Hock Cliff – 30 June 2019
On Sunday the 30th July, UKAFH embarked on a fossiling foray in Gloucestershire, on the River Severn near a small village called Frampton on Severn – a location known as Hock Cliff.
We met at a private car park for which we were very grateful to have received permission to use. (Please note that if visiting Hock Cliff individually parking would need to be sourced elsewhere along the road heading north west from Frampton on Severn). From here, group leader Lizzy Hingley guided us on a short walk through fields before arriving on the riverbank where a cliff, at most 20m tall, is exposed for a stretch of just over 1000m.
The cliff and foreshore exposed at Hock Cliff are made up of the lower portions of the Blue Lias formation, an early Jurassic formation about 200 million years old. As well as Gloucestershire, The Blue Lias formation is exposed along the Somerset Coast, in Dorset near Lyme Regis, South Wales and even parts of Yorkshire, so the material here was not unfamiliar to many of the seasoned hunters on this trip. The Blue Lias consist of repeating bands of a blueish limestone and darker shale (The name Blue Lias is derived from the blue colour of the limestone, the colour itself is derived from the high concentration of pyrite in the stone). These sediments were formed in a warm, placid, shallow sea which would have been much closer to the equator than present day. The surprisingly formulaic nature of the repetitive limestone and shale bands can be interpreted as resulting from a Milankovich Cycle. This is a cycle in which the earth wobbles on its longitudinal axis – a result of gravitational influence from other celestial masses – which result in earth’s polar regions becoming closer, then further from the sun and so effecting global climate and sea level; and consequently the sedimentation of this Jurassic sea as finer sediments travel further from their source, effecting the type of sediment produced. Amazingly we can calculate the length of this cycle and so can interpret each section of limestone and shale to represent about 90,000 years of time, which unlike more homogeneous strata allows us to visualise the passage of time throughout the formation.
This location is incredibly fossiliferous with well-preserved examples from much of the familiar early Jurassic marine forna abundant here. Most notably the bivalve Gryphaea (Often called Devil’s Toenails) can be found in quantity but are remarkably well preserved, often with both valves intact and with pronounced growth rings, allowing you to calculate its age when it died. Ammonites are also common, often small and preserved in pyrite, hints of gigantic ammonites can be spotted in the limestone ledges. There is also a crinoidal bed where, if exposed, mudstone packed full of crinoid beautifully preserved in calcite can be found. Hock cliff also has its share of vertebrates including fish, Hybodus sharks and marine reptiles such as ichthyosaurs and plesiosaurs. Further down the River Severn the pterosaur Dimorphodon was found in the Blue Lias above Aust cliff so the chance of terrestrial fauna is not unimaginable.
After a short introduction to the geology and paleaontology of the area, fossils were being found by the group almost immediately. Matilda Brewer was first in with a fantastic Promicroceras ammonite preserved in pyrite and Barry Taylor found an exceptional example of crinoid bed which was not very abundant on this trip. I think everyone found a great example of Gryphaea, some with the tube casts of creatures who made their home on the shell preserved. Rob Howe struck lucky when he found a small, fairly worn but significantly uncommon ichthyosaur vertebra. However, the star of the show was by far the exceptionally rare and beautifully preserved Hybodus shark tooth found by Tegan Watts.
We were not just lucky with our fossil finds but also lucky with the fine weather, which had cooled slightly from the 30C+ the previous day. Importantly the ground was solid – if visiting this site in winter months or after heavy rain it is important to take extra care as the solid ground becomes dangerous mud.
Hock Cliff is a Site of Special Scientific Interest (SSSI) and so the cliffs and foreshore must not be hammered or dug into. However, there are plenty of loose fossils to be collected along the foreshore.
A big thank you to everyone who attended our hunt at Hock Cliff, it was a great day with a great group of fossil hunters! See you all again soon.