The Coastline between Staithes and Port Mulgrave

FORMATION

MEMBER

      Whitby Mudstone

Alum Shale
Mulgrave Shale (includes the Jet Rock and overlying Bituminous  Shales)
Grey Shale

      Cleveland Ironstone  

Kettleness
Penny Nab

      Staithes Sandstone 

       Redcar Mudstone 

Cowbar Nab

Perhaps one of the most famous Jurassic sites in Northern England is on the North Yorkshire coast between Staithes and Port Mulgrave. The rocks exposed are Lower Jurassic, all of which were deposited in marine conditions of varying depth. The formations and members are illustrated in the table above. Check the tides before doing this traverse!

 

The Staithes Sandstone Formation (SSF) consists of shallow marine sandstones and siltstones. Notable features, as seen in the illustration to the left, are well bedded coarser sandstones alternating with finer sediments where the bedding has been destroyed due to bioturbation. Cowbar Nab is the type locality for this formation.

Staithes Sandstone Formation

Jet Wyke
Ironstone seams

 Above the SSF is the Cleveland Ironstone Formation (CIF), so named because of the ironstone seams which were once quarried or mined.
        The seams, as seen in the illustration to the left, are clearly visible, usually being coated in a white clay mineral, probably dickite. In downward sequence they are the Main, Pecten, Two Foot, Raisdale and Avicula seams, the last one, in this picture, resting on the wave-cut platform.  Each seam forms the top of a coarsening up cycle. The cave in the Jet Wyke illustration is due to a plane of weakness caused by a small fault, downthrow to the right. Some of the ironstones are oolitic, as seen in the cross polarised thin section. The extinction cross is a characteristic feature of concentrically layered ooliths.

   Another interesting feature to be seen within the the CIF in Jet Wyke are the so called "striped beds" with infilled gutters. The gutters can be seen below the Raisdale Seam in plan view, to the right, and cross section, as illustrated below. The striped beds and gutters are believed to have been caused by storm conditions, and are known as "tempestites".

Gutters (plan view)
Gutters seen below the Raisdale Seam in plan view

Gutters in striped beds
Striped beds with gutter casts

Rhizocorallium
Rhizocorallium burrow

The coastline between Staithes and Port Mulgrave is quite fossiliferous. It is, however, a Site of Special Scientific Interest and therefore collections must only be made from loose material. Examination of the Main Seam will reveal many horizontal U shaped burrows, above right, believed to have been made by a shrimp like creature named Rhizocorallium. The Grey Shale Member is best exposed in Brackenberry Wyke. Here, specimens of the zonal ammonite Dactylioceras tenuicostastum can be found.


Dactylioceras tenuicostatum

Oolith
Oolith

Many of the Jurassic ironstones contain ooliths consisting of concentric layers of chamosite, an iron silicate, or their derivatives, berthierine or goethite. They are formed by the precipitation of minerals around a nucleus,  e.g. a shell fragment being rolled around by oscillating currents. The concentric structure of the mineral results in a cruciform extinction pattern when viewed under the microscope with cross polarised light. The illustration to the left shows an oolith which is 1mm in diameter.

From Brackenberry Wyke to Rosedale Wyke at Port Mulgrave the overlying Mulgrave Shale and Alum Shale Members are exposed in the cliffs. Pieces of jet, a hard black type of lignite derived from wood belonging to auracarian trees resembling the modern day Monkey Puzzle tree, can occasionally be found among the rocks on the shore in Rosedale Wyke.

 

Rosedale Wyke, Port Mulgrave
Rosedale Wyke near Port Mulgrave

More information about this  area can be found in the following excellent guides: the G.A. guide "The Yorkshire Coast", 
 the Y.G.S. guide "Yorkshire Rocks and Landscape". Details can be found on the Recommended Reading page.

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