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

Junction Bed

Overview

The Marlstone Rock Bed forms the top formation of the Middle Lias. It consists of light grey sandstones and sandy greenish-grey or oolitic red-brown limestones, and is often conglomeritic. It is actually part of the rock band known as the Junction Bed, forming the lower part of it.

The upper layer Upper Lias Junction Bed is found as a thin dull white-pink fine-grained limestone overlaying, and cemented to, the lower (and older) layer, the Middle Lias Marlstone Rock Bed. It is a condensed deposit indicating shallowing of the surrounding sea, with several non-sequences and planed surfaces, containing ammonites representative of several horizons on the Lower and lower Upper Toarcian. 

The bed appears to consist of four different ages of formation. The lowest layer contains ammonites from the topmost zone of the Middle Lias. The other layers include ammonites from the Upper Lias. These four layers separate along planes which appear to have been eroded by sea action before deposition of the next layer.

Only 2m thick at Thorncombe Beacon, this formation is over 100m thick when examined on the Yorkshire coast at Whitby. It is also rich in ironstone, in many places the top 5-25cm of the formation has been weathered and altered, the original green iron minerals (such as glauconite, chamosite and vermiculite) having been oxidised to hydrous iron oxides (limonite). The formation also shows variations in the sand content.

According to 'Jenkyns and Senior', there is evidence for a minor palaeofaulting of this bed in the Eype Mouth area. This is the 'youngest' formation currently covered by this website. 

Exposures

Junction Bed as found between Seatown and Eype Mouth:

Junction Bed as found between Eype Mouth and West Bay:

Fossils

The ammonite Pleuroceras salebrosum and the brachiopod Tetrarhynchia tetrahedra have been found fossilised in the Marlstone Rock Bed.

Zones, sub-zones and fossils

Marlstone Rock Bed Zone
Pleuroceras spinatum
Sourced from Kato's Collections

Junction Bed Zones

 Sub-zone Fossils
Grammoceras thouarsense    
Haugia variabilis    
Hildoceras bifrons    
Harpoceras falciferum    
Dactylioceras tenuicostatum    

Beds, geology and fossils

Junction Bed

 Name Geology Fossils found
Striatulum   Hard grey-yellow limestone or earthy marl with limestone nodules. Top of bed planed off by erosion. Traces of variabilis Subzone welded to undersurface.
Bifrons   Hard limestone mottled yellow pink-red. Fine-grained to conglomeratic. Well marked erosion plane separates this layer from above layer. Ammonites Hildoceras bifrons and Dactylioceras commune.
Falcifer   Tough yellow-pink limestone, sometimes mottled red or green. Usually rests on planed-off top of the Marlstone Rock Bed. Ammonite Dactylioceras commune.

Marlstone Rock Bed

 Name Geology Fossils found
Upper Serrata Bed
Paltus Bed		 Fine-grained pale limestone. Richly fossiliferous.
Coarsely ribbed amaltheid ammonites of genus Pleuroceras spinatum.
Brachiopod Quadratirhynchia.
Lower Media Bed
Spinatum Bed		 Hard brown ironshot Marlstone

Photographs


A piece of Junction Bed picked up from the western end of the beach at Eype Mouth. The upper surface of the bed shows several thin layers, believed to be the fossilised remains of algal growths (the scum from the surface of a drying out lake?). Embedded within the bed are many Ammonite and shell fossils, as well as many small pebbles.

All images copyright © Graeme Caselton