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RIGS of the Month [May] - Portishead foreshore

RIGS of the Month - May
Portishead foreshore

SITE SPECIFIC INFORMATION

Accessibility: Park on esplanade near Lido. All public access.

Risks: Best accessed on a falling tide as tides can reach base of cliff.
Hard hats should be worn near cliffs
Slippery rocks in tidal areas, steep slopes on Battery Point.
Strong tidal flows.

 

 Location map (numbers refer to sites in text). BGS Sheet ST 47 Clevedon and Portishead.

All photos from this post can be viewed in a larger format - https://picasaweb.google.com/charly.stamper/PortisheadPhotos

 

The foreshore at Portishead offers an opportunity to walk through a sedimentary succession from the Lower Carboniferous through to the Mid-Lower Devonian. There is excellent exposure of the Portishead beds, the rocks that form the core of the Mendips, and the area has been known for many years for the famous Woodhill fish beds where examples of early Devonian fish fossils have been found. Interesting features along the coast include Variscan folding, fossils (including fish scales from the Devonian fish beds), cross bedding, faulting and calcrete formation. 




1 – Battery Point

Black Rock Dolomite (BRD). Part of the Black Rock Group of the Carboniferous Limestone Series from the Tournasian Stage (359 – 345 Ma) of the Carboniferous Period (equivalent to the Lower Dolomite in the Forest of Dean). Here, the strata dips 40/50º to the North East.

Near to the old lighthouse there is an outcrop of hard Black Rock Dolomite (BRD). This limestone was laid down in a shallow calm sea and was later dolomitised by magnesium saturated fluids, resulting in partial replacement of the calcium with magnesium and destruction of much of the shelly matter. In places the rock is stained red-brown from the overlying Triassic sediments and also contains some chert horizons. The BRD is surrounded by Dolomitic Conglomerate which consists of limestone clasts cemented in a matrix; this material was eroded from the BRD and deposited in a skirt around Battery Point during the Triassic. 

 

Contorted strata can be seen from the seaward side of the old lighthouse. The beds were deformed as part of the Variscan Orogeny and are thought to be part of a south verging anticlinorium (a vast elongated anticline with its strata further folded into anticlines and synclines) with a normal northern limb and an overturned southern limb.

The Merchant Navy memorial stone on Battery Point is a large chunk of Roach Stone from the Portland series. Many fossils can be seen in it, especially bivalves and examples of the ‘Portland Screw’ -  a turreted gastropod - Aptyxiella portlandica.

2 - Battery cliff

The Lower Limestone Shales are also from the early Carboniferous and consist of alternating lenticular limestones and calcareous siltstones. The limestones contain skeletal debris and well preserved fossils whereas the siltstones are intensely bioturbated. The beds are dolomitised and red stained throughout.

3 - Woodhill Bay

The transition from Carboniferous to Devonian cannot be seen as there is a shallow valley to the South of Battery Point which is infilled with Triassic sediments. The limestone units in Woodhill Bay are made up of a mixture of highly abraded hematic skeletal debris and well preserved fossils of articulated crinoid stems and fairly common examples of the tabulate coral Vaughania (Cleistopora) which is the index fossil for the Lower Limestone Shales. These have been affected by the Triassic alteration and so have been heavily dolomitised and stained red by the iron oxides. There are many small folds to be seen on the beach which appear to be part of the larger fold system. 

  Triassic sediments in Woodhill Bay containing well-preserved crinoid stems from the Lower Limestone Shales

4 - Kilkenny Bay 

At the Southern end of the sea wall, cliffs of the Upper Old Red Sandstone (ORS) (including and including the Woodhill Bay fish beds) are well exposed. This is the Portishead formation which dips at low angles to the North. This means that the rocks get progressively older to the South, eventually changing into the Black Nore Sandstones (BNS) of the Lower Old Red Sandstones which continue to be exposed as far to the South as Charlecombe Bay. The total thickness of the ORS in this area is thought to be 900 – 1200 m.

The Woodhill Bay fish bed formation consists of sandy siltstones and sandstones where beds vary in thickness between 0.5 m and 3 m. Many fish have been discovered here in the past and fish scales can still be found.

The ORS is associated with the erosion of the Caledonide mountains formed by the collision of Avalonia, Baltica and Laurentia to form the ORS Continent, an event known as the Caledonian Orogeny which occurred about 425 to 395Ma. Most of the ORS deposits seen in this area exhibit complex cross bedding. Plotting the current directions has shown that the main source of the pebbles in the deposits is to the North West. It has been proposed that the source of many of these pebbles is the Precambrian Mona complex of Anglesey

Two post-Triassic faults can be seen, with the Triassic conglomerate being down thrown with respect to the Devonian strata forming a graben between them. A discontinuous cover of Triassic Dolomitic Conglomerate blankets the ORS strata and, in places, excellent examples of angular unconformity are present.

Angular unconformity between Devonian Old Red Sandstone and coarse Triassic Dolomitic Conglomerate in Kilkenny Bay

Calcrete at Portishead
Calcrete can been seen in the Lower Devonian cliff exposures of BNS.  This calcrete formation is known in the South Pembrokeshire area as the Chapel Point Calcrete. It extends across South Wales and can be seen on the shore cliffs of the Severn at Lydney, where it is known as the Bishop’s Frome Limestone. For more info on this topic, click here.
 
Richard Kefford & Charly Stamper

References

- Savage R.J.G. 1977. Geological Excursions in the Bristol District.
- Barclay W.J. 2005.  Introduction to the Old Red Sandstone of Great Britain (GCR) Chapter 1. 

- BGS. 1968 Geological Sheet ST 47  Clevedon and Portishead. 

- BGS. 2004 England and Wales Sheet 264  Bristol.