Photographic feature: Gypsum nodules at Whitmore Bay, Barry

Gypsum nodules at Whitmore Bay, Barry

This photographic feature discusses the gypsum nodules that can be found at Whitmore Bay, Barry, Wales, UK. Whitmore Bay is a popular sandy beach on the South Wales (UK) coastline. In the west of Whitmore Bay, the cliff sections expose red-brown mudstones and siltstones of the Triassic Branscombe Mudstone Formation (Mercia Mudstone Group, formerly known as the Keuper Marl).

Whitmore Bay, Barry

Whitmore Bay, Barry. Copyright Gareth James (Creative Commons Licence)


Within the sub-horizontal mudstones and siltstones are white to pinkish brown gypsum nodules, some of which have dissolved leaving hollows or voids, others have been partially replaced by calcite. The nodules are locally known as ‘potato stones’. Similar voided areas can be found at Jackson’s Bay, approximately 1km to the east (Strahan and Cantrill, 1902). Nodules of gypsum up to 0.63m have been reported within the South Wales area (Bevins, 1994).


Partially dissolved gypsum nodule (part replaced with calcite) within the red-brown mudstones and siltstones at Whitmore Bay, Barry. Copyright Tom St. John



Nodule of gypsum from Whitmore Bay, Barry. Copyright Tom St. John.


Engineering Implications

The dissolution of sulfate minerals such as gypsum can lead to a number of problems for civil engineering, including leaching of sulfates and development of voids/cavities within a rock mass. Hawkins (1979) reported on the presence of cavities created through sulfate dissolution in the Triassic Keuper Rocks of the Bristol (UK) area and the associated engineering problems encountered during tunnel excavation. Further information on dissolution and subsidence problems relating to gypsum-bearing strata in the UK can be found in the article “A hole lot of trouble” by Farrant and Cooper (2014). See below for references and further information.

further information

Bevins, R.E. (1994). A mineralogy of Wales. National Museum of Wales. Geological Series No. 16, Cardiff.

Farrant, A., and Cooper, T. (2014). A hole lot of trouble. Geoscientist. 24 (6), 12-27. Click here to read article.

Hawkins, A.B. (1979). Case histories of some effects of solution/dissolution in the Keuper rocks of the Severn Estuary region. Quarterly Journal of Engineering Geology and Hydrogeology. 12, 31-40. Click here to read the abstract.

Lawrence, T., and Hamilton, W. Vale of Glamorgan Field Trip Report. Click here to read report online.

Strahan, A., and Cantrill, T.C. (1902). The Geology of the South Wales Coal-field; The country around Cardiff (Sheet 263 of the map).

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