Belemnite Marl	Black Ven Marl	Down Cliff Sands	Eype Clay Bed
Green Ammonite Bed	Junction Bed	Shales with Beef	Thorncombe Sands	Three Tiers Bed

Black Ven Marl

Overview

Originally known as the Black Marl Beds.

Black Ven Marls on Black Ven 

The Black Ven Marls (averaging 43m in thickness), are a succession of blue-black mudstones and shales containing the occasional limestone bands and nodules. Although the colour is due to the presence of finely disseminated iron pyrite, it does not denote that the sea-bed, at the time of the formation of theses beds, was always in an anaerobic state.

This bed is best examined to the east of Charmouth, between Evan's Cliff and Stonebarrow Hill, where these marls appear above the Birchi Tabular Bed (Bed 76a). 

Birchi Tabular Bed (Bed 76a)

The Hummocky Bed forms an intermittent sloping shore platform or foreshore ledge called Hawkfish Ledge, that is revealed at low tide. 

At 11m and 15m above the base of this division are the calcareous concretions belonging to the Lower Cement Bed (Bed 80) and the Upper Cement Bed (Bed 82 - sometimes called the Pavior Bed). At 17.6m above the base is the Flatstone Bed (Bed 83d). This is well renowned for its fine specimens of the Asteroceras turneri ammonite, which are preserved in yellow and brown calcite. Above this bed the remaining shales are richly fossiliferous with ammonites, Rhynchonellids and some insect and fish remains.  

The Coinstone Bed (Bed 89) at 0.8m above the nodules marks the top of the Obtusum Zone.

The horizons forming bed 83, are collectively known as the 'Obtusum Shales'. 

Although formed during a time when the sea level was fairly deep and often anoxic , the Coinstone Stone bed, the Stellare Nodule Bed (Bed 88f) were formed during a time when the sea had shallowed quite significantly, combined with erosion leading to some non-sequence occurring.

Exposures

Black Ven Marl as found between Lyme Regis and Charmouth:

Black Ven Marl as found between Charmouth and St. Gabriel's Mouth:

Fossils

The 0.5m thick Pentacrinite Bed (Bed 84b), is hard to find but has well preserved fossils of the crinoid Pentacrinites fossilis. The Pentacrinite stems, with their distinctive star shapes were known in the past as 'St. Cuthbert's Beads'. About 10m about the Flatstone Bed (Bed 83d)is a line of nodules called the Stellare Nodules (Bed 88f). 2.1m below the nodules is the Limestone with Brachiopods Bed (Bed 87) containing Rhynchonellids and bivalves. 

The Hummocky Bed (Bed 103), 13.7m above the Coinstone Bed (Bed 89) is taken as the top of the Black Ven Marls division. This bed contains the ammonite Echioceras. The Watch Ammonite Bed (Bed 99) is 4m below the Hummocky bed and also contains the ammonite Echioceras.

Echioceras raricostatum

The ammonite Asteroceras obtusum is also known as the Obtuse Star Ammonite, the Asteroceras stellare is also known as the True Star Ammonite and the Microderoceras birchi is known locally as both the Tortoise ammonite and the White Ammonite. Other ammonites found in this bed include Oxynoticeras oxynotum and Echioceras raricostatum.

Zones, sub-zones and fossils

Beds, geology and fossils

It is thought that there may be a non-sequence bed at this point...

Bed	Bed name	Geology	Fossils found
89	Coin Stone Bed (Coin Stone Ledge)	5" parallel sided impersistant nodular limestone with pittings and borings on upper surface	Promicroceras sp Cymbites sp Plagiostoma giganteum Fish remains
88f	Stellare Nodular Bed forming the Mumbles reef	1' regular compressed spherical nodules	Asteroceras stellare Promicroceras sp Inoceramus faberi
88e	-	10" pale marl	No fossils found in this bed
88d	-	3" pale marl with 4-5 beef strings	No fossils found in this bed
88c	-	4-8" irregular limestone nodules	No fossils found in this bed
88b	-	18" small irregular limestone nodules in pale marl	No fossils found in this bed
88a	-	18" bedded marl	No fossils found in this bed
87	Limestone with Brachiopods Bed originally called the Upper Cement Bed after Thomas Wright	10" tabular limestone	Xipheroceras aff dudressieri Cymbites ? Asteroceras ? sp Promicroceras aff planicosta Cymbites l vigatus Epopioceras sp Belemnites sp Plagiostoma giganteum Rhynchonella cf lineata Piarorhynchia gryphitica juvenis Tropiorhynchia sp
86g	-	4" conchoidal marl	No fossils found in this bed
86f	-	2" indurated marl	No fossils found in this bed
86e	-	2" conchoidal marl	No fossils found in this bed
86d	-	4" small irregular nodules in conchoidal marl	No fossils found in this bed
86c	-	2'10" conchoidal marl & occasional beef	Asteroceras aff margaritoides Asteroceras aff stellare Cymbites neglectus
86b	-	4" strings of beef in conchoidal marl	No fossils found in this bed
86a	-	3" conchoidal marl	No fossils found in this bed
85	Mudstone Bed	2-4" occasional small lenticular nodules in conchoidal marl sometimes containing zinc sulphide crystals	Eparnioceras flavum Eparnioceras semicostatum Xipheroceras dudressieri Xipheroceras aff dudressieri Xipheroceras cf rasinodum Xipheroceras sp Asteroceras margaritoides Asteroceras aff margaritoides Asteroceras aff stellare Asteroceras sp Promicroceras aff planicosta Promicroceras aff perplanicosta Promicroceras aff capricornoides Promicroceras sp Cymbites l vigatus Cymbites neglectus Cymbites sp Inoceramus faberi Scelidosaurus harrisoni
84e	-	3-9" conchoidal marl	Asteroceras sp
84d	-	3" occasional small nodules in conchoidal marl	Fish remains Xipheroceras aff dudressieri
84c	-	5'6" conchoidal marl, paper shales & beef	No fossils found in this bed
84b	Pentacrinite Bed	1' paper shales, beef, FeS and 1" crinoid remains	Pentacrinite fossilis Extracrinus briareus
84a	-	3'8" paper shales, beef, FeS and lignite	No fossils found in this bed
83i	-	5' 4" paper shales, occasional bedded shales and beef	Asteroceras sp Promicroceras aff perplanicosta
83h	5" bed formally known as Stonebarrow Flatstone Bed or Upper Flatstone Bed. Recent geological surveys have since failed to find this bed
83g	-	1' bedded conchoidal marl	No fossils found in this bed
83f	Woodstone Bed	2' paper shales & lenticular limestone nodules	Asteroceras obtusum
83e	-	2' bedded conchoidal marl	No fossils found in this bed
83d	Flatstone Bed (Lower Flatstone Bed)	18" paper shales & lenticular limestone nodules	Xipheroceras aff dudressieri Asteroceras obtusum Promicroceras aff precompressum Cymbites aff l vigatus Chalamys calva
83c	-	18" conchoidal marl	No fossils found in this bed
83b	-	10-12" hardened shales	No fossils found in this bed
83a	-	3'6" conchoidal marl & occasional nodules	Chalamys calva
82	Upper Cement Bed (Pavior Bed)  forming Dover Ledge reef	1' impure & irregular limestone	Promicroceras sp Chalamys cf lohbergensis
81q	-	6-8" conchoidal marl	No fossils found in this bed
81p	-	1-6" very irregular limestone with stellate nodules	No fossils found in this bed
81o	-	5" conchoidal marl with cruciform & stellate nodules	No fossils found in this bed
81n	-	3'6" conchoidal marl	No fossils found in this bed
81m	-	1" indurated marl	No fossils found in this bed
81l	-	2'3" conchoidal marl	No fossils found in this bed
81k	-	2" strong indurated marl	No fossils found in this bed
81j	-	3" conchoidal marl & beef with layer of FeS	Promicroceras sp Promicroceras capricornoides Promicroceras pyritosum Microderoceras inexpectans
81i	-	15" conchoidal marl & beef with layer of FeS	Promicroceras sp Promicroceras capricornoides Promicroceras pyritosum Microderoceras inexpectans Arietites sp
81h	-	1" strong indurated marl	No fossils found in this bed
81g	-	3" conchoidal marl	No fossils found in this bed
81f	-	2" indurated marl with flattened nodules	No fossils found in this bed
81e	-	5" conchoidal marl	No fossils found in this bed
81d	-	2" indurated marl	No fossils found in this bed
81c	-	3' conchoidal marl	Microderoceras cf inexpectans
81b	-	1" indurated marl	No fossils found in this bed
81a		4" conchoidal marl	No fossils found in this bed
80	Lower Cement Bed forming Black Rock reef	1' tabular persistent limestone [actually an argillaceous dolomite of 84% (by weight) of dolomite and 2% calcite]	Promicroceras cf capricornoides Promicroceras aff pyritosum Promicroceras sp capricornoides
79	-	-	-
78	Blocky Marlstone	3" blocky indurated marl	Promicroceras cf capricornoides Promicroceras pyritosum
77b	-	13' conchoidal marl (coarser above/finer below)	Promicroceras pyritosum Promicroceras capricornoides Belemnites acutus
77a	-	0.5" beef seam with pyritised ammonite layer on upper surface	Promicroceras capricornoides Microderoceras inexpectans Cymbites neglectus Squamirhynchia belemnitica Rhynchonella sp
76g	-	1.5" conchoidal marl	Cymbites suncostulatus Arietites cf turneri
76f	-	4" small flat nodules of indurated marl & impurities	Promicroceras sp Arietites sp
76e	-	7" conchoidal marl	Arietites cf turneri
76d	-	5" larger flat nodules of indurated marl & limestone	No fossils found in this bed
76c	-	3" conchoidal marl	Arietites cf turneri Plagiostoma giganteum
76b	forms Mouth Rocks at extreme low tide under Stonebarrow	1-2' paper shales & conchoidal marl	Pholidophorus sp Promicroceras capricornoides Promicroceras perplanicosta Cymbites l vigatus Arietites cf turneri Microderoceras birchi Plagiostoma giganteum Inoceramus faberi
76a	Birchi Tabular Bed	1' irregular tabular to lenticular limestone with FeS on upper surface	Promicroceras capricornoides Cymbites l vigatus Cymbites cf semicostulatus Arietites turneri Microderoceras birchi

Photographs

The Black Rock Reef.
Formed by the Lower Cement Bed (Bed 80).
This can only be seen if the shingle has been removed from the 
beach under Stonebarrow Hill.

The Black Rock Reef seen at a low spring tide. The 'beach section' is covered by shingle.

The Lower Cement Bed (Bed 80) provides an easy marker along Evans Cliff
and Stonebarrow Hill.

Lower Cement Bed (Bed 80) as found in-situ along Evans Cliff and Stonebarrow Hill.

Birchi Tabular Bed (Bed 76a) provides an easy marker along the Black Ven cliff face,
just to the west of Charmouth.

A partially uncovered Flatstone from the Flatstone Bed (Bed 83d).

The relationship between the Birchi Tabular Bed (Bed 76a) in this division, and
the lower Birchi Nodular Bed (bed 75a), from the Shales with Beef division. 

The top of the Shales with Beef division meeting the bottom of the Black Ven Marl just to the west of Charmouth. The constant erosion reveals new part of the beds every time there is a landslip.

Black Ven Marl when it gets wet!
In this state it cannot support any weight, hence the landslips.

All images copyright © Graeme Caselton