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, 13 July 2012

RIGS of the Month [July] - Cliff Quarry, Compton Martin

RIGS of the Month – July 2012
Cliff Quarry – Compton Martin

Dedicated to the work of Cliff Salter.

General view of Cliff Quarry East face
Picture credit Richard Kefford
A larger picture here.

Geological map showing location of Cliff Quarry.
Picture credit - BGS.
                                                              A larger picture here

Gallery of quarry, fossils and crystals here.
Picture credits Richard Kefford.

Please follow the Geologist's code.

Location: Near Compton Martin ST 541 568

Access: Turn South into lane in Compton Martin called ‘The Coombe.’ Follow lane to end and then follow short section of footpath ahead up into Compton Wood where Cliff Quarry is on the left. Parking is limited in the village.

Risks: Keep away from rock faces. Hard hats should be worn.

Topography: Rough ground, steep slopes and unstable cliff faces.


Oxwich Head Limestone at East Cliff Quarry. 
Picture credit Richard Kefford.

A larger picture here

General description
        There are two parts of Cliff Quarry.  Access is better at the East as the West Quarry is overgrown.

       The East Quarry is adjacent to the 36 mile (58 km) ‘Limestone Link’ footpath. This path connects the Carboniferous Limestone of the Mendip Hills with the Jurassic Limestone of the Cotswolds and so bridges more than 100 million years of geological time.

       The quarry was used to supply hard road stone but was closed in 1956 as the big Eastern Mendip quarries had better transport links to the main markets in the South East of England.

        Slightly to the East at ST 543 566 are the Compton Martin ochre mines.
        The ochre mining industry of the Avon RIGS area will be the subject of a future feature on this blog.

Geological context.
        The quarry is cut into the Oxwich Head Limestone (previously known as Hotwells Limestone) which was deposited in a shallow shelf sea during the Asbian – Brigantian  stage of the Early Carboniferous, 327 – 334 million years ago. The Southern part of the British Isles then lay just South of the equator. It had a tropical seasonal climate, probably driven by monsoons.

        The upper part of the quarry exposes Triassic Dolomitic Conglomerate rocks of the Mercia Mudstone Group. These form a drape of varying thickness around most of the Mendips. They also form infilled wadis that were cut by flash floods during arid Triassic times. The adjacent ochre mines were developed in this formation.

        The quarry is located on the North Eastern limb of the Blackdown pericline which is one of four that are set en echelon to form the structure of the Mendip Hills. 

        The rocks in the quarry dip about 400 to the North East. The strata of the Blackdown pericline dip more steeply to the North than to the South as it is an asymmetric anticline.

        The Oxwich Head Limestone Formation is up to 183 m thick in the area of the quarry. The type section is at Oxwich Head on the Gower. It is part of the Pembroke Limestone Group.

A      Cross section of Blackdown Pericline.      B
Picture credit - BGS

A larger picture here
Key to geological units on cross section. B à A

MMG -       Mercia Mudstone Group
CM    -       Coal Measures
DCG  -       Dolomitic Conglomerate - can be seen in quarry
QSG  -       Quartzitic Sandstone Formation
OHL  -       Oxwich Head Limestone - can be seen in quarry
CHI   -       Chinastone
CDL   -       Clifton Down Limestone
ChO  -       Cheddar Oolite Member
ChL   -       Cheddar Limestone Member
BO    -       Burrington Oolite Subgroup
VL     -       Vallis Limestone
BRL   -       Black Rock Limestone Subgroup
AvG   -       Avon Group ( Lower Limestone Shales.)
PO    -       Portishead Formation ( Old Red Sandstone )

Scientific importance
        This quarry is important because of the previous fossil content and the knowledge that this has added of the geology of this early part of the Carboniferous. This includes the discovery of a new species.

        The work was carried out on the fossils by Cliff Salter, who lived in Compton Martin. His collection of fossils is now held by the British Geological Survey (BGS) at Keyworth, Notts..

        This work has been recorded by Murray Mitchell and published by the Mendip Society.
        The many fossils that have been found in a section of this formation in the past are listed here pps 489 - 490

        A scientific paper based on the work carried out by Cliff Salter is here. It describes a new species, Cyclus Martinensis. It is named after the village of Compton Martin. See picture on page 489 here

          A common coral fossil called Lithostrotion ( Siphonodendron ) can be found in this quarry.

      There is some further information on the implications of Cliff Salter's research here and here reproduced from  'Thornbury Geology Group Newsletter February 2012'. 
There is also a paper here that follows on from this.

The above texts, following on from Cliff Salter's work, show that the existing numbers of fossils from the Carboniferous seas may be badly skewed and may result in parts of the palaeontology text books being rewritten.


Thanks to Richard Ashley and Tarquin Bolton for their advice on identification of the fossils in the gallery.


The Salter Collection, from Cliff Quarry. Murray Mitchell. The Mendip Society.

A Walker’s Guide to the geology and lanscape of western Mendip. 2008. Andy Farrant. BGS.

Thornbury Geology Group Newsletter February 2012.

Richard Kefford. July 2012.

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