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Benbulben

County Sligo, Ireland

""Spectacular Scenery
Benbulben

Ireland’s very own table mountain, the whole Benbulben Plateau exemplifies a sequence of Carboniferous age limestones and shales that best represent this stratigraphical sequence found across much of northwest Ireland.

BenBulbenThe Benbulben Shale Formation (which overlies the Mullaghmore Sandstone Formation) is well exposed on the lower slopes of Benbulben and in some stream sections. 

Limestones defining the base of this formation are succeeded by a 90m thick package of thinly bedded black shales with occasional impure limestone bands. The shales are rich in fossils including brachiopods, caniniid corals and bryozoa. 

The top of the formation is characterized by the reappearance of limestone beds. A transitional phase from predominantly deltaic sedimentation to predominantly carbonate sedimentation is reflected in the overlying limestone and shale of the Glencar Limestone Formation. The gradual upward transition from the Benbulben Shale Formation to the Glencar Limestone Formation is well exposed in a stream section at Tievebaun. 

Individual Glencar limestone beds average 10cm-20cm in thickness. Some beds are bioturbated (reflecting the burrowing activity of the organisms that lived in the sediment) and fossils are locally abundant. 

Overlying the Glencar Formation are the blue/grey limestones of the Dartry Limestone Formation. The limestone here is rich in fossils and also contains blue/grey nodules of chert, which formed during compaction and lithification early in the rock’s history. A mud-bank limestone mound caps the Benbulben range. This upstanding structure could have formed in an environment analogous to modern day coral reefs.

Landscape shaped by ice

BenbulbenAs well as excellent exposures of Carboniferous rocks, Benbulben displays important Quaternary structures including nunatak landscapes (where isolated rock peaks project through an ice-sheet) and scree slopes formed by freeze-thaw weathering.

Ben Bulben is an iconic landmark in Sligo, and as a representative geological section, the whole plateau is important, with individual parts such as Ben Whiskin, as significant as Ben Bulben itself. The whole plateau is already an SAC/NHA for its biological interest, and it is recommended as a geological NHA. The site is expected to be part of a large multi-interest geological NHA comprising Benbulben and the Truskmore Plateau (including parts in Leitrim).

Text: Sarah Gatley 

100 Great Geosites

Related Links

References

  • The Geological Heritage of Sligo. Geological Survey of Ireland. 2004

Images (top to bottom): 

  • Benbulben © David Plunkett
  • Benbulben © BigBear_Martin, Public Domain, source Wikipedia
  • Benbulben © David Plunkett
Earth Science Week 2017

Earth Science Week
October 7-15
Theme: 'Our Restless Earth'