Previous SiteNext Site

CultraDown
Magnesian Limestone overlain by Permian Upper Marls at western end of section, Cultra foreshore, Co.Down.
Summary Full report
Site Type: Coastal section
Site Status: ASSI
District: North Down Borough Council
Grid Reference: J413810
Rocks
Rock Age: Permian, Carboniferous (Zechstein)
Rock Name: Magnesian Limestone, Magnesian Limestone (Durham), Lower, Permian Upper Marls
Rock Type: Breccia, Dolomite, Marl
Interest
Fossil Groups: Bivalve, Gastropod, Microfossil
Other interest: No data, Marine sediments

Summary of site:

There are only three small surface exposures of Permian rocks in Northern Ireland and the outcrop on the foreshore at Cultra, about 320 m long, is the most extensive and varied. It is a fragment of a much more extensive succession of rocks that have been badly mauled by earthquakes and faulting.

During the Permian the area that is now Northern Ireland was equatorial, at the eastern end of an extensive mountainous region on the continent of Laurentia. The climate was hot and arid and in this desert environment the mountains shed angular fragments of Ordovician and Silurian rocks, draping their flanks in screes and debris fans. Infrequent but locally intense rainfall spread this coarse breccia into the valleys where, in places, it was hundreds of metres thick. Cultra appears to have been marginal to the lowlands and only 1.5m of these rocks (now called the Enler Group) are present here. Later in the period a limb of the Zechstein Sea, called the Bakevillia Sea, inundated the lowest areas and a fine dolomitic limestone, the Magnesian Limestone, was deposited on to the breccia sea bed. At Cultra this limestone forms thick yellow beds, 8.4m thick, containing over 30 species of fossils, mostly sea snails and bivalve molluscs. Examples can be seen in the collections of the Ulster Museum. The sea then shallowed and lime-rich, sandy and muddy marls, reddish-brown in colour, settled on to the limestone. These are the Connswater Marls and, at Cultra, 3m can be seen before they are lost beneath the waters of Belfast Lough. There is fossil evidence from borehole cores to suggest that the Magnesian Limestone and Connswater Marls are later, perhaps by as much as 6 million years, than the Enler Group. Much can happen in 6 million years but there is no evidence at Cultra to indicate what. The rocks are extensively overgrown by shellfish but there are no immediate threats to the site. Shores along the lough are prone to the sudden appearance of slipways, so there is a need for continuous vigilance.


Previous SiteNext Site

|Home|