Doniford Farm Park with their kind permission, since the nearby public car park was closed at the start of the year. This allowed us to park and gather easily as well as providing a wonderful opportunity to purchase delicious lunches and a variety of local produce and wares as well as meeting the farm animals!
In the glorious sunshine of a late September heatwave, we walked as a group to the nearby beach access and descended to the bay. Sam set off to check the terrain and rock exposures on the beach and Aidan, the group leader for today’s event, provided some information to the group on the geology of the area and what the group might expect to find. Directing our gaze to the distant cliffs in the direction of Watchet, Aidan explained that the red layers which were clearly visible displayed a history of interchanging desert and aquatic conditions, with water encroaching on the landmass then retreating to give way to desert conditions. These varying states, occurring as Pangaea broke up and the Triassic period came to an end, finally gave way to full inundation by the ocean as the Jurassic period commenced. The change in condition brought marine life to the area which leaves its record in the rocks beneath and around us in this location.
The rocks at Doniford Bay represent the very earliest part of the Jurassic period beginning 201 million years ago. The most abundantly evident fossil is the ammonite Psiloceras planorbis, which is a zone fossil, which means it is recognised as being the defining biological marker for the start of the Hettangian stage 201.3 ± 0.2 million years ago, the earliest stage of the Jurassic period. All ammonites with the exception of the genus Psiloceras went extinct at the end of the Triassic so all Jurassic and Cretaceous ammonites are descendants of this genus.
Aidan continued to explain that the other ammonites we were likely to see demonstrate the evolution of this ammonite as it adapted and evolved to different niches of the newly opened up marine environment following the mass extincions of the end Triassic. Firstly we will notice Caloceras johnstoni, which, like Psiloceras planorbis is always crushed flat but maintains aragonite (mother-of-pearl) shell preservation which often demonstrates spectacular rainbow iridescence but which evolved a ribbed shell which would have afforded advantages, possibly affecting buoyancy and swim control or resistance to predation or other damage. This was succeeded by larger and more ribbed species like Arnioceras and Coroniceras which we hoped to see in situ in the wave-cut shale platforms.
The post-extinction sea quickly refilled with new life and Aidan informed the group that alongside the abundant ammonites we could also hope to find fishes, marine reptiles such as ichthyosaurs and plesiosaurs and plant remains.
We headed off as a group towards an area of loose rocks on the foreshore where we knew ammonites could be commonly found. Aidan pointed out some examples that were clearly visible and demonstrates how best to split the fragile shales, reminding group members to be safe using goggles and ensuring others were not close and vulnerable to flying chippings. He added that the delicate ammonites benefitted from preservation to bring out their colours and prevent deterioration and shared a tip that a smear of lemon juice was often effective in helping bring out the colour. Finally Aidan reminded members that they should only collect a few examples as no-one needs many identical specimens and there should be plenty left for others.
The group dispersed over the pebbly area and were quickly finding examples of Psiloceras and some Caloceras ammonites as well as some fragments of 3D ammonite. Chris Tait found a beautifully preserved example of Brachyphyllum, a cone- bearing plant which is known from the late Carboniferous to the Cretaceous.
After the group searched the loose foreshore pebbles Aidan took the group to the ledges of shale along the beach where we could observe the later, larger ammonites like Arnioceras and Coroniceras. The site is SSSI so hammering if in-situ rocks is mot permitted and these specimens cannot be extracted but we were all able to view, photograph and enjoy the many beautiful examples visible on the rock surfaces.
We were fortunate to observe, on close inspection, a small fossilised fish exposed in the rocks and one lucky group member found a small ichthyosaur vertebra. Also visible were crinoid ossicles and sea urchin spines.
As we returned to the slipway to leave the beach accompanied by the sound of the steam train whistles there was still a surprise yet to come – a beautiful, unusually preferved brachiopod found by Jonah.
UKAFH would like to thank Doniford Farm Park for allowing us to park for the duration of our excursion. Your pasties and pies make delicious lunches and we wish we could have stayed for the delicious looking carvery!
We hope everyone had an enjoyable day at Doniford and we hope to see you all again on future field trips.