The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Friday 22 June 2012


Fossils of Avon RIGS region 

Please read the Geologist's code here:-

Name: Siphonodendron (formerly Lithostrotion) martini
Phylum: Cnidaria
Order: Rugosa
Horizon: Carboniferous

Photo credit - T. Bolton

Source rock: Clifton Down Limestone Formation

Age: Tournasian stage of the Lower Carboniferous Period (345 - 359Ma)

Locations: Avon George, Broadfield Down, Mendip Hills. Tickenham Ridge

Note: The underlined area is the type location for this Formation

Description of fossil

Siphonodendron martini corals were colonial, cylindrical and showed growth increment bands. Increment bands arise as a result of daily changes in light (which would affect the algae living in the outer cells of the organism) or temperature as well as monthly changes, associated with the lunar cycle. In the cool season or at night the corals secrete less calcite and so monthly and daily bands are visible on the surface of the corallites. Devonian corals studied at a different location show 400 daily bands in a year while these in the Lower Carboniferous have 391. This would indicate that the earth is slowing down on its rotational axis with tidal friction thought to be a major causal factor. The decrease in speed of the Earth's rotation is shown to accelerate in the Lower Carboniferous due to widespread occurrence of shallow shelf-seas.
Siphonodendron martini has a solid rod columella, major and minor septa (28 septa in each order) and dissepiments. In some areas many corallites did not reach maturity, only growing to 3mm. in diameter. Three ecotypes of the fossil are identified probably representing growth stages in the coral. The third ecotype is characterised by small radius, comprising only twenty septa and a single row of dissephnents. Some samples showed rejuvenescence i.e. the corallites reached adult dimensions then stopped growing only to recommence with smaller radius and more juvenile features. Measured growth was reduced from 4 mm a month in the limestone down to 2 mm a month in the calcareous shales where stunted growth is also observed to affect the diameter of the corallites. In the shale, rejuvenescence took place and there was little evidence of asexual breeding or monthly bands as the increase in nutrients from high levels of sedimentation is likely to have restricted breeding. Alternatively, a decrease in light levels  light may have affected the algae living in the outer cells of the organism, have reduced breeding and thus eliminated the monthly banding.

Photo credit - T. Bolton

Description of source rock ( Clifton Down Limestone )


During the Lower Carboniferous the British Isles were just south of the equator and under water. As the Laurasian continent drifted north a large delta carrying eroded sediments from the north deposited its load in the sea, eventually killing the coral reefs and starting the Millstone Grit beds – forming the Quartzitic Sandstone Formation in the Avon RIGS area.

The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.

Richard Kefford


- British geological Survey – Lexicon of rock units
- Natural History Museum – British Natural History
- The Black Country Geological Society – Newsletter No. 129.  June 1998.
- BGS. 1:50.000 series. Bristol 264 (S & D)
- BGS. 1:25,000 series Clevedon and Portishead. Geological Sheet ST 47.
- Post by T. Bolton on www.ukfossils.co.uk/ forum - photo credits

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