Fossils from the Avon RIGS area.
Gryphaea arcuata
Please read the
geologist's code here:-
Name
Gryphaea arcuata
Phylum
Mollusca
Class Bivalvia
Order Pterioidea
Genus
Gryphaea
Species
G. arcuata
Gryphaea
arcuata showing 'toenail' and ‘lid’.
Photo credit – Richard Kefford
Other, larger, pictures of this fossil can be found here
Source rock Lias
Group
Age range Triassic/Jurassic, Rhaetian 203 Ma – Aalenian 171 Ma
Locations Several
outcrops in area. The one pictured above was found at Hock Cliff in the
Sinemurian strata of the Lower Jurassic from about 194 Ma. SO 722 094
Please note that there are additional risks at this site which include:- unstable cliffs, difficult rocky and muddy foreshore, fast flowing currents and tides which can reach parts of the cliff.
Check tide times before venturing onto foreshore.
Please see comment below from Andrew Mathieson.
Please note that there are additional risks at this site which include:- unstable cliffs, difficult rocky and muddy foreshore, fast flowing currents and tides which can reach parts of the cliff.
Check tide times before venturing onto foreshore.
Please see comment below from Andrew Mathieson.
Description
of fossil
in the family Gryphaeidae. Commonly known as Devil’s
Toenails.
Their temporal range is from late Triassic
to early Cretaceous but their fossils are most commonly found in Jurassic strata.
These oysters lived on the sea bed
in shallow waters, possibly in large colonies. The complete fossils consist of
two articulated valves; a larger gnarly-shaped shell ( the “toenail” ) and a
smaller, flattened shell, the “lid”. The soft parts of the animal occupied the
cavity between the two shells, just like modern oysters. The shells also
feature prominent growth bands. The larger, curved shell sat within the mud on
the sea floor. These shells are sometimes found in fossil plates along with Turritella, Pentacrinites, Plagiostoma, clams, and sometimes shark’s
teeth and fossilized fish scales.
Description
of source rock in area – Lias Group.
Paleoenvironment
Nearing the
end of the Triassic, the landmasses remain united, in the form of the
supercontinent Pangea. The Tethys Ocean is actively spreading. The British
Isles are at about 300 North.
During the early Jurassic, Pangea breaks
up resulting in Gondwana and Laurasia separating as the South Atlantic starts
to rift open. The British Isles continue to drift Northwards to about 400
North. Lithospheric extension and passive rifting occurs to the East, starting
to form the North Sea and to the West where the North Atlantic will later open.
During the early Jurassic, sea levels
continue to rise across the British Isles so Triassic desert sediments are
replaced by a cyclic succession of fossiliferous dark grey marine mudstones,
marls and limestones.
Lithology
Predominantly
grey, well bedded, marine calcareous mudstone and silty mudstone; thin tabular
or nodular beds of argillaceous limestone, particularly in the lower part;
thicker units of siltstone and sandstone, particularly in the upper part, and
ironstone, particularly in the middle part. Marginal limestone facies also
occur.
Richard Kefford
Richard Kefford
References
- British geological Survey – Lexicon of rock units
I found Hock Cliff to be quite safe for children, with sensible precautions. A change of shoes and a carrier bag to put muddy boots in is welcomed by coach and car drivers. Over the years I have taken several thousand primary school age children there and it was always very popular.
ReplyDeleteAndrew Mathieson